On the evolution of tactile stimulatory actions in longhorned beetles

(Cerambycidae, Coleoptera)

By AXEL MICHELSEN~)

With 3 f igures

Rece ived I f i . 12. 1965

<: o n t e n t s : The ,timulatory actions p. 257. - Some examples of behaviour patterns p. 259. - The origin of signal actions p. 260. - The neurophysiological mechanisms p. 261. - The

origin of the tactile stimulatory actions p. 262. - The evolution of the stimulatory accions from l i k i n g p. 263. - Discussion p. 264. - Summary p. 265. - Zusammenfassung p. 265. - References p. 266.

Introduction

The sexual behaviour of longhorned beetles is dominated by the exchange of tactile stimuli between male and female (MICHELSEN 1963 and 1966). When mounted by the male the female is very restless, runs about, raises her abdomen, and kicks the male with her hindlegs. But during mating the male performs some movements which have a calming effect upon the female. When the male performs these actions it is usually able to control the female from the start of amplexus until the middle of copula. However, the female becomes more and more restless and finally dislodges the male.

Several types of stimulatory actions have been found, all of which seem to transfer the same sort of information, i. e. the effect upon the female is the same. Further, stimulatory actions carried out with different effector systems (i. e. different groups of muscles) may be combined in a great variety of ways, and several intermediate types between the individual actions can be found. Therefore, the stimulatory actions of longhorned beetles represent a system well suited for t h e study of the evolution of tactile signal actions with special reference to the mechanisms involved.

The stirnulatory actions

The tactile stimulatory actions observed in the males during mating can be divided into three groups, according to the part of the body used (see fig. I):

Oral actions

The most common type is a “ 1 i c k i n g ” of the female thorax or elytra executed by means of the maxillary and labial palps. In Cerambyx cerdo rhyth- mical movements of the mandibles may occur synchronously with the “licking” movements of the palps. Another common type is a “ t a p p i n g ” action in

‘) Zoophysiological Laboratory A, Juliane Mariesvej 32, Copenhagen (D, Denmark.

2. f . Tierpsyihol. Band 23 , Heft 3 17

25 8 AXEL MICHELSEN

which the head is lowered rhythmically. In anumber of species the back and forth movements of the “tapping” action predominate, and little or no vertical movement is seen. When the male is very active the “tapping” action may become more violent, gradually developing into a “ s c r a p 1 n g ” action, i. e. the head is moved form ard, depressed very rapidly, and then moved backward with the widespread mandibles pressed against the back of the female. A loud crackling sound is produced by this action. Normally the “scraping” action does not start as a “tapping” action but is performed as a short series of pure “scraping” actions. In some species the “scraping” action may be combined w i t h a b i t i n g o f t h e f e m a l e ’ s p r o n o t u m o r e 1 y t r a . A c o m - plicated series of actions involving b i t i n g o f a f e m a 1 e a n t e n n a has been observed in two species.

Abdominal actions

In several species rhythmical m o v e m e I I t s o f t h e m a 1 e ’ s a b - d o m e n and genital organ (push and pull) occur during copula. In a number of species the males p u 1 1 out t h e ‘‘0 v i p o s i t o r” during copula. This action may be rhythmical or nonrhythmical.

Movements of the legs

The males of som’e species s t r i k e t h e f e m a 1 e’s a b d o m e n with their stretched hindlegs. During precopulatory amplexus the Phyrnatodes testaceus male bends his hindlegs, striking the abdomen of the female with his tibia1 spurs. In Donacia aquatica the male performs a “ w i p i n g ” action with his foremost tarsi, i. e. the tarsi placed upon the head or pronotum of the female move rhythmically “out of phase”, one tarsus beginning to move forward when the other starts to move backward. The wiping action may move the female antenna or it may be performed upon the eyes of the female. K. OHBAYASHI (personal communication) found that the male Leptura arcuata tsumagurohana Ohb. f i x e d t h e h i n d t i b i a of the female in copula, using his dilated and carinated tibiae in apical half.

I

’ ‘&

\ a b d o m i n a l movem. s t r i k i n g 9’s ‘knee’ w i p i n g p u l l i n g of ‘ovipos.‘ a b d o m e n movem.

F i g . 1: The stimulatory display and other movcmenrs in the male longhorned beetle. +-+ a movement in the plane of the paper, a movement perpcndicular to the plane of the paper

On the evolution of tactile stirnulatory actions in longhorned beetles 259

It should be noted that several effector systems (i. e. different groups of muscles) are involved: both pairs of palps, sometimes moved independently of each other (licking), the neck region and the pro-mesothorax region (tapping, scraping, biting), the mandibles (licking in Cerumbyx cerdo, biting), the ab- domen and legs (movement of abdomen, pulling of ovipositor), the hindlegs (striking), and the forelegs (wiping). The biting of a female antenna has not been analysed into single effector systems, but this action is very complex (see MICHELSEN 1963).

As a rule the stimulatory actions are rhythmical, and normally when two or more of these actions are combined they are synchronized. In order to describe in detail the patterns of synchronization it is useful to split up the rhythmical actions to obtain the following single components:

licking

tapping

scraping

biting

abdominal movements pulling of ovipositor

wiping

1 1 : 1 2 : t l : t 2 : s l : s 2 :

b l : b 2 : a l : a 2 : 0 1 : 0 2 :

w l : w 2 :

the palps move outward the palps move inward head moves down and/or forward head moves up and/or backward head moves forward and down head moves backward, the mouthparts pressed against the female's back t 1 and mandibles separate the bite and t 2 the abdominal t ip moves forward and down the abdominal tip moves backward and up a 1 and hindlegs bend very violent a 2, hindlegs stretched, ovipositor pulled backward and upward right tarsus forward, left backward left tarsus forward, right backward

During states of excitement the male moves his antennae. The function of these movements is not known, but they may be divided into seven types and one posture (MICHELSEN, 1966). In most species it is possible to describe the movements as composed of one or more of these types. Though the antenna1 movements may be synchronous with the tactile stimulatory actions this is not normally the case.

In several species sounds may be produced during sexual activity, and generally the stridulation is coupled to the tactile stimulation (see below). In a few species both sexes may stridulate during mating.

Some examples of behaviour patterns

A combined action of two or more synchronous rhythmical movements has been found in several species (MICHELSEN 1963 and 1966). Only a few cxamples should be given here. In the following the synchronous couplings will be indicated as: (1 1, t 1 * 12, t 2), which means that first the palps move outward while the head moves down and/or forward. Then the palps move inward and the head up and/or backward, and so on (this is the most common combination found).

In Clytunthus sartor the pattern of synchronization may vary during the copula. In all cases the licking and tapping actions are synchronized as indicated

17"

AXEL MICHELSEN 260

above, but these actions may be coupled to the abdominal movements and the weak pulling of the ovipositor in two alternative ways:

T y p e 1: (1 1, t 1, a 1, o l , + 12, t 2 , a 2 , 0 2 ) T y p e 2: (1 1, t 1, a 2 , o 2, + 12, t 2, a 1, o 1)

In type ( I ) the male bends and stretches its body, in type (2) the male rocks its body. In Plagionotus floralis, Phymatodes testaceus, and Clytus arietis, type (1) occurs, whereas type (2) has been observed in Clytus rhamni and Tetropium castaneum.

In Oeme costata abietis CHEMSAK (1965) found that the male (after coupling of the genital organs) underwent a series of rapid, jerky back and forth motions pulling at the female genitalia. No licking was observed. T h e jerks were rhythmical (CHEMSAK, personal communication).

In Obrium brunneum a pure licking is seen during periods of very active stimulation, whereas a pure tapping, which may be combined with abdominal movements (t 1, a 2, + t 2, a l), occurs during very calm periods. During the rest of copula licking and tapping occur together, but they are not synchronous, the licking being performed much faster than the tapping.

In some Leptura rubra males the “knees” of the forelegs move rhythmically forward - backward during copula. Though very similar to the ’wiping’ action these actions are not identical, because the movements of the knees are in phase, and the tarsi are immobile. This movement is synchronous with the licking, tapping, and abdominal movements: (k I , t I , 1 I , a 2, -+ k 2, t 2, 12, a I), where k 1 and k 2 mean a movement of the ’knees’ in the forward and back- ward direction respectively.

The origin of signal actions

Many displays are ’derived’ movements. TINBERGEN (1952) considered two main sources of derived movements, viz. intention movements and displacement activities. MOYNIHAN (1955) divided the sources of displays into activities belonging to the drive producing the display (intention movemerits of low or higher intensity, redirection activities) and ‘irrelevant’ activities (displacement activities). The process by which these ‘ordinary’ movements and acts have been modified to produce signals is called ’ritualisation’ (HUXLEY 1914, TINBERGEN 1952). The process of ritualisation has been discussed by several authors (e. g. BLEST 1961; TINBERGEN 1962). I t includes changes in the structure and behaviour of the animal. While some displays a re so similar to the movements from which they were derived that their origin can easily be recognised, others have been changed ’beyond recognition’ (LORENZ 1935).

BLEST (1961, modific;ition from MORRIS 1957) listed the possible changes in co-ordination between the individual actions for the process of ritualisation: 1. Intensity changes, 2. In- crease o r decrease in speed of performance, 3. Omission of components, 4. Changes in com- ponent co-ordination, 5. Changes in sequence of components, i. e. in the order of their per- formance, 6. Differential exaggeration of components, 7. Development of rhythmic repetition. As an eighth possibility BLEST (1961) proposed tha t a signal function might be transferred from one set of effectors to another.

BLEST (1961) comments that ’these categories are arbitrary, and are not wholly in- dependent’. T o this s ta temmt TINBERGEN (1962) remarks that “there should be n o question of replacing a classification o f the characteristics of overt movements (and structures) acting as signals by a classification of the types of neurophysiological changes involved in the evolution of signals”. The set of criteria was thus intended to be descriptive in the papers of TINBERGEN (1959) and MORRIS (1957). However, the characteristics of signal actions compared with those of ’ordinary’ actions are likely to reflect the neurophysiological changes involved (cf. TIN- EERGEN 1952). A consideration of the points listed by BLEST might thus be useful for an understanding of the mechanisms underlying the process of ritualisation.

When a number of displays with the same signal function is found in a group of related species, two e x t r e m e cases of evolution may be imagined

On the evolution of tactile stimulatory actions in longhorned beetles 261

(fig. 2). A number of signal actions (A, B, C -) may have evolved from the ‘ordinary’ actions (a, b, c, -) by a process of ritualisation (ful l line arrows) involving changes of the types 1-7 in the list of BLEST (fig. 2 : I). Two or more of the signal actions may then combine in a complex of actions (in the figure: AB). In this case the origin of the display can be found, if the secondary changes are ’peeled off’ (TINBERGEN 1962).

In the other e x t r e m e case one rhythmical ‘ordinary’ action (a) evolved into a rhythmical signal action (A) by a process of ritualisation (fig. 2 : 11). This original signal action then activated another effector system (defined in this context as a muscle or a functional group of muscles), a new rhythmical action B being performed synchronously with A (broken line arrows). Gradually B became more intense, and simultaneously the signal content of action A was

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IQI 1‘

Icl

El 1‘

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m

\ 7 / -._ \ -. / -\- / /

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= 7“ Fig . 2: The two extreme models proposed for the evo- lution of signal actions. Explanation in the text

transferred to the complex AB. From the combined signal actions AB action A may have been lost, or a third effector system may have been activated, a new rhythmical action C being performed synchronously with A and/or B, and so on (cf. fig. 2 : 11).

It will be demonstrated here that the patterns of stimulatory behaviour found in the present-day species of longhorned beetles are in reasonable agreement with the latter extreme case, but probably the actual evolution was a combination of the two extreme cases.

The neurophysiological mechanisms

For several years there has been a gap in ideas between ethologists and neurophysio- logists concerning the extent of control of the behaviour patterns by the external world (see TINBERGEN 1963). LORENZ (1935, 1937) was the first to point out that animals carry out “Instinkthandlungen”, relatively stereotyped motor patterns which are controlled by internal as well as external factors. A t the same time von HOLST (1934-1936) studying the swimming movements of fish suggested the presence of an automatic mechanism within the central nervous system controlling the rhythmical co-ordinated movements. Most neurophysiologists however, were working on rather simple reflex systems and did not offer much help to the understanding of so complex phenomena as instincts. Later, LISSMANN (1946) and GRAY (1950) concluded that the evidence for a central automatic mechanism was not conclusive. During recent years however, the ideas of von HOLST have been supported by several studies,

262 AXEL MICHELSEN

e. g. the experiments of TAUB and BERMAN (1964) on almost totally deafferentiated monkeys. Studies on the behaviour of :;ingle neurones has also supported the theory of a central control (see BULLOCK 1961).

In insects, most simultaneous actions were thought to be performed more or less in dependently of each other (see V o a m s 1961), and it had been generally accepted that the central nervous system is composed of a number of reflex systems controlled by phasic sensory input. In 1961 however, WILSON showed that the central nervous mechanism responsible for the maintenance of flight in ;I locust is of an oscillatory type not dependent upon phasic input. Further, it seems probable I hat the oscillating system consists of a number of independent oscillators (WILSON, 1962; BLEST, COLLETT and PYE 1963). The individual oscillators are nor- mally synchronized in some way, and the entire oscillatory system must be triggered by some non-phasic sensory input in (order to oscillate. The whole subject has been reviewed by HoYI.~: (1964) and by WEIS-FOGH (1964) who also considered the advantages of such systems coni- pared with reflex systems.

A synchronous coupling of two or more rhythmical movements is also known from vertebrates. VON HOLST (1934-1936) observed that a 'dependent' rhythm of a fin in a swimming fish may be forced to follow a 'dominant' rhythm of another fin, and he also demonstrated ( 1 934) that iridcpendent rhythms may become synchronized during weak nar- cosis. Recently, K R U I J T (1964) found that ground-scratching in birds may be 'caught' by the rhythm of trampling or kicking.

T h e origin of the tactile stimulatory actions

In the present day species of longhorned beetles all the actions described as stimulatory in the male seem to transfer the same sort of information, i. e. the effect upon the receiving animal is the same (they all inhibit the restless behaviour or equivalent behaviour in the female). In other words: we are considering a number of actions all having the same signal content, but being carried out with different effector systems.

In the stimulatory signal actions considered here the synchronization between the individual movements suggests that some sort of coupling exists between the spatially separated parts of the CNS, which are primarily respon- sible for the individual, rhythmical movements. When actions are combined the relative intensity of the components may vary considerably. Though in a few species movements may be non-rhythmical or non-synchronous with other movements (e. g. in Obrium brunneum, Toxotus meridianus and Spondylis buprestoides) this is thc exception.

It is well known that electrical stimulation of rather large parts of the brain may bring about complicated rhythmical or non-rhythmical movements in insects (HUBER 1962; ROEDEK 1963). Rhythmical actions may also be released by an external non-rhythmical stimulus (see e. g. WEIS-FOCH 1956; BLEST 1957; BARTH 1964). This also seems to be the case here, e. g. the sti- mulatory behaviour of the male is often released by a single kick from the female.

In several species it has been observed that at a certain phase of the mating one or more new rhythmical actions may be introduced synchronously to the action which was performed during the preceding phase. Furthermore, in Spondylis buprestoidcs after a period of rhythmical abdominal movements carried out by the male the female may start to perform rhythmical movements with her ovipositor. This suggests that in the individual animal excitation may spread in the C N S from the part responsible for one rhythmical action to other parts, or rhythmical actions may be induced by external stimuli.

In the evolution of the actions considered here the transfer of the signal function from one set of effectors to others (the eighth of the possible changes listed by BLEST, see above) is proposed to be the fundamental mechanism, but

011 the evolution of tactile stimulatory actions in longhorned beetles 263

the points one to five in the list of BLEST also play an important role. How- ever, the concept of exaggeration (point six of BLEST) refers to an ’ordinary’ action, so this point does not seem relevant. In contrast to former theories rhythmic repetition is considered an original quality of an action (cf. point seven of BLEST), and a non-rhythmical action or a non-synchronous combi- nation of actions are thought to represent more advanced types.

The evolution of the tactile stimulatory actions from licking

The licking action is simple, and it is identical with the movements of the palps seen during feeding and cleaning behaviour. Further, it occurs in morpho- logically primitive species, and combined with other actions in almost all species. If this action is considered the original one, the first step in the evolu- tion might. have been the synchronous combination with weak, horizontal,

Fig. 3: The probable phylogenetic relationship between the individual stimu- latory actions

tapping movements (as seen in Rhagium mordax). The tapping action then grows more violent, becomes vertical, and the licking action may be lost (see above and MICHELSEN, 1963).

Fig. 3 illustrates, in a somewhat simplified way, how the evolution may have taken place.

Similarly, movements of the abdomen may be introduced synchronous to the ‘licking and tapping’. From this complex the licking may be lost (Obrium brunneum, Cerambyx scopoli) or a pulling of the ovipositor may be intro- duced (in two different ways, see above). Then the licking and tapping may be lost, a rhythmical pulling of the ‘ovipositor’ being the only stimulatory action (as in Oeme costata abietis, see above). Finally, this may evolve into a non- rhythmical pulling of the ‘ovipositor’ as in Toxotus meridianus (MICHELSEN 1963), and the Cerambycines Hesperophanes griseus F. and Strornatium fulvum Vill. (PICARD 1929).

The scraping action may often be indistinguishable from a violent tapping action (Clytus mysticus, Plagionus floralis), and the transition is also gradual

264 AXEL MICHELSEN

from the scraping to the biting action, the two actions often alternating (Cerambyx scopoli).

The combined licking and movement of the mandibles demonstrated in Cerarnbyx cerdo may have evolved from the licking action, but other explan- ations are possible.

T h e licking action performed by the maxillar palps only (in Spondylis buprestoides) may have evolved from the normal licking action, and it represents one of several examples where an action or a part of an action has been lost.

The asynchronous licking and tapping found in Obrium brunneum is an example of the loosening of the synchronization between two rhythmical actions and probably evolved from the synchronous ‘licking and tapping’ action.

The synchronous coupling of sounds, which are produced by almost the same movements as thiose of the tapping action, to tactile stimulation in a number of species (see MICHELSEN 1966) suggests that the sounds were intro- duced as an incidental “background music” produced by the stimulating move- ments. The sounds may then gradually have obtained a signal value and the coupling to the tactile stimulation has been lost.

As for the introduction of sounds into the life of insects, most theories have been concerned with the relationship between flight noises and stridulation (ZEUNER 1934 and 1939; ANDEK 1939; HUBER 1962). Though this idea offers a probable explanation for some groups of insects, the origin of most types of sound production has not been explained. PRINGLE (1 957) suggested that the tymbal method of sound production in Hemiptera arose as a development of movements made by the insect during actual copulation, but later he dis- carded the idea (LESTON and PRINGLE 1963). Since tactile stimuli play an im- portant role in the sexual behaviour of insects, a probable explanation in several cases might be a sort of evolution similar to that proposed here.

Discussion

The extreme model for the evolution of signal actions i n longhorned beetles presented above is based upon the assumption that not all actions found today were present in the common ancestors of the present species and that the tendency for a coupling of motor systems (operating by spread of excitation in the central nervous system) is genetically determined. T h e restless behaviour of the female during amplexus will prevent a copulation in almost all species if the male is not able to perform an effective stimulation. Therefore, during the evolution of these signal actions a strong selection pressure has been operating for effective stimulatory behaviour. But still more information is needed about the constancy of the behaviour patterns of the male, the genetic mechanisms involved, and the effectiveness of the individual actions to calm the female.

Some of the tactile stimulatory actions have a widespread occurence throughout the group (e. g. licking, tapping, abdominal movements). Further- more, they are extremely similar in the different species, and it might therefore be suggested that they are homologous, all having derived from a similar behaviour in a common ancestor. However, the extreme model presented above implies that this might not be the case. It was suggested above that the licking

O n the evolution of tactile stiniulatory actions in longhorned beetles 265

action (derived from feeding or cleaning movements) was the original tactile stimulus. We do not know and probably will never know if this is correct. The abdominal actions, for example, have been postulated to have a physio- logical function in the transfer of semen (DOHRING 1949), so perhaps they were the original actions. In fact, one may start with any action or combination of actions and build up a model for their evolution similar to that postulated here. I t is therefore very probable that not all the apparently identical actions are homologous, and that convergent evolution has been important too. This idea is supported by a comparison of the behaviour patterns of the species: the occurrence of these actions do not follow the systematic groups, whereas the distribution of several other actions of the sexual behaviour is in reasonable agreement with the phylogeny based upon morphological structures. It may therefore be suggested that a comparative analysis of signal actions is unlikely to contribute to the understanding of phylogeny, if an evolution similar to that postulated here has occurred.

Acknowledgments

The author is most indebted to the following persons for discussions, advice, and com- ments upon the manuscript: Dr. A. D. BLEST, Dr. F. BRA:STRUP, Dr. 1 . A. CHEMSAK, Professor E. FABRICIUS, Dr. H. LIND, Professor E. G. LINSLEY, and Professor T. WFIS-FOGH. The study was supported by grants from The Japetus Stecnstrup Foundation and Statcns almindrlige Videnskabsfond.

Summary

T h e tactile stimulatory actions of the male longhorned beetle are des- cribed. These actions are carried out with different parts of the body, and they all seem to transfer the same information. In the individual species, two or more rhythmical actions may be performed simultaneously. The origin of these signal actions is discussed. It is suggested that the performance of rhythmical signal action(s) synchronously activated other motor system(s) in the animal, and gradually the signal content was transferred to the new rhythmical action(s). This theory, which also explains the origin of sound signals, is sup- ported by recent neurophysiological findings. I t implies that the apparently identical actions in the different species are not necessarily homologous, and that convergent evolution might have been important. In the case of an evo- lution similar to that postulated here, a comparative analysis of the signal actions may be unlikely to contribute to the understanding of phylogeny.

Zusammenfassung

Es werden die Beriihrungsreize setzenden Handlungen der Bockkafer-d d* beschrieben. Diese Handlungen, die alle den gleichen Signalwert zu haben scheinen, werden von verschiedenen Korperteilen ausgefuhrt. Bei den einzelnen Arten konnen zwei oder mehr Handlungen synchron-rhythmisch ausgefuhrt werden. Der Ursprung dieser Signalhandlungen wurde besprochen. Vermutlich hat die Ausfuhrung der rhythmischen Signalhandlungen andrre Bewegungs- systeme im Tier synchron aktiviert, und der Signalwert ist gradweise auf die neuen rhythmkchen Handlungen iibertragen worden. Diese Theorie, die auch das Entstehen der Lautsignale erklart, wird durch jiingste neurophysiologische Befunde erhartet. Aus ihr geht hervor, daf3 die offensichtlich gleichen Hand-

266 AXEL MICHELSEN

lungen bei den verschiedenen Arten nicht unbedingt homolog zu sein brauchen und daf3 eine konvergente Evolution stattgefunden haben kann. Im Falle einer derartigen Evolution mag wohl eine vergleichende Analyse der Signalhand- lungen dem Verstandnis der Phylogenie einer Gruppe kaum dienlich sein.

References

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