1980 - Sissom - Life Histories of Two North American Scorpions Centruroides Vittatus and Vaejovis us

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LIFE HISTORIES OF TWO NORTH AMERICAN SCORPIONS

CENTRUROIDES VITTATUS (SAY) (BUTHIDAE) AND

VAEJOVIS BILINEATUS POCOCK (VAEJOVIDAE)

by

WILLIAM DAVID SISSOM, B.S.

A THESIS

IN

ZOOLOGY

Submitted to the Graduate Facultyof Texas Tech University in

Partial Fulfillment ofthe Requirements for

the Degree of

MASTER OF SCIENCE

Approved

<r

August 1980



C^fi * ^ ^ ACKNOWLED GMENTS

I w ish , in particular, to express my thanks and appreciation

to Dr. Oscar F. Franc k e, the chairman of my committee, who gave

so freely of his time throughout the course of this study. His

encouragement, willingness to share knowledge and ideas in discu<;sion,

and friendship made all of this work possible. I also wish to

thank the other members of my committee, Drs . Robert W. Mitchell and

Willi am P. Morri son for serving in that capacity. Their reviews

and comments on the manuscript are sincerely appreciated.

Several individuals helped in collection and care of specimens,

and I am grateful to them: James C. Cokendolpher, Dr. 0. F. Franck e,

Jane Gro en, Steve Jones, Dale K ing, Dr. Robert W. Mitche ll, James V.

Moo dy, and Fred W. Wagner. Mr . Cokendolpher also brought to light

many interesting ideas concerning life histor ies.

I also wish to extend a very special thanks to Lisa Nelms for

her help and encouragement throughout the latter portions of this

study. Her time and consideration were given unselfishly during

that time and are truly appreciated.

Finally, my parents provided m e w ith support, both moral and

financial, and understanding while I was a student at Texas Tech

University. To them I am extremely grateful and appreciative, for

without them all of this would have been impossible.

1 1



1 1

TABLE OF CONTENTS

ACKNOWLEDGMENTS

LIST OF TABLES iv

LIST OF FIGURES vi

INTRODUCTION 1

Purpose and Scope of the Thesis 1

Review of Previous Research 3

The Hypotheses 13

MATERIALS A ID PROCEDURES , 17

LIFE HISTORY OF CENTRUROIDES VITTATUS (SAY) 23

Introduction , 23

Results 23

Discussion 40

LIFE- HISTORY OF VAEJOVIS BILINEATUS POCOCK 50

Introduction 50

Results 50

Discussion 58

GENERAL DISCUSSION 69

Tests of Hypotheses 69

Field vs . Laboratory Observations 72

SUMMARY , 30

LIST OF REFERENCES 83

iii



LIST OF TABLES

TABLE Page

1. Measurements of adult females and second instar young, and

growth factors from previous life history studies . . . . 10

2. Estimation of the number of molts (N) from second instar

to adult in three litters of Centruroides vittatus (Say)

based on the prediction formula 14

3. Estimation of the number of molts (N) from second instar

to adult in five litters of Vaejovis bilineatus Pocock

based on the prediction formula 15

4. Duration of instars and cumulative age at time of molting

in days in laboratory-reared Centruroides vittatus (Say) . 31

5. Measurements in mm and growth factors of observed instars

(mean + standard deviation); predicted 95% confidence

intervals for instars not observed in the laboratory; and

selected measurements in mm of field-collected specimens

of Centruroides vittatus (Say) 32

6. Duration of instars and cumulative age at time of molting

in days in laboratory-reared Vaejovis bilineatus Pocock • • 56

7. Measurements in mm and growth factors of observed instars

(mean + standard deviation) ; predicted 95% confidence

intervals for instars not observed in the laboratory; and

selected measurements in mm of field-collected specimens

of Vaejovis bilineatus Pocock 5^

iv



TABLE Page

8. Results of Duncan's multiple range test to determine

significance of size differences between litters of

Vaeiovis bilineatus 68

9. Measurements and growth factors of five structures

from an immature Paruroctonus mesaensis collected

near Indio (Riverside Co.), California 76

10. Measurements of adult females of Paruroctonus mesaensis

from the 0. F. Francke collection 78

V



LIST OF FIGURES

FIGURE Page

1. Diagrammatic representation of carapace length used in

life history studies 22

2. Diagrammatic representation of chela length used in

life history studies 22

3. Diagrammatic representation of metasomal segment V length

used in life history studies 22

4. Dorsal view of late embryo of Centruroides vittatus . . . 26

5. Ventral view of late embryo of Centruroides vittatus . . . 26

6. Bar graph showing the presumed correlation between occurrence

of deaths in second instar Centruroides vittatus to times

of exuviation 30

7. Internal face of pedipalp femur of second instar Centruroides

vittatus 36

8. Internal face of pedipalp femur of third instar Centruroides

vittatus 36

9. Movable finger of pedipalp chela in third instar specimen of

Centruroides vittatus 39

10. Movable finger of pedipalp chela in fourth instar specimen of


11. Movable finger of pedipalp chela in fifth instar specimen of


12. Plot of carapace length vs , chela length in Centruroides

vi



FIGURE âee

vittatus 43

13. Plot of carapace length vs . metasoma V length in Centruroides

vittatus 46

14. Bar graph showing the presumed correlation between the

occurrence of deaths in second instar Vaejovis bilineatus

to times of exuviation 5^

15. Plot of carapace length vs . chela length in Vaejovis

bilineatus 64

16. Plot of carapace length vs . metasoma V length in Vaejovis

bilineatus 66

V I 1



INTRODUCTION

Purpose and Scope of the Thesis

There are two basic methods for determining the number of instars

required by scorpions to reach maturity. The better method is the

direct method, where scorpions are reared from birth to maturity and

the number of instars counted. Because this is usually difficult,

indirect methods w ere developed to estimate the number of instars,

and these methods are used both in the laboratory and in the field.

In the laboratory the number of instars can be indirectly

determined if the rearing of young fails to yield a complete life

history. Measurements are taken from all the instars observed, and

by extrapolation confidence intervals for dimensions of the structures

measured are obtained for succeeding instars. These results are

then compared w ith specimens know n to be adults and the instar

representing the adult stage is ascertained.

In the field morphometric methods are used. Large samples are

measured and grouped into modal size groups which presumably

represent the number of instars in the life cycle. However, actual

ecdyses are not observed, and growth is based on estimations of

population averages.

The primary purpose of this research is to present a third

indirect method and to test its accuracy by comparisons with data

from the literature and with observations obtained from laboratory



rearings of two North American scorpion species.

It has long been k nown that arthropod growth is limited by the

presence of an exoskeleton and that dimensions of body structures grow

at a near-constant increment at each ecdysis. This latter characteristic

was first reported by Dyar (1890), and the growth increment has since

been know n as Dyar's constant. In scorpions there is generally an

increase of 25-35% in dimensions between consecutive instars (Francke

1976) , although some structures on a given molt can increase in a

linear dimension by as much as 60% . Thus, each structure has its

own growth increment and this can vary with each molt. For these

reasons I prefer to use the term "growth factor" rather than "Dyar's

constant".

If this growth factor is known or estimated (and expressed as

the dimension of a structure divided by its dimension in the previous

instar), and measurements of adults and young of known instar are

available (which is often the case in scorpions), then an equation

can be derived to determine the number of molts between the known

immature stage and adult. If A is the dimension of the adult

structure, Y is the dimension of the same structure in a young

specimen of k nown age , G is the growth factor of the measured

structure, and N is the number of molts required by the young

specimen to reach adulthood; then the equation may be expressed as

follows:

NA = Y ( f / ) ,

that can be transformed into logarithms, giving:



log A = log Y + N log G.

By rearrangement the work ing formula can be obtained:

^ log A - log Ylog G

This equation is a modification of original growth equations

by Huxley (1932) and Teissier (1960), and w as put into this usable

form for scorpions by Francke (personal communication).

A secondary purpose of this thesis is to report observations on

the life histories of two common North American scorpions:

Centruroides vittatus (Say) and Vaejovis bilineatus Pocock. The

number of instars to maturity will be predicted and tested by rearing

specimens. Various instars w ill be compared morphologically and

actual growth factors of three measured structures will be calculated.

Review of Previous Research

Studies on the post-birth development of the order Scorpiones

are relatively few in number, but the last decade has produced a

surge of interest in this area. At present species of five families

have been investigated: Buthidae, Chactidae, Diplocentridae,

Scorpionidae, and Vaejovidae. These investigations will be considered

herein according to the methods used in determining the number of

instars to maturity for the particular species.

In Table 1 measurements are given for second instar young and

adult females. These measurements were obtained from all published

sources in which adequate measurements (for calculation of growth

factors) were presented.



Direct Methods

The first account of a scorpion reared from birth to maturity

was that of Schultze (1927), who reared Heterometrus longimanus (Herbst),

a scorpionid. He reported that the first two molts occurred in the

first 18 days after birth and made careful observations on instar

duration and behavior. He recognized eight instars to adult, but

reported no measurements . Consequently, the rate of growth of each

stage is unknown.

A rather detailed study of the biology of the chactid Euscorpius

italicus (Herbst) included a report on its life history (Angermann

1957). He was the first to observe maturation molts to occur at two

instars — the sixth and seventh. Only one female required the

additional molt to attain maturity in his study. Measurements of

carapace length were included in this paper.

Another scorpionid, Pandinus gambiensis Pocock, was reared to

maturity by Vachon ejt al . (1970), and males of that species were found

to mature at the seventh or eighth instar. They divided morphological

characters into two groups based on life history information: (1)

those stabilized at birth (such as the number of pectinal teeth,

trichobothria, and tarsal spines on the legs) and (2) those changing

with age (such as size and setation). Characters stabilized at birth

are considered very important in taxonomy. Measurements of the

metasoma, chela, and body (prosoma and mesosoma) were given in this

study for several instars . Because these measurements are inadequate

for calculation of growth factors, they are omitted from Table 1.



The remaining direct-method studies were done on buthids. Shulov

and Amitai (1960) raised Orthochirus innesi Simon to adulthood at the

sixth instar. The number of hairs on the pectinal fulcra was observed

to increase during the immature stages in that species. Total body

length of all stages was measured and reported.

Buthus occitanus Amoreux w as reared in the laboratory by Auber

(1963). She reported first instars ("larvae") to lack claws on the

tarsi and to possess well-formed pectines with the full complement of

teeth. She also reported sexual maturity to be attained with the

seventh instar. Later (Auber-Thomay 1974), she reared Androctonus

australis (L.) and again observed the constancy in pectinal tooth

counts throughout development. A.. australis was found to mature at

the eighth instar. In both contributions measurements of chela

length of all stages in the life cycle were presented.

Several studies have been done with members of the genus Tityus.

Both Matthiesen (1962) and San Martin and Gambardella (1966) found

T. serrulatus Lutz and Mello to be parthenogenetic, females giving

birth at the sixth instar. Matthiesen presented no measurements, but

San Martin and Gambardella provided measurements of some of the

instars. The measurements given do not allow calculation of growth

factors and will be omitted from Table 1. Matthiesen (1970) was

able to raise several specimens of T_. bahiensis (Perty) and found

females to attain maturity at the sixth instar. In this contribution

he presents measurements of the movable finger of the pedipalp chela

for the instars.



Lourengo (1978) reared six individuals of Tityus trivittatus

fasciolatus Pessoa to the sixth instar and found males to mature at

either the fifth or sixth instar. Some field-collected males were

noticeably larger and were estimated to be seventh and eighth instars.

Females were considered to be mature at the sixth instar. Measurements

of carapace length and-metasomal segment V length were reported

in :this contribution.

A very detailed account of the life history of Isometrus maculatus

(De Geer) was presented by Probst (1972). ^. maculatus achieves sexual

maturity at the seventh instar, but 5-10% of the females were found

to mature at the sixth instar. Not only did Probst present measurements

(carapace length, chela length, and metasoma V length), but he

calculated growth factors for each of the structures and compared

growth of males and females. He found that allometric sexual

dimorphism, which is strong in this and many other buthids, is not

distinct until the fifth or sixth molt.

A detailed morphological analysis of the different instars in the

life cycle w as done on Buthotus minax occidentalis (Vachon and Stockmann)

by Stockmann (1979). He observed increases in the number of hairs on

the fulcra and middle lamellae of the pectines and in the number of hairs

around the basitarsal spurs. In addition he noted increases in the

number of sensillae on the pectines and in the number of lyrifissures

on the articulations of the appendages. Males of B . minax occidentalis

reach maturity at the sixth or seventh instar and females do so at the

seventh or eighth. No measurements were reported in this study.



Francke (unpublished data) was able to rear two litters of

Centruroides gracilis (Latreille) to maturity w ith very low mortality

permitting detailed analysis of the life history of this species. C_.

gracilis was found to mature at the seventh instar (some males mature

at the sixth) with an average growth rate of 27.5%. Measurements

of carapace length, pedipalp femur, tibia, and chela lengths, metasoma

V length, and telson length were used in that study.

Indirect Methods

The number of instars in the life history of eight scorpion species

has been estimated by the use of indirect methods. Five of these

species were reared through part of their life history in the laboratory

and three species have been observed in the field (two studies have

been done on the same species). By necessity the number of instars

was determined morphometrically, and measurements w ere presented in

all studies except that of Smith (1966) . In that paper was presented

a graph plotting the logarithms of measurements obtained from two

structures, and the measurements can be estimated from this graph.

Laboratory Method

A subadult specimen of Diplocentrus spitzeri Stahnke

(Diplocentridae) molted to maturity in the laboratory (Francke, in

press) and by using the growth factors obtained from that molt, it

was hypothesized that D. spitzeri attains maturity at the sixth

instar. The observations presented in this paper represent the

only observations made on this family of scorpions to date.





size class for first instar young, which do not appear on the surface

with the rest of the population) , and stated that P_. mesaensis has

eight instars. Dissection revealed that P. mesaensis matures at the

presumed seventh instar, and this was interpreted to mean that P_.

mesaensis has a postmaturation molt , something that has never been

observed in scorpions.

Polls and Farley (1979) in a more detailed study found seven

distinct size classes (including first instars) and concluded that

P. mesaensis has seven instars in the life cycle. They reported

that there was definitely no postmaturation molt and calculated an

average growth factor of 1.38, which is rather high in comparison

with other scorpions. This large growth factor was attributed to

differences between field and laboratory data.

Fox (1975) also analyzed populations of Paruroctonus baergi

(Williams and Hadley). There were presumably seven distinct size

classes in its popula tion, or a total of eight instars (including

first instars). Again a postmaturation molt was hypothesized.

Finally Smith (1966) published information from field studies

on Urodacus abruptus Pocock (Scorpionidae) and recognized six instars

in that scorpions's life cycle. He included no measurements but

plotted logarithms of measurements of two structures on a graph.



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13

The Hypotheses

Earlier, I presented a derivative of the well-known growth equations

of Huxley (1932) and Teissier (1960), which will allow prediction of the

number of molts to maturity. This formula wi ll be used for the first

time in hypothesizing the number of molts to maturity in two scorpions:

Centruroides vittatus and Vaejovis bilineatus. The hypotheses wil l be

tested by rearing litters of these species in the laboratory.

For Centruroides vittatus the values for dimensions of the

structures of second instars and adults are presented in Table 2.

The growth factor used in the formula is the average value of all

observed growth factors from published laboratory studies (Table 1)

wh ere growth and ecdyses are measurab le. This average growth factor

is approximately 1.29.

On the basis of the obtained values of N (rounding to the nearest

molt) , the prediction is that C _. vittatus should molt five times between

the second instar and adult. Including the first molt (from first to

second instar) this gives a total of six molts in the life cycle.

The ref ore , I hypoth esize that C_. vittat us reaches maturity at the

seventh instar.

For Vaejovis bilineatus the data are presented similarly in

Table 3. From the obtained v alues of N the prediction is that V;.

bilineatus should molt four times between the second instar and adult.

Including the first molt a total of five molts should occur in the life

cycle. Ther efor e, I hypothesize that V. bilineatus matures at the

sixth instar.



14

Table 2. Estimation of the number of molts (N) from second instarto adult in three litters of Centruroides vittatus (Say) based on theprediction formula.

Specimen orParameter

LITTER #1Female

log A2nd instar

log Y

mean

Growth Factorlog G

CalculatedLITTER #2

Femalelog A

2nd instarlog Y

N

mean

Growth Factorlog G

CalculatedLITTER #3

Femalelog A

2nd instarlog Y

N

mean

Growth Factorlog G

Calculated

Average_Alog AAverage Y

log YGrowth Factor

log GCalculated N

N

All measurements are

FormulaValue

A

Y

G

N

A

Y

G

N

A

Y

G

N

A

Y

G

N

AverageAverage

Average

NN

N

CarapaceLength

4.810.681.69

0.231.290.114.09

5.330.731.620.211.290.114.73

4.680.671.670.221.290.114.09

4.940.691.660.221.290.114.27

for LITTERfor LITTER

for LITTER

in mm.

ChelaLength

8.130.912.66

0.421.290.114.46

9.150.962.410.381.290.115.27

7.980.902.560.411.290.114.46

8.420.932.540.401.290.114.82

#1: 4.46#2: 5.33

/ 3: 4,55GRAND AVERAGE N (ALL LITTERS): 4.78

Metasoma VLength

5.640.751.66

0.221.290.114.82

6.800.831.480.171.290.116.00

5.600.751.540.191.290.115.09

6.010.781.550.191.290.115.36



15

Table 3. Estimation of the number of molts (N) from second instarto adult in five litters of Vaejovis bilineatus Pocock based on theprediction formula.


LITTER #1Female

log A2nd instar mean

log YGrowth Factor

log GCalculated N

LITTER #2Female


log YGrowth Factor

log GCalculated N

LITTER #3Female


log YGrowth Factor

log G-*• ** o

Calculated NLITTER #4

Femalelog A

2nd instar meanlog Y

Growth Factorlog G

Calculated NLITTER //5

Femalelog A

2nd instar mean

log YGrowth Factor

log GCalculated N

All measurements are

FormulaValue

A

Y

G

N

A

Y

G

N

A

Y

G

N

A

Y

G

N

A

Y

G

N

CarapaceLength

4.150.621.52

0.181.290.114.00

3.550.551.370.141.290.113.72

4.150.621.410.151.290.114.27

3.750.571.420.151.290.113.82

3.530.551.410.151.290.113.64

in mm.

ChelaLength

4.900.691.70

0.231.290.114.18

4.200.621.500.181.290.114.00

5.050.701.570.181.290.114.73

4.630.671.610.211.290.114.18

4.270.631.590.201.290.113.91

Metasoma VLength

4.360.641.46

0.161.290.114.36

3.800.581.260.101.290.114.36

4.500.651.260.101.290.115.00

4.230.631.350.131.290.114.54

3.810.581.320.121.290.114.18



Table 3. (Continued)

16


FormulaValue

A

Y

G

N

CarapaceLength

3.830.581.430.161.290.113.89

ChelaLength

Metasoma VLength

Average Alog A

Average Ylog Y

Growth Factorlog G

Calculated N

Average N for LITTER HAverage N for LITTER #2Average N for LITTER #3Average N for LITTER /MAverage N for LITTER #5

4 . 6 10 . 6 61 . 6 10 . 2 11 . 2 90 . 1 1

4 . 2 0

4 . 1 84 . 0 34 . 6 74 . 1 83 . 9 1

4 . 1 40 . 6 21 . 3 40 . 1 31 . 2 90 . 1 1

4 . 4 9

GPxAND AVERAGE N (ALL L IT T E R S ): 4 , 1 9



MATERIALS AND PROCEDURES

Approximately 250 specimens of Centruroides vittatus and Vaejovis

bilineatus were available for life history studies. This number not

only includes females and their lit ters, but also approximately 50

specimens from the 0. F. Francke collection used for comparative

purposes. Only specimens taken near or at the collecting localities

of the pregnant females were used for comparison.

Specimens of C_. vittatus came from several sources: (1) four

pregnant females from Rio Grande Village, Big Bend National Park,

Brewster Co., Texas on 29 July 1978 by 0. F. Francke and J. V. Moody

(only two of these females gave birth, n = 24 on 3 August, and n = 21

on 21 August 1978); (2) one pregnant female from Glenn Springs, Big

Bend National Park , Brewster Co. , Texas on 23 March 1979 by R. W.

Mitchell (aborted 28 young in June 1979); (3) one pregnant female

from White River Lak e, Crosby Co. , Texas on 3 September 1979 by S.

Prien (gave birth 20 September 1979, n = 32); (4) one immature female

from two miles north of Wink , Winkler Co. , Texas on 12 November 1977

by J. Groen (molted to maturity in the laboratory); and (5) a freshly

molted adult male from 20 miles east of Bracketville, Kinney Co. ,

Texas on 4 June 1980 by J. C. Cokendolpher (found underneath a rock

with its exuvia).

Five pregnant females of V. bilineatus were collected at Villa

Hidalgo, San Luis Potosi, Mexico by R. W. Mitchell et al. in March

17



18

1977. All females gave birth in the laboratory during that summer.

In addition an immature specimen collected with the pregnant females

molted to maturity in the laboratory.

All specimens were maintained in an environmental chamber as

described by Francke (1976, 1979, in press). Temperature was constant

( 2 6 . 6 + 1 C) except for a one month period (June 1979) when the

heating system failed. During this time scorpions were moved to room

temperature (approximately 21° C ) .

Young scorpions born in the laboratory were separated and

placed in individual containers (3 oz. jars with lids) as soon as

the litter dispersed from the mother's back. A bit of paper towel

or a layer of soil was placed on the floor of the jar to provide

a substrate, and a moistened sponge was used for watering. A variety

of food was offered to the young scorpions, including newly hatched

crickets, newly emerged wingless Drosophila sp., young cockroaches

of two species [ Nauphoeta cinerea Saussure and Supella supellectilium

(Serville)], and termites. All of these prey items were introduced

into the containers alive, and all were accepted quite readily.

In addition chopped "steaks" of Nauphoeta were fed to the earlier

instars when living prey of small size were not readily available.

That young scorpions accept such food matter has been documented

by Francke (1979). All litters of scorpions were fed at least

twice week ly (more often depending upon the availability of food),

and adults were generally fed three to four times per month.



1<

Data were collected with each molt or death of a laboratory

specimen . Exuviae from the molts were stored in two and three dram

via ls , and dead specimens w ere preserved in 80% ethanol. With each

a label was placed bearing identification information, instar number,

and date of the event.

Tw o methods w ere used in predicting the number of instars to

matur ity. At the onset of the investigation, second instar young and

adult females were measured, and the values obtained were substituted

into the proper place in the formula N = °^ ^ ~ ^ ° " — . The growthlog G

factor used was the average of all reported values (1.29) from

laboratory studies including measurements. The value of N obtained

represented the predicted n umber of molts between the second instar

and adu lt, and the total number of molts w as obtained by adding one

(the mo lt from first to second instar).

The failure of specimens from the litters to reach maturity

required the use of the indirect method for estimating the number of

instars to maturi ty. From the size of the last instar observed 95%

confidence intervals we re used to predict the sizes of the next instars,

as explained in detail by Francke (1979). Known adults are compared

with the ranges of predicted measurements so that the instar at which

maturity is reached can be determined. Structures used are the same

as by Francke (carapace len gth, chela lengt h, and metasoma V length),

and the dimensions measured are illustrated in Figures 1-3. All

measurements were taken on an American Optical Model 569 binocular

dissecting microscope equipped with an ocular micrometer calibrated



20

at 20X. Illustrations were done with a Wild M7A dissecting scope

equipped with a drawing tube (TYP 256575).



21



22

1



LIFE HISTORY OF CENTRUROIDES VITTATUS (SAY)

Introduction

Of three litters of Centruroides vittatus (Say) born in the

laboratory during 1978 and 1979, two litters (n = 21 and n = 24)

were the pale yellow form formerly referred to as £. pantheriensis

Stahnke, and one litter (n = 32) was the typical striped form of

C _. vittatus. Five specimens from these litters were kept alive well

into the fifth instar (three specimens are still alive), one immature

specimen molted to adult after its capture and return to the

laboratory, and one adult specimen was captured with its last exuvia.

These and additional specimens from the 0. F. Francke collection were

used to elucidate the details of the life history of this common

North American scorpion.

Results

The young of Centruroides and other buthids are born surrounded

by a "birth membrane", and they escape from this structure before

ascent to the mother's back (Baerg 1961; Shulov and Amitai 1960;

Stahnke 1966; Williams 1969) where they stay until shortly after

their first mol t. The abortion of 28 young in the laboratory by

a female C_. vittatus captured in August 1979 allowed the opportunity

to observe young scorpions within their "birth membranes". Although

these specimens are of little value in studying the postbirth

development of C . vit tatus, there have been few descriptions or

illustrations of the development or position of scorpions in their

23





25



26

H



27

At birth the young scorpions are creamy white , and the appendages

possess a yellowish tinge. Portions of the carapace (particularly the

interocular area), pedipa lps, metasoma, and legs have considerable

dusky mark ings . The tergites bear rudiments of the two submedian

stripes which are so characteristic of typical C_. vittatus. The

ventral surface of the cauda has median, and sometimes two lateral,

dark bands; and metasomal segment V is completely darkened. Color

of the young gradually darkens until the next molt .

The lack of a number of typical adult characters at this stage

is rather distinctive (for a discussion relative to this, see Francke

1976:255-256). For this reason the stage has often been referred to

as a larval stage by French observers. A number of species at this

stage have been described (Auber 1963; Francke 1976; Lourengo 1978;

Polls and Farley 1979; Shulov and Amitai 1960; Williams 1969), and

the appearance of first instar C_. vittatus agrees very w ell with

other scorpions. Pectines are fully developed and contain the

full complement of pectinal teeth; the tarsi are blunt and lack claws;

the aculeus is soft and b lunt; trichobothria are absent; and except

for two or three fine setae on the tarsi of the legs, these cuticular

structures are absent as well. In addition granulation and carinal

development are lacking, pedipalp chelae lack dentate margins, and

the cheliceral teeth appear as barely discernible protuberances on the

inner margins.



28

Second Instar

The second instar (first nymphal stage of French observers) is

the stage at w hich young scorpions become independent from their

mothers. It is this stage which has caused problems in many life

history studies, because mortality due to starvation, molting

difficulties, or other causes is very high. This is true of the

present study, as well as several others (Francke 1976, 1979;

Hjelle 1974; Lourengo 1979). Mortality of second instar £ . vittatus

in this study was approximately 80%. Upon examination of the

specimens from two of the litters (n = 21 and 24) , it became probable

that many of the deaths were caused by molting difficulties, as

evidenced by separation of the old cuticle from the prosoma and by

the correlation of dates of death and dates of molts (Figure 6 ) .

Mortality in the third litter (n = 32) was largely due to starvation,

as specimens apparently refused the food offered them.

Specimens surviving the second instar (n = 15) molted to the

third instar at an age of 152.7 + 3 7 . 3 days. The second instar was

found to last 143.1 + 37.5 days (range 104-256 days). For actual

chronologies of specimens molting in the laboratory see Table 4. For

measurements of second instar structures consult Table 5.

At the second instar, young scorpions possess most of the

characters found in adults. The body is now covered with setae, and

trichobothria are found in normal numbers on the pedipalp chela and

tibia; but the internal face of the femur in C. vittatus lacks one —

trichobothrium i «, according to Vachon's (1974) terminology, is missing





30

MOLTS

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(Fig. 7 ) . This same trichobothrium w as found to be missing in second

instar Buthotus minax occidentalis by Stockmann (1979), in Tityus

bahiensis and T . serrulatus by Stewein and Delgado (1968), and in

Centruroides gracilis by Francke (unpublished data). According to

Vachon (1974), this trichobothrium is missing in all second instar

buthids.

Carinae are present and moderately developed, though crenulations

and serrations are quite feeble. Development of carinae is rather

gradual throughout the instars, and it is very difficult to age

specimens using this character.

Other structures are also well developed at the second instar.

The aculeus is now hardened, sharp, and useful in subduing prey. The

teeth of the chelicerae are hardened, and the nodules are present on

the ventral surface of the cheliceral fingers. The dentate margins of

the pedipalp chela fingers are well developed, and internal and

external lateral granules are present. Supernumerary granules are

absent in this stage and do not appear until the fourth instar (Figs..

9-11). Color of this and succeeding instars will be discussed later,

as comparisons w ill be made between the pale and striped forms of this

species.

Third Instar

Of 15 specimens reaching the third instar, six died during this

stage (mortality = 4 0% of second instar survivors, or 7.8% of the

original number). One of these specimens died shortly after its



35



36

7



37

second molt before it was able to expand its exoskeleton fully, and it

was omitted from third instar measurements for this reason.

Specimens surviving the third instar (n = 8, and one specimen

remains alive) molted to the fourth instar at an age of 223 + 45.3 days.

The third instar w as found to last 87 + 29.3 days (range 38-118 days).

For individual chronologies consult Table 4. For measurements of

third instar structures consult Table 5.

Other than s ize, there were few differences between the second and

third instar scorpions. The previously missing trichobothrium on the

pedipalp femur internal face is now present (Fig. 8 ) . Supernumerary

granules on the fingers of the pedipalp chela are still absent (Fig. 9 ) .

Fourth Instar

Five of the eight specimens reaching the fourth instar completed

this stage and one remains alive at this time. The fourth molt was

accomplished at an age of 342 + 127.7 days, and the duration of the

fourth instar w as found to be 128 + 74 days (range 59-210 days). For

chronologies of the individuals surviving this instar consult Table 4.

For measurements of fourth instar structures consult Table 5.

The only noticeable structural difference between third and

fourth-instar scorpions, other than size increase, is the presence of

supernumerary granules on the pedipalp chela fingers (Fig. 10). At the

fourth instar there is usually only one (but sometimes tw o) of these

granules betw een consecutive lateral granules, both on the internal

and external sides of the dentate margin (Fig. 10 ). The number of

these granules increases at the fifth instar (Fig.11).



38



39

. sg -

4 4 .

9 11



40

Fifth and Succeeding Instars

Four of the five remaining specimens died during the fifth instar,

and the fifth specimen remains alive. Therefore, the duration of this

stage is unknown at present, but three specimens in different stages

are still alive and could possibly survive this instar, or even reach

maturity.

Structurally, fifth instars resemble fourth instars rather closely.

At the fifth instar there are usually more supernumerary granules on

the fingers of the pedipalp chela, but due to the small sample sizes

of known fourth and fifth instars, I do not know how reliable this

character will be in separating these two age groups. Probably the

best external criterion to differentiate fifth instars from subsequent

stages is size, as specimens of these age groups are difficult to

identify by other means. Information pertinent to the late instars,

including predictions of the number of stages in the life history of

£. vitt atus , will follow immediately in the discussion.

Discussion

Growth

Even though laboratory-reared litters of C. vittatus failed to

yield a complete life his tory, the data obtained from them, from one

specimen which molted to maturity after its capture, and from an

adult found with its last exuvia are sufficient to observe growth in

this species, and predict 95% confidence intervals for instars not

observed in the laboratory. For each structure (carapace length, chela



41

length , and metasoma V length) a growth factor was calculated for the

second through fifth instars (see Dyar 1890; Francke 1976), and this

growth factor was used in predicting 95% confidence intervals for

sixth, seventh, and eighth instars.

The carapace growth factor of C_. vittatus is fairly steady through

the third molt but tapers at the fourth. Francke has found that the

carapace growth factor of C_. gracilis continues to taper through the

sixth, and final , mol t. The same cannot be said for the chela growth

factor, as it remains relatively constant throughout all observed

instars of £. vittatus. In C_. gracilis, however, Francke found that

chela growth factor shows rather definite allometry: Males experience

a significant increase in chela growth factor at the maturation mol t,

whereas in females it remains rather constant. Proper comparison

cannot be made here due to the small sample size of fifth instars in

this study, and the confidence limits obtained for succeeding instars

have been based on male and female chelae lengths considered together.

The values for these confidence limits (Table 5, Fig. 12) may be

slightly distorted for this reacon. Indeed, field-collected adult

males show a slight trend toward a higher ratio of chela length to

carapace length than females of the same age (Fig. 12) . Therefore,

it is probable that males of C . vittatus show considerable allometry

at the maturation molt.

It has long been know n that Centruroides scorpions show drastic

allometry in the metasoma, and this character is used to sex adults of

these scorpions on sight. Only metasoma V was analyzed in this





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44

contribution , but it should be pointed out that allometry involves all

five metasomal segments. Growth of m.etasoma V in C . vittatus was

found to be relatively constant until the fifth instar, when it

becomes possible to sex specimens due to a slight increase in male

growth factors. Of two specimens molting to maturity (one male from

Kinney Co ., and one female from Winkler Co., Texas), more drastic

allometry occurred at the maturation molt. The metasoma V growth factor

of this molt w as 1.61 for the male , and 1.33 for the female. Even

though this information came from only two specimens, Francke

(unpublished data) observed the same in £. gracilis: Male metasoma

V growth factors averaged 1.58, while females averaged 1.36. Because

such drastic allometry occurs in metasoma V, separate confidence

intervals were predicted at sixth, seventh, and eighth instars for

males and females using 1.58 for the male growth factor and 1.29

(average value for the observed growth factor in this study) for

the female growth factor. The confidence limits thus obtained for

the males are based on observations of C . gracilis and a maturation

molt for a male £. vittatus. These confidence intervals are subject

to error , but the available field observations of C . vittatus indicate

that growth of this structure in the two species is quite similar. It

should be noted at this point that field-collected specimens agree

closely with the predicted confidence limits (Fig. 13 ) , and there is no

reason to doubt that these limits are reasonably accurate.



45



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47

Number of Molts to Maturity

In predicting the number of molts to maturity, it was necessary to

use the indirect approach developed for scorpions by Francke (1976, 1979)

and used later by Lourengo (1978, 1979). Confidence limits for the size

of the three measured structures were obtained for late instars using

the method described in detail by Francke (1979) and are presented in

Table 5. Measurements from field-collected specimens were compared

with these confidence limits (obtained from the laboratory-reared

specimens) , and the number of molts to maturity is based on these

comparisons.

Maturity of field-collected males was determined by dissection of

hemispermatophores. This was done by making a longitudinal incision

in the pleural membrane of the mesosoma and looking inside for the

hemispermatophore. By using this technique the specimen remains

undamaged. Field-collected males fit predicted confidence limits for

both sixth and seventh instars. This is demonstrated in Figures 12

Cwhere chela length is plotted against carapace length) and 13

Cwhere metasoma V length is plotted against carapace length). Table 5

provides direct comparisons between measured specimens and the

confidence intervals.

The adult male from Kinney C o., Texas, which had just molted to

maturity prior to its collection, falls within seventh instar confidence

intervals for all three structures. The exuvia collected with it fits

sixth instar confidence intervals for all three structures.



48

Maturity of females was determined by means other than dissection

to prevent damage to specimens. Six females that gave birth were

found to be seventh instars, and females of comparable size were judged

also to be mature. One specimen is slightly larger than predicted size

for seventh instars (Figs. 12 and 13), but is far too small to be an

eighth instar, and I consider it to be a large seventh instar.

The female from Winkler Co. , Texas, which molted to maturity in the

laboratory, fell within seventh instar confidence intervals for all

three adult structures. Carapace measurements from the two exuviae of

this specimen were slightly lower than sixth (-0.09 mm) and fifth

C-0.14 mm ) instar confidence intervals; chela measurements w ere also

lower than predicted sixth (-0.07 mm) and fifth (-0.18 mm) instars; but

metasoma V fell within those levels. Therefore, I consider this

specimen to be a small adult at the seventh instar.

It should also be considered whether or not £. vittatus attains

maturity at the eighth instar. Because no field-collected adults

Cn = 26) fit near the predicted eighth-instar confidence intervals

(Table 5 ) , it becomes unlikely that this species matures at the eighth

instar.

In light of the above evidence, I conclude that males of C . vittatus

mature at the sixth or seventh instar, and that females do so at the

seventh instar. The possibilty exists that females may also mature at

the sixth instar, but no evidence here suggests this to be true.





LIFE HISTORY OF VAEJOVIS BILINEATUS POCOCK

Introduction

Five pregnant females of Vaejovis bilineatus Pocock from Villa

Hida lgo, San Luis Pot osi , Mexico gave birth in the laboratory (n = 17 ,

20, 22 , 25 , and 2 6 ) , and their litters we re reared. The litter of

one female (n = 17) died as second instars, and specimens were used

only for litter size inform ation, duration of first instar, and

meas ure ment s of second instar structures. Fourteen specimens from the

other litters entered the fourth instar and one of those entered the

fifth. An immature female collected w ith the adults at Villa Hidalgo

molted twice and reached maturity in the laboratory and remains alive

at the time of w ritin g. Theref ore, by combining these two sources

a complete life history was obtained. As in the study of C _. vittatus

the 0. F. Franck e collection provided useful comparative material

for life history predictio ns. These are the first observations on life

history for the genus Vae jov is, which is the most diverse genus of

scorpions in North America . Only three other vaejovids have been

studied: Uroctonus mordax by Francke (1976); Paruroctonus baergi

by Fox (1975); and Paruroctonus mesaensis by Polls and Farley (1979).

Comparisons will be made between all four vaejovids where relevant.

Results

First Instar

The young of Vaejovis bilineatus are born enclosed by the "birth

mem bra ne" as are many other scorpions. Upon freeing themselves from

50



51

this membrane, they climb onto the mother's back , where they stay until

after the first mol t. Once on the mother's back, the neonates orient

themselves in an organized fashion with the prosoma directed forward

and the metasoma back ward. This neat layering effect was reported

first by Williams (1969) for several Vaejovis and is apparently

characteristic for the genus. In this laboratory the same was

observed for Vaejovis coahuilae Williams and Vaejovis waueri Gertsch

and Soleglad.

The first molt occurred at an age of 9.8+0.45 days. Molting

of all the young took place in a single day. Dispersion of the young

took two to four days to complete, and this process began at an age of

16.75 + 1.71 days (7 + 1.63 days after the first molt). The young

were sorted after all were dow n, and this occurred when they were20.4 + 3.78 days of age.

First instars of V_. bilineatus are creamy white at birth but

gradually darken until time of exuviation. As in all other scorpions,

they lack many adult features. Tarsi are blunt and lack claws, the

aculeus is soft and b lunt , trichobothria and setae are absent,

granulation and carinae are absent, and the denticles on the margins

of the pedipalp chela fingers are absent. Pectines, however, are

well developed and contain the complete number of pectinal teeth.

Second Instar

As in £ . vittatus, mortality was very high in the second instar.

Only 40 of 110 specimens (36.4%) survived this stage, and deaths were



52

probably due to a number of causes. Several individuals died early in

this stage, probably due to starvation; a number of others died during

or shortly after peak molting periods; and the remainder simply never

molted (Fig. 14 ) . The average age at the second molt was 106 days, but

11 of the specimens which died failed to molt after 160 days. These

specimens probably died because of problems in initiating molting, but

the exact nature of these problems cannot be discerned because

environmental conditions w ere constant and equal for all specimens.

The young Vaejovis were fed chopped roach "steaks" because they

shied away from available living prey. Although this food is accepted,

there is one disadvantage to its use. VThen left in the container for

very long (sometimes even overnight), it molds easily and the hyphae

may not only trap the young scorpion, but may also damage the specimen

to where measurements are not possible. Fortunately, this was the case

primarily in the second instars where sample size was rather large.

Another problem resulted in the loss of specimens. The litters

of all _V.. bilineatus were reared on a soil substrate, and a number

of specimens were lost as a result. These "missing" specimens probably

dug into the soil and died, where they dessicated and crumbled so that

recognition was difficult and measurements impossible. This was

especially true of second and third instars, in which the specimens are

very small.

Specimens surviving the second instar (n = 40) molted to the third

instar at an age of 106 4 15.62 days. The duration of the second instar

was 96.2 + 15.69 days, and the range of observations was 60-140 days



53



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58

instar structures are presented in Table 7.

The single specimen that molted to the fifth instar did so at

an age of 323 days and the duration of its fourth instar was 147 days.

To ensure better accuracy for instar predictions, 95% confidence

intervals for measurements were calculated from fourth instar specimens

(n = 9 ) . Therefore, confidence intervals on measurements for the

fifth instar were calculated as well as for sixth, seventh, and eighth

(Table 7 ) . The use of these confidence intervals in prediction of

the number of instars to maturity is discussed in the following section.

Discussion

Growth

Although the laboratory-reared litters of v;. bilineatus failed to

reach maturity , it w as possible to observe relative growth of measured

structures (carapace length, chela length, and metasoma V length) at

the early instars. Information on growth at later instars was primarily

taken from a single specimen in one litter which molted to the fifth

instar and from a field-collected female that molted twice (and

reached maturity) in the laboratory.

The carapace growth factor of V . bilineatus was steady throughout

life averaging 1.26 _+ 0.05. This observation is based not only on the

litters but also on the specimen molting to maturity in the laboratory

The magnitude of the growth factor and its constancy is consistent with

reports on Uroctonus mordax (Francke 1976). The observed average growth

factor of that specimen, reared to the fifth instar, was 1.31.



59

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61

Fox (1975) reported carapace measurements for his recognized

population size classes in Paruroctonus baergi and P. mesaensis.

Whereas the measurements were not considered to represent the limits

of carapace size in instars, they do represent approximations of

"instar" size and growth factors can be calculated from them. In

both species carapace length was reported to decrease steadily throughout

life. The calculated growth factor for P_. baergi decreases from an

average of 1.41 (which is rather high) between second and third

"instars" to 1.11 (which is very low) between seventh and eighth

"instars". In P_. mesaensis the growth factor decreases from 1.37

(from second to third "instars") to 1.17 (from seventh to eighth

"instars") . Not only does this differ from reports on Vaejovis

and Uroctonus, but it is in conflict with reports of Polls and

Farley (1979) for _P. mesaensis.

Polls and Farley (1979) did not report carapace length but

measured two other prosomal characteristics instead. They reported

seven size classes (or instars) for P . mesaensis and calculated growth

factors between these size classes. The growth factors fluctuated

considerably and ranged from 1.30 to 1.58. The mean overall growth

factor reported w as 1.38, which is rather high in comparison to other

scorpions. The differences observed between these two field studies

will be discussed fully in the general discussion, and comparisons

between field and laboratory methods w ill be made.

Chela growth is also relatively constant throughout life in V .

bilineatus, and averages 1.28 + 0.05. The average growth factor



62

in U. mordax w as observed to be 1.26 (Francke 1976) but was rather

low in the molt from second to third instar (1.20). It is quite common

for a single specimen to show considerable fluctuation in the growth

factor betw een instars, and this is not reflected in the averages.

Neither Fox (1975) nor Polls and Farley (1979) reported on growth in

the pedipalp chela, so comparisons cannot be made with Paruroctonus.

Metasomal segment V in Vaejovis does not show the allometry that

members of Centruroides exhibit, and growth in this structure in V_.

bilineatus is rather constant between instars. The average growth

factor was 1.32 +. 0.07. Several specimens showed high growth factors,

but these occurred betw een the second and third instars, and no

inferences could be made from those observations. The growth rate of

metasoma V was slightly higher than that in the other structures, and

this was found to be true in IJ. mordax as well (Francke 1976).

Again, metasoma V w as not analyzed in either field study of Paruroctonus

and comparisons cannot be made.

Number of Instars to Maturity

As stated previously, methods for predicting the number of instars

to maturity are the same as used by Francke (1976, 1979) and Lourengo

(1978, 1979). Measurements of observed instars and 95% confidence

intervals for instars not observed are presented in Table 7.

When measurements of field-collected adults are compared to the

confidence intervals obtained from the laboratory-reared litters

(Table 7, Figs 15 and 16) , almost all of them fit easily within the

confidence intervals predicted for sixth instars. However, three adult



63





65



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67

females (two giving birth to laboratory-reared litters, and the immature

specimen molting twice in the laboratory) fall withi n the lower limits

of the seventh instar. Although it is possible that these three females

are indeed seventh i nsta rs, I consider that they are actually large

sixth instar adults based on the following observations. Young of

diffe rent litter s of V_. bilineat us bo rn in the laboratory w ere

significantly different in size (Analysis of Variance gives an F value

of 6.03, d = 0.05). Young of one litter were shox»7n to be significantly

larger than the others (by Duncan's mutliple range test; see Table 8)

and extrapolation from the average size of second instars to adult

(using the growth factor obtained from this litter) shows that a

specimen of this litter could easily fall into the predicted seventh

instar confidence intervals after only four mol ts. I hypothesize that

the specimens in question came from litters of large individuals and

represent sixth instar adults.

Only two adult males were available. These w ere determined to

be mature by dissecting for hemispermatophores, and their size fell

w ithin predicted confidence intervals for the sixth instar.

In concl usion, maturity in V. bilineatus is reached at the

sixth instar for both males and females based on available laboratory

observation.



68

Table 8. Results of Duncan's multiple range test to determinesignificance of size differences between litters of Vaejovis bilineatusAny two means not underscored by the line are significantly different.The number of the litter corresponds with the females in Table 3; nis the number of individuals of each litter measured; x is the meancarapace length of individuals w ithin the litter; s is the standarddeviation of that mean .

n= 16 n = 18 n = 15 n = 11 n = 26

X = 1.37 X = 1.41 X = 1.41 X = 1.42 x = 1.52

s = 0.034 s = 0.025 s = 0.042 s = 0.037 s = 0.029

litter #2 litter #3 litter #5 litter #4 litter H



GENERAL DISCUSSION

Tests of Hypotheses

Laboratory Test

In the introduction I presented an equation which enables prediction

of the number of molts to maturity of any scorpion, provided adults

and immatures (of kn ow n instar) are available. Based on litters of

two s peci es, Centruroides vittatus and Vaejovis bilineatus , born in

the labora tory , I hypothesized the number of molts required by those

species to reach maturity.

For Centruroides vittatus I hypothesized that there should be six

molt s (and seven insta rs) in the life cycle. The best test of that

hypothesis would have been the rearing of litters to maturity where

the numbe r of instars could have been counted. Unfortunately, no

specimens from the three litters have yet reached the adult stage, and

indirect met hod s w ere used. Analysi s by the indirect method indicates

that females of C_. vitta tus m atur e at the seventh instar and that

males do so at the sixth or seventh. That males mature at two instars

cannot be predicted at the onset of investigation since size ranges of

instars are unkno w n at that time. If sixth instar males are used in

the formula in the place of the adult female measurement, then the

resulting number of molts from the formula would reflect six instars

to adu lt. The ref ore , the formula is strongly supported by the evidence

obtained from the indirect meth od. Because three specimens of £.

vittatus remain a liv e, evidence from the direct method may soon be

available.

69



70

For Vaejovi s bilineat us I hypothesized five molts (six instars)

in the life cycle . Aga in, direct methodology was not possible and

the number of instars w as calculated by the use of the indirect method.

The hypothesis w as again supported rather strongly by the results

obtained. Both males and females fit 95% confidence intervals for

sixth in star s, and this w as the same value as predicted.

Test b^; Comparison w ith Literature Data

Measure ments from published life histori es, where available,

have been tabulated (Table 1 ) . For each structure the measurements

of those studies were substituted into the formula and a value of N

is predi cted. These calculated values can be directly compared to

the observed or inferred values of N as reported by the studies.

Only measure ments of adult females were used in the prediction formula,

and the number of mol ts calculated is relevant primarily to females.

Wher e a number of different measurements were given in publications

the standard mea sure ment s as used in this study (carapace length , chela

length, and metasoma V leng th) w ere chosen for the table. If the

standard measure ments w ere not w ere not available other structures

w ere used. For each structure a value of N was obtained, and from

these values for a single species an average N was calculated. These

values of N w ere compared w ith the observed values using a paired

T-test (Sokal and Rohlf 1969). The observed values did not differ

significantly from the predicted ones (t = 0.739; df = 17; a = 0.05

level) . Therefore, the predictive power of the formula is



71

substantiated by previous observations and can be useful in future

studies, both in the laboratory and in the field.

Use of the Formula

In the use of the formula there are several considerations.

First , structures w hich show allometric sexual dimorphism, such as

metasomal segment V in Centruroi des, should be used very carefully

in predic tions . The predicted number of molts w ill be accurate for

one sex but w ill show considerable error in the other depending upon

the degre e of allometry involved. Second, structures that show

ontogenetic allometry (i.e., grow noticeably faster or slower than

others, or show sharp declines or increases in growth at the late

•instars) should also be carefully analyzed. An example of this is

carapace length in Centruroides. As observed in C_. gracilis (and

partially in C _, vitta tus) the average grow th factor decreases at the

late instars and drops to about 1.25 (and sometimes less). In my

predictions of instar number based on carapace length for C_. vittatus

(Table 2) , all calculated values of N w ere lower than for chela and

meta soma V leng th (N = 4.23 as compared to N = 4.77 and 5.32,

respectively). In Vaejovis bilin eatus , where growth of the various

structures w as si milar, the predictions were very good. In light of

this, it is good to obtain an average value for N from several structures

in the final formulation of the hypothe sis.

It is important to note that variation in the values of N (in

the special cases outlined above, as well as in general prediction)





73

w ere favored for the following reasons: (1) difficulty in duplicating

field environme nts in the laboratory, including food intake and

temperat ure; (2) presumed differences in growth between field and

laboratory speci mens; (3) presumed differences in duration of intermolt

perio ds; and (4) presumed inhibition of ecdysis under laboratory

conditions. At the same time, they realized disadvantages of the

field method: Grow th, matur ity, and ecdyses of individuals is not

observed and can only be estimated based on population averages.

I must concur that many parameters of the life history occurring

in natural popu lati ons, particularly those of daily, seasonal, or

generation to generation occurrence cannot be accurately determined

in the laboratory. At the same time, certain parameters of life

history are more accurately determined in the laboratory, and should

be determined thus. It is these param ete rs x rith w hic h this contribut ion

is primarily concerned.

The estimations of these parameters in the field differ somewhat

from laborat ory r esu lts. The growth factor obtained in the field is

based on presumed population averages and is subject to error,

depending on the accuracy of the size classes considered to represent

instars. Due to large varia bilit y in size of scorpions (even between

litters at b i r th ) , grouping of scorpions from field data into size

classes is risky because of inevitable overlap between groups. The

error can be compounded in the calculation of the growth factor, which

has been done in the two field studies of ?_ , mesaensis.



74

Fox (1975) recognized eight instars for P. mesaensis (eight size

classes) but did not calculate growth factors. As I have already

shown, the estimated growth factor is relatively high between the

second and third instars , and declines steadily until it reaches

1.17 betw een the seventh and eighth instars. The latter value is

obviously low'w hen compared to other scorpions, particularly when it

is realized that this value is the presumed population average at

that molt . Polls and Farley (1979) recognized only seven distinct

size classes, or seven instars, and calculated growth factors for

them. As may be expected, these growth factors were higher than

those obtained from Fox's (1975) data, ranging from 1.30-1.58

(average 1.38). Therefore, from two field studies on the same

species, there are conflicting interpretations. Because the two

studies did not use the same structural measurements, the relative

sizes of adults between the two studies cannot be determined. It

also seems unlikely that geographical variation causes this disparity.

Fox studied populat ions from Palo Verde (Riverside Co.) , California

and Mohawk (Yuma C o . ) , Arizona, whereas Polls and Farley studied

populations not far away (approximately 110 miles from Palo Verde),

near Palm Springs (Riverside C o . ) , California. All three localities

are located centrally within the known distribution of _P. mesaensis

(Francke, personal communica tion). I find it doubtful that populations

so close together could produce conflicting data.

An immature specimen of P. mesaensis from near Indio (Riverside

Co.), California was collected as a second instar and has molted three



75

times to date in this laboratory. The exuviae and specimen have been

measured and growth factors calculated for five structures (including

those of field studies). These measurements and growth factors are

presented in Table 9. The overall average growth factor was 1.30

(range 1.23-1.38) w hich is intermediate betw een the two published

studi es. Even thoug h this observat ion is based on only one specimen

(and fifteen growth factors) , it is obvious that the growth factors

calculated here and those of the two field studies are different.

Structures measured by Polls and Farley (1979), other than total

length, are not the standard ones used in systematics, and therefore,

are hard to compare w ith other measurem ents. Problems are inherent

in two of the measure ments selected. The first of these, distance from

the edge of the prosoma to the medial ocelli , produced an extremely

large grow th factor of 1.58 betw een the first and second instar. It

has been previously noted (Pavlovsky 1924 ; Laurie 1896) that the

medial ocelli shift posteriorly in other species and that this growth

is not indicative of grow th in later instars and other structures.

Secondly, total length (as measured with a millimeter ruler) is subject

to error due to stretching of the intersegmental membranes of the

mesosoma in well- fed specimens. For these reasons these measurements

will be excluded from remaining calculations in this study.

Because a laboratory growth factor was obtained and adult females

of sizes com parable to those of the field studies are available, the

number of instars w as calculated using the prediction formula. The

average value of N using the standard measurements (carapace length.



76

Table 9. Measurements and growth factors of five structures

from an immature Paruroctonus mesaensis collected near Indio(Riverside Co.) , California. "X" denotes measurement of theinterlateral ocellar distance, and "Y" denotes that from the edgeof the prosoma to the medial ocelli.

2nd instar exuvia

Growth factor3rd instar exuvia

Growth factor4th instar exuvia

Growth factor5th instar specimenAverage growth factor

Carapacelength

2.11

1.262.651.253.301.364.481.29

Chelalength

3.50

1.274.431.255.551.337.381.28

MetasomaV length

2.13

1.262.681.323.551.344.751.31

X

1.30

1.231.601.282.041.292.631.27

Y

0.81

1.351.091.201.401.381.931.34

Overall average growth factor = 1.30



77

chela l engt h, and metasoma V length) w as 5.74 (range 5.40-6.00). These

resul ts ar e tabul ated (Table 10) and values of N for structures reported

by Polls and Farley (1979) are presented there. It appears that P_.

mesa ensi s re aches maturity at the eighth instar, which agrees with the

results of Fox (1975). The disparity of the growth factors calculated

from Fox (1975) and from this contribution are probably attributable

to error in estimating the boundaries of the size classes of the former

study.

To the obvious argument that growth rates may differ in the

laboratory and the field, one bit of evidence is available for

discussion. The male Centruroides vittatus from Kinney Co. , Texas

that wa s collected w ith its exuvia gave the following growth factors:«

carapace growth factor, 1.22; chela growth factor, 1.27; metasoma

V growth factor, 1.61. The first two values were the same as observed

in the litters of C_. vitt atus reared in the labo ratory, and the latter

value is the allometric growth in metasoma V that has been observed in

the labora tory (in C_. gracil is by Fra nck e, unpublished data). The

question still rem ain s, but this bit of evidence suggests that growth

rates in the laboratory and in the field do not differ, and that more

evidence is at least obtainable.

In conclusion the majo r differences between laboratory and field

studies is in interpretation. Both methods have distinct advantages,

and excellent results have been and can be obtained from both.

Depending upon the questions being ask ed, one method may be favored

over anot her: but it w as and is my purpose to show that determination



78

Table 10. Measurements of adult females of Paruroctonusmesaensis from the 0. F. Francke collection. For each structureof each female, a value of N (obtained from the predictionformula) is given. The other variables in the formula are from

the second instar exuvia and growth factors from succeeding moltsof the immature specimen from Indio, California (Table 9 ) . Agrand average value (N) for the number of molts is given for thefirst three structures.

1.

2.

3.

4.

5.

Carapace lengthN value

Chela lengthN value

Metasoma V lengthN value

Interlateral ocellarN value

Edge of prosoma toN value

distance

medial ocelli

Fem. 1

8.505.54

15.406.00

10.405.875.055.683.755.23

Fem. 2

8.215.40

14.355.72

10.005.735.055.683.555.05

Fem. 3

9.055.79

14.855.85

10.205.805.305.883.905.37

Avg.

8.595.58

14.875.85

10.205.805.135.753.735.22

Grand average value of N = 5.74



79

of the nimber of instars to maturity in scorpions, and of growth

rates betw een those instars , is more accurately done in the laboratory

because individuals are actually observed. With this in mind, studies

should be attempted in which both methods are used in elucidating

the details of the life history of scorpions



SUMMARY

Tw o basic met hod s of determining the number of instars in the

post-bir th development of scorpions were recognized: the direct method

and indirect method. The direct method requires that scorpions be

reared to mat uri ty, the better metho d. Indirect methodology is of

two types : laboratory and field. The laboratory indirect method

w as developed for instances in which scorpions w ere reared but failed

to yield complete life histories. The number of instars to maturity

is obtained by extrapolation and subsequent comparison with adults

collected in the field. The field method requires estimation of size

classes in natural populations but is not as effective as the previous

methods.

A n ew indirect method for determining the number of instars to

maturity was presente d, A formula, derived from previous growth

equations (Huxley 193 2; Teissier 1960) allows the calculation of the

number of instars and may be expressed as follow s:

_ log A - log Y^ " log G

where A is the dimension of a structure in a known adult specimen, Y

is the dimension of the same structure in an immature specimen of

known age , G is the growth factor, and N is the number of molts

required for the immatu re specimen to reach the size of the adult.

Iti scorpio ns females are sometime s collected p regn ant, or with first

or second instars on their bac k s, mak ing the formula a practical one.

80



SI

The formula w as tested by rearing two species, Centruroides

vittatus (Say) and Vaejovis bilineatus Pocock in the laboratory and

by comparisons w ith data from the literature. In each test the formula

was found to be highly accurate.

In addition to the number of instars typical life history

information for the two reared species is given. Growth in C ,

vittatus w as found to be quite similar to that of £. gracilis.

Carapace grow th declines w ith age and growth in metasomal segment

V is found to show drastic allometric sexual dimorphism at the

maturation molt . Sexual dimorphism wa s noticeable in specimens at

the fifth instar, but the growth factor showed a drastic increase

only at the matura tion molt . Chela growth also showed slight

indications of allometric sexual dimorphism based on field-collected

adults. Because no specimens as yet have reached sexual maturity

in the laboratory, the indirect method was used to predict that

male s of C . vit tatu s mat ure at the sixth or seventh instar and that

females do so at the seventh.

Grow th in V, bilin eatus v/as found to be rather constant in all

three structure s. No distinct sexual dimorphism was noticed in any

of the struc tures , and Indirect methods indicated that both males and

females matu re at the sixth instar.

The validity and predictive pow er of the formula is discussed,

and comment is made concerning choices for values of G. From

measure ments and grow th factors presented in laboratory studies an

average grow th factor (1.29) for all scorpions wa s calculated. This



82

growth factor was used for all predictions and was considered more

effective than Przibram's rule.

Observed discrepancies in field reports for one species,

Paruroctonus mes aen sis , are discussed and compared w ith laboratory

life history data on the same species. The field method was determined

to be less relia ble in estimation of growth factors and the number of

instars to matu rity . Because laboratory methods enable direct

observation of ecdyses and growth of individuals, this method gives

better accuracy than field methods and should be used when possible

in determining these paramete rs.



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Auber, M. 1963. Reproduction et croissance de Buthus occitanus Amx.

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Hjelle, J. T, 1974. Observations on the birth and post-birth

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Laurie, M. 1896. Further notes on the anatomy and development of

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Paruroctonus from Nex>7 Mexirn anri T « ^ « „ fo_ lu iNcw .lexico and Texas (Scorpiones, Vaejovidae).

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Documents

1980 - Sissom - Life Histories of Two North American Scorpions Centruroides Vittatus and Vaejovis us