Morphology and ultrastructure of paired prototergal glands in the adult rove beetle Philonthus varians (Coleoptera, Staphylinidae)

Morphology and ultrastructure of paired prototergal glands in the adult

rove beetle Philonthus varians (Coleoptera, Staphylinidae)

Andre Quennedeya,*, Didier Drugmandb, Jean Delignec

aDeveloppement et communication chimique, Universite de Bourgogne, UMR CNRS 5548, 6 Boulevard Gabriel, F-21000 Dijon, FrancebDepartment d Entomologie, Institut royal des Sciences naturelles de Belgique, 29 Rue Vautier, B-1000 Bruxelles, Belgium

cLaboratoire de Biologie Animale et Cellulaire, CP160/11, Universite Libre de Bruxelles, 50 Avenue F.D. Roosevelt, B-1050 Bruxelles, Belgium

Received 27 May 2002; accepted 29 August 2002

Abstract

Philonthus and other genera of Philonthina possess a pair of prototergal glands located in the first abdominal tergum and hidden at rest by

hind wings and elytra. In Philonthus varians they occupy the whole length of the tergum and form a pouch-like invaginated reservoir with a

scaly glandular zone and a smooth outlet. A grille of long setae covers the opening of each gland.

The fine structure of these glands is given for the first time. Three types of cells are found in the glandular epithelium. Epidermal cells

underlie the cuticular scales, numerous class 1 secretory cells open in the centre of calyces made of finger-like processes of the cuticle, and

class 3 cells are connected to pored tubercles. A cytological comparison is made with the diverse class 1 cells described to date in Coleoptera.

In these cells different evolutionary trends are shown in the structure of the cuticular apparatus, particularly in the number, size and position

of the cuticular apertures as well as in the length and abundance of epicuticular filaments.

A possible defensive function of the prototergal glands against pathogens and their interest for the phylogenetic study of Staphylininae are

discussed. q 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Comparative cytology; Abdomen; Exocrine gland; Insecta

1. Introduction

Jordan (1913) discovered in two staphylinid species,

Philonthus splendens (F.) and Heterothrops nigra (Kr.), a

pair of thoracic glands, the position of which is unexpected

since they are covered at rest by the hind wings and elytra.

Sulc (1922) described the morphology of these glands in ten

or so species of Staphylinidae and especially in some

Philonthus species. According to him, they are situated on

both sides of the metanotum and should be called

osmeteria. As discussed subsequently both terms thoracic

glands and osmoteria are inappropriate. As the glands are

in fact situated on the first abdominal tergum, i.e. the

prototergum, we named them prototergal glands.

Since these old morphological works an impressive

number of ultrastructural data have accumulated on the

exocrine glands of insects and in particular of Coleoptera.

For the family Staphylinidae alone many papers have been

published on the fine structure of various glands involved in

functions like defence (Happ and Happ, 1973; Araujo, 1978,

1981, 1985; Araujo and Pasteels, 1985, 1987; Kellner and

Dettner, 1992; Steidle and Dettner, 1993), prey-capture

(Kolsch and Betz, 1998; Kolsch, 2000) or communication

(Skilbeck and Anderson, 1994).

Due to the high diversity of these glands, their

comparative anatomy is still a difficult topic and new

works on their fine structure are still needed to improve the

present state of their morphological classification. (Noirot

and Quennedey 1974, 1991; Quennedey 1998). In this

context preliminary observations convinced us that the

prototergal gland harboured interesting features and

deserved a closer examination.

Using histology and especially scanning (SEM) and

transmission electron microscopy (TEM), we thus investi-

gated the organisation of these peculiar glandular organs

and we compared them to glands of other Coleoptera with

the aim of getting more detailed information on the typology

of gland cells.

Although comparative anatomy is the main concern of

1467-8039/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.

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* Corresponding author. Tel.: 33-380-39-62-98; fax: 33-380-39-62-89.

E-mail address: [email protected] (A. Quennedey).

this work, the possible functions of the prototergal glands

and their interest for the phylogenetic study of Staphylininae

are also considered.

2. Material and methods

2.1. Animals

Philonthus varians (Paykull, 1789) is a small Holarctic

rove beetle (about 8 mm long) living mainly in decaying

matter, including dung. This species belongs to the subtribe

Philonthina in which the prototergal glands exhibit a greater

complexity than in other taxa of Staphylinidae (Sulc, 1922;

Drugmand, unpublished data). Adults of P. varians were

collected in Viroinval (South Belgium). Males and females

were studied separately but all the results were finally

pooled as no sexual dimorphism could be detected, neither

in the outer nor in the inner morphology of the prototergal

glands.

2.2. Light microscopy

For histological observation specimens were fixed in

Fig. 1. Scanning electron microscopy of P. varians in dorsal view with elytra and hind wings removed. (A) Position of the paired prototergal glands (pg) on the

first abdominal tergum (arrow) behind the first abdominal spiracle (S1). (B) Opening of the left gland, covered with a grille of long setae. Anterolateral zone

with a scaly cuticle (c); posterior zone and outlet with a smooth cuticle (arrow). (C) Opening of the left gland in oblique view. The left row of setae is

suppressed to show the marginal row of four basiconic sensilla (arrowheads). The scaly zone (c) and smooth zone (arrow) continue forwards in the covered part

of the reservoir. (D) Side view of one basiconic sensillum inserted in a shallow depression along the external side of the outlet. (E) Scaly glandular cuticle seen

obliquely from behind. The openings of C1 gland cells are partially covered by digitated scales (sc); they are each surrounded by a calyx made up by finger-like

processes (ca). The C3 gland cells open into pored tubercules (arrowhead).

A. Quennedey et al. / Arthropod Structure & Development 31 (2002) 173183174

alcoholic Bouin and embedded in paraffin. Serial transverse

sections were cut at 10 mm and stained with haematoxylinphloxin-light green.

2.3. Scanning electron microscopy

After dissection the glands and surrounding integument

were cleaned in a 2 M aqueous solution of HCl for 1 day,

rinsed, dehydrated in alcohol, dried, coated with gold and

examined either with a ISI 130 SEM (Universite Libre de

Bruxelles) or a Philips XL30 ESEM (Institut royal des

Sciences naturelles de Belgique).

Specimens were prepared for observing both sides of the

integument. In some cases, the surface of the cuticle was

scratched with a thin pin to reveal internal structures. Some

glands were cut with a microscalpel to observe transversal

sections of the integument.

2.4. Transmission electron microscopy

Beetles of both sexes were anaesthetised and dissected.

Samples were fixed for 16 h in a cold 2% paraformalde-

hyde3% gluteraldehyde mixture in 0.2 M pH 7.4 caco-

dylate buffer and postfixed 1 h in osmium tetroxide in the

same buffer. After dehydration, specimens were embedded

in an EponAraldite mixture and sectioned. Ultrathin

sections stained with an alcoholic solution of uranyl acetate

and lead citrate were examined with a Hitachi H-600

electron microscope (CMAB, Universite de Bourgogne).

3. Results

3.1. Scanning electron microscopy and histology

3.1.1. Dorsal view

The two prototergal glands are situated on each side of

the body, near the first abdominal spiracle (Fig. 1(A)). Each

gland is a deep pouch-like invagination of the integument,

the reservoir, which expands forwards under a fold of the

tergite (Fig. 1(B) and (C)). The opening of the reservoir is

elongate and slightly curved. Its posterior part looks like a

gutter-shaped outlet. It becomes less deep posteriad and

ends at the posterior edge of the tergite. The reservoir is

flanked on each side by a row of elongated trichoid setae

measuring 60120 mm in length. These setae are inclinedtowards the reservoir and form a kind of grille over its

opening. Three or four short basiconic sensilla also line the

external side of the outlet (Fig. 1(C) and (D)). Each of them

is articulated inside a shallow depression of the integument.

The floor of the reservoir shows an anterior zone with a

scaly cuticle and a posterior zone with a smoother surface

(Fig. 1(B)). These two zones continue forwards, respec-

tively, in the lateral and medial halves of the covered

reservoir (Fig. 1(C)).

In the scaly zone (Fig. 1(E)) the scales lie in parallel rows

which overlap like tiles on a roof. Their free edge is directed

posteriad and in most cases bears several blunt spines.

Between the rows of scales and under the spines two

characteristic structures, calyces and pored tubercles, can be

observed. Calyces are formed by six or seven short cuticular

finger-like processes standing in a circle around a cavity

about 1.5 mm in diameter. Pored tubercles are smallrounded elevations bearing an apical pore the diameter of

which is about 0.5 mm. These tubercles are about 1/10 asnumerous as calyces.

3.1.2. Inside and cross-sectional views

Inside the body the prototergal glands are located in the

enlarged lateral part of the first abdominal tergite and extend

from the anterior edge of this sclerite to the posterior one

(Fig. 2(A)). In each gland the inner face of the cuticle shows

two clearly different zones (Fig. 2(B)). The anterolateral

zone is rough, bearing about 600 little knobs, called here

glomerules, as well as canalicules, while the rest of the

cuticle is entirely smooth. The glomerules are spheroidal

bodies measuring about 3 mm in diameter disposed alonginternal ridges of the cuticle (Fig. 2(C)). At their base a tiny

circular structure, the cupule, can often be observed (Fig.

2(C)(E)). When a glomerule is gently scratched one

discovers underneath a duct in the cuticle, which has a star-

shaped aperture, with generally six narrow rays separated by

infoldings of the wall (Fig. 2(D)). The canalicules (Fig. 2(C)

and (E)) are about 1/10 as numerous as glomerules. They

measure about 0.6 mm in diameter and up to 40 mm inlength. They enter the cuticle through a circular hole.

Cross-sections show that the smooth zones of both sides

of the cuticle correspond to each other while the scaly zone

of the outer side corresponds to the rough zone of the inner

side. More precisely each canalicule enters the cuticle at the

base of a pored tubercle and each glomerule is located under

an outer calyx. The central cavity of the calyx communi-

cates with a short duct going through the cuticle and ending

into a glomerule (Fig. 2(F)). The finger-like processes of the

calyx extend into the duct taking the form of inward folds of

its wall. The duct is thus fluted and looks like a bunch of

gutters draining the glomerule.

3.1.3. Histology

A cross-section of the gland (Fig. 2(G)) clearly shows the

twofold ceiling of its anterior part and the two different

zones of its floor. In the anterolateral zone covered by a

scaly cuticle and dotted with glandular pores the epithelium

is thicker (about 25 mm) and more vacuolated than that ofthe medial zone, where the cuticle is smooth and devoid of

pores.

Only one muscle, inserted on the side of the glandular

epithelium, is directly associated with the gland. It takes its

origin lower on the lateral integument of the first abdominal

segment (Fig. 2(H)).

A. Quennedey et al. / Arthropod Structure & Development 31 (2002) 173183 175

3.2. Transmission electron microscopy

In the glandular epithelium three categories of cells were

observed, the epidermal cells and two types of glandular

cells called C1 and C3 according to the classification of

Noirot and Quennedey (1974). Each category is connected

with peculiar cuticular structures seen in SEM or histology.

They are described according to their abundance in the

glandular epithelium.

3.2.1. C1 cells

These cells are the most abundant. They reach about

25 mm in length and 10 mm in width (Fig. 3(A)). In theirapical part they surround an extracellular space which itself

contains a spherical glomerule (3 mm in diameter)connected to the glandular cuticle.

Each glomerule is a part of a sophisticated cuticular

apparatus (Fig. 3(B)) the outer part of which is a calyx

surrounding the opening of a short duct. The wall of the duct

is deeply wrinkled into longitudinal folds connecting

together to form a bunch of short epicuticular gutters

(1 mm long and 0.2 mm of inner diameter) which look likeducts in TEM sections. These gutters slightly diverge before

opening into inner pockets of the glomerule (Fig. 3(C)), the

major part of which is made of aggregated epicuticular

filaments. A thin epicuticular cupule (4 mm in diameter and0.01 mm thick) is fixed to the bunch of epicuticular guttersand forms a hemispherical cap over the upper half of each

glomerule (Fig. 3(B)(D)).

Adjoining C1 cells are united by septate junctions. Their

apical cytoplasm is particularly thin (0.02 mm) and closelyapplied to the convex side of the cupule (Fig. 3(D)). The

extracellular space is very wide (Fig. 3(A)) and is lined by a

deep invagination of the apical cell membrane showing

many infoldings but no microvilli. Free ribosomes, smooth

endoplasmic reticulum and mitochondria are also found in

the cytoplasm. The cell nucleus (5 mm long) is located in thebasal part of the cytoplasm (Fig. 3(A)).

Numerous strands of secretory material are scattered into

the extracellular space (Fig. 3(A), (E) and (F)). Along the

apical cell membrane, they are closely associated with dense

patches (Fig. 3(F)). Near the glomerule, they lose their

linear structure (Fig. 3(D) and (E)) before taking an

amorphous appearance. Afterwards, the secretion is stored

in the inner pockets of the glomerule before reaching the

glandular reservoir via the bunch of epicuticular gutters and

the opening of the calyx.

3.2.2. Epidermal cells

These cells are generally reduced to a thin cytoplasmic

layer (0.2 mm thick) contained between the glandularcuticle and the top of C1 cells (Figs. 3(D) and 4(A)). In

the vicinity of the nucleus they are, however, thicker and

penetrate for a short distance between the C1 cells (Fig.

4(A)).

3.2.3. C3 glandular units

These glandular units are much less abundant than C1

cells. They are composed of 3 cells laid along a common

cuticular duct measuring about 40 mm in length (Fig. 4(B)).The canal cell contains the outer part of the cuticular

duct, i.e. the conducting canal made of dense outer and inner

epicuticles which opens at the top of a pored tubercle. This

canal is about 25 mm long and 0.5 mm in diameter. It issurrounded by a very thin cytoplasm and runs towards the

base of the epithelium. Apically, intercellular junctions

connecting the canal cell to epidermal cells are strongly

contorted and reinforced with septate junctions and an

apical desmosome (Fig. 4(A)).

The intercalary cell overlaps the inner part of the canal

cell (Fig. 4(B)) but its direct connection with the cuticular

tube is greatly reduced (Fig. 4(C)). Compared with the

conducting canal, the intercalary part of the duct is much

shorter (1 mm long) and pitted with small channels near theentrance of which bundles of fibrillar material are gathered

(Fig. 4(C) and (D)). It bathes in a reduced extracellular

space lined by an invagination of the cell membrane bearing

short microvilli (Fig. 4(C)). Septate junctions bind the

intercalary cell to the two other cells.

The terminal cell, with a diameter of 1520 mm, is thelargest cell of the glandular unit and it entirely surrounds the

intercalary cell (Fig. 4(B)). It also contains the inner part of

the cuticular duct, i.e. the receiving canal, inside a wide

extracellular space (Fig. 4(B), (C) and (E)). The receiving

canal has a slightly larger lumen and its epicuticular wall is

perforated by curved splits (Fig. 4(E)). The cytoplasm of the

terminal cell is particularly rich in vesicles of rough

endoplasmic reticulum containing material of different

densities (Fig. 4(F)). Small mitochondria, Golgi bodies

and free ribosomes are also present. The extracellular space

Fig. 2. Scanning electron microscopy of the inner side of dorsal integument (A)(F) and histology (G), (H). (A) Position of the paired prototergal glands (pg)

on the first abdominal tergum (arrow) behind the large first abdominal spiracle (S1). (B) Rough secretory zone (arrow) and smooth zone (arrowhead) of the

gland; first abdominal spiracle (S1). (C) Rough secretory zone. Glomerules (g) and cupule (arrowhead) of C1 gland cells along internal ridges of the cuticle;

canals of C3 glandular units (arrow). (D) Scratched glomerules showing the inner fluted duct with lateral gutters (arrow) separated by infoldings (i) of the wall;

cupules (arrowheads) surround the ducts. (E) Conducting canal (arrow) of a C3 glandular unit going through the cuticle; glomerules (g) and cupule

(arrowhead). (F) Cross-section of the rough glandular zone showing the fluted duct (arrow) of a glomerule (g) opening between the processes of a calyx (ca).

(G) Cross-sections of the right prototergal gland seen from behind. The anterior part of the glandular reservoir (gr) is covered by a twofold ceiling (tc) under the

folded hind wings (hw). The lateral thick secretory epithelium (arrow) underlies a scaly cuticle (c) while the medial thin epithelium underlies a smooth cuticle

(arrowhead). (H) Muscle inserted, at upper end (arrowhead), on the glandular cuticle lining the glandular reservoir (gr) and, at lower end, on the lateral

integument (arrow) below the first abdominal spiracle (S1).


contains loose material also seen in transit in the lumen of

the receiving canal (Fig. 4(C) and (E)).

The structural features of the three types of cells found in

the prototergal glands are summarised in Fig. 5.

4. Discussion

4.1. Position and name of the glands

The glands are located in a segment characterised by a

particularly large spiracle and by a deep emargination at the

middle of its anterior margin (Figs. 1(A) and 2(A)). In

comparison with other Staphilinini studied in detail by

Dajoz and Caussanel (1968) and by Naomi (1989a,b)), this

segment is without doubt the first abdominal segment. It

cannot be the metathorax as supposed by Jordan (1913) and

Sulc (1922).

The term osmeterium proposed by Sulc (1922) is no

longer appropriate as it designates eversible glands in recent

literature. We preferred the term prototergal glands

referring only to the precise location of the organs.

4.2. Ultrastructure and comparative anatomy of C1 cells

According to their number and size the C1 cells may be

considered as the main secretory cells of the prototergal

glands. Three cytological features deserve a discussion.

1. The infoldings of the apical cell membrane found in

place of a typical brush border has been rarely found in

C1 cells of insects (Quennedey, 1998). In Philonthus this

lack of microvillous surface seems balanced by the deep

invagination of the apical cell membrane and its

numerous infoldings.

2. The abundance of smooth endoplasmic reticulum

suggests that the secretion is at least partly made of

small molecules.

3. The strands of secretion found in the extracellular space

disintegrate into an amorphous material when reaching

the glomerule. This modification is probably the

morphological sign of a chemical transformation.

Besides these features, the cuticular apparatus is the most

noteworthy characteristic of prototergal C1 cells. This

apparatus shows several features that recall at first sight C3

glandular unit: (i) the cuticular duct opens outwards through

a large single pore; (ii) the duct and the glomerule are

surrounded by a subcuticular space recalling the extracellu-

lar space formed inside the terminal cell of class 3 cells; (iii)

the glomerule has the same spherical shape as the receiving

bulb which forms the receiving canal inside C3 terminal

cells in numerous defensive glands of Hemiptera (Quenne-

dey, 1998); (iv) the glomerule and the receiving bulb both

have a fibrillar structure and they could thus both serve as

porous support for enzymatic activities as hypothezised by

Araujo (1978) for C3 cells.

These common features and particularly the similarities

between the glomerules and the receiving bulb of C3 cells

would lead to the inclusion of the C1 cells of Philonthus

into the class 3 gland cells as initially defined by Noirot and

Quennedey (1974) and Noirot and Quennedey (1991)). In

fact, the origins and chemical compositions of the glomerule

and the receiving bulb are drastically different. The

glomerule results from the aggregation of numerous

epicuticular filaments made of liquid crystals of lipid

water (Locke, 1974), while the receiving bulb is composed

of numerous fibrils of inner epicuticle made of polymerised

lipoproteins.

By comparison with data previously published for

several glands of beetles (Fig. 6), the gland cells of P.

varians provided with a glomerule can be easily considered

to belong to class 1 cells according to Noirot and

Quennedeys classification as updated by Quennedey

(1998) and Quennedey (2000)). In beetles, the common

features of all these C1 cells are the invagination of the

apical cell membrane allowing the development of a sub-

cuticular space and the piercing of the cuticle by apertures

which may vary in size and number. In Dendroctonus (Happ

et al., 1971; Fig. 6(A)) the cuticle is pierced by a number of

minute perforations containing epicuticular filaments. More

often the epicuticular filaments increase in length and

number and occupy a greater volume as in Eusphalerum

(Araujo, 1978; Fig. 6(B)) and Semiadalia (Barbier et al.,

1992; Fig. 6(C)). In the latter genus the minute perforations

are further replaced by a lesser number of small pores.

Another modification is the invagination of the cuticle into

the subcuticular space. The cuticle can form a tube opening

outwards by a single large pore but with an inferior blind

ending crossed only by epicuticular filaments through

minute perforations as in Bruchidae (Biemont et al., 1990,

1992; Pierre et al., 1997; Fig. 6(D)) and Ceutorhynchus

Fig. 3. Structure (TEM) of C1 cells. (A) Low magnification showing cuticular apparatus (arrows) against the glandular cuticle (c). Below, strands of secretory

material (arrowheads) are seen inside the large extracellular spaces (es). Note the basal location of the nucleus (n). (B) Sagittal section of cuticular apparatus

showing two epicuticular gutters giving them a duct-like aspect (arrows). A spheroidal structure, called here glomerule (g), is capped with an epicuticular

cupule (arrowheads). (C) The glomerule (g) contains several inner cavities full of amorphous secretion (s) which are connected to the epicuticular gutters

(arrowhead). (D) A thin strip of cytoplasm (black arrows) covers each cupule (open arrow). Beneath the glandular cuticle (c), epidermal cells (e) insert between

C1 cells for a short distance. In the extracellular space (es), the secretory material is amassed around the glomerule (g). (E) In the extracellular space (es),

strands of secretion (arrowheads) crumble in the vicinity (arrow) of the glomerule (g). (F) Electron-dense patches (arrows) lining the apical cell membrane are

often associated with the strands (arrowheads) of secretion. Cytoplasmic infoldings (large arrow) and smooth endoplasmic reticulum (small arrows) are also

seen.


(Ferguson et al., 1999; Fig. 6(E)). The tube can also open

outwards through a few small pores while its bottom is

entirely open as in Drusilla (Araujo, 1978; Fig. 6(F)),

Choleva, Speophyes (Martin, 1977; Fig. 6(G)) and Aleo-

chara (Skilbeck and Anderson, 1994; Fig. 6(H)). In

Philonthus, the cuticular invagination is different again

and more sophisticated. This short duct is crowned by a

calyx of finger-like processes. Its wall is deeply wrinkled

and looks like a bunch of epicuticular gutters opening within

a large protruding glomerule (Fig. 6(I)). In addition, the duct

supports a cupule overlapping the upper half of the

glomerule. This cupule appears like a funnel, which

reinforces the apical part of the cell and conducts its

secretion through the glomerule towards the external

opening.

The ultrastructural study of prototergal glands thus

provides new evidence of the large plasticity of C1 cells.

In particular these single cells can produce a cuticular

apparatus with elements like distinct pores, an evacuating

duct and even a bulb-like glomerule that are produced by

different cells in C3 glandular units. As a consequence C3

gland cells cannot be distinguished from C1 cells on the sole

criterion of a single large outer pore as supposed in some

original descriptions (Fig. 6(D) and (G)) but according to

the number of cells found in each glandular unit.

4.3. Ultrastructure of C3 glandular units

On the contrary to the canal cell, both intercalary and

terminal cells are involved in the synthesis of secretion. The

intercalary cell develops microvilli around a short part of the

cuticular duct, as is generally the case in defensive and

other integumentary glands of beetles (Tschinkel, 1972;

Delachambre, 1973; Martin, 1975; Araujo, 1978, 1981;

Dailey and Happ, 1982; Bartlet et al., 1994; Weis et al.,

1999; Kolsch, 2000). The occurrence of these microvilli

together with the piercing of the receiving canal provide

evidence of the secretory activity of this small cell.

The terminal cell is, however, the main glandular cell

of the C3 unit, as can be inferred by its much greater

volume and by several cytological features. The large

extracellular space contains patches of loose material

secreted by numerous vesicles of rough endoplasmic

reticulum. The absence of microvilli in Philonthus can be

explained by the strong exocytotic activity of these

vesicles which allows the transport of proteinaceous

secretions into the extracellular space without requiring

an increase in cell surface.

Fig. 5. Diagram summarising the ultrastructural features of the prototergal

glands. The glandular cuticle is composed of large digitated scales (sc)

produced by the epidermal cells (e). The apical digitations partly overlap

the openings of the two types of glandular cells. Class 1 cells (C1) are

columnar and show a central extracellular space (es) lined by the

invagination of the apical cell membrane. Strands of secretion are filtered

through a glomerule (g), capped with a cuticular cupule (arrowhead). Then,

the secretion gathers into the glandular reservoir (gr) after being conveyed

through short cuticular gutters opening in the calyx (ca) defined by a crown

of finger-like processes. Class 3 cells (C3) correspond to a glandular unit

with three cells laid along a common cuticular duct (open arrow). It drains

the secretion outside and opens at the top of a pored tubercule (t). The duct,

or conducting canal, is surrounded by a long canal cell (1), shortly

overlapped by a small intercalary cell (2) which is entirely surrounded by a

large terminal cell (3). The terminal part of the duct, or receiving canal

(arrow), is immersed in the central extracellular space (es) of the terminal

cell.

Fig. 4. Structure (TEM) of C3 cells. (A) Longitudinal section of a conducting canal just before its connection with the apical pored tubercule (t). Note also the

contorted path of intercellular junction (arrow) linking epidermal cell (e) and canal cell (1). (B) General organisation of C3 cells. The glandular unit is

composed of three cells: a lengthened canal cell (1), a small intercalary cell (2) and a large terminal cell (3) with a broad extracellular space (es). (C)

Longitudinal section of the cuticular duct draining the glandular unit. Boundaries of canal (1), intercalary (2) and terminal (3) cells are underlined (arrows). In

the intercalary cell, a narrow extracellular space is lined by short microvilli (arrowheads) and bundles of fibrillar material (open arrows) surround the duct in

which dotted secretion is seen (star). (D) Transverse section of the intercalary cell (2) and its duct surrounded by bundles of fibrillar material (open arrow). (E)

Transverse section of the terminal cell (3). The receiving canal is pierced by curved splits (arrowheads) and located in the middle of the extracellular space (es).

(F) Numerous vesicles (v) of rough endoplasmic reticulum (magnified in the insert), free ribosomes (arrow), a Golgi body (open arrow) and the cell nucleus (n)

are observed.


4.4. General functioning of the glands

C1 and C3 gland cells pour out their secretions,

respectively, through calyces or pored tubercles in the

anterolateral zone of the prototergal glands. In this same

zone the epidermal cells lay down a cuticle provided with

numerous digitated scales and are therefore implicated in

the mode of delivery of the secretions. This scaly surface

recalls the sophisticated cuticular area produced by the

epidermal cells in odoriferous glands of bugs (Crossley and

Waterhouse, 1969). In Philonthus it can retain the liquid

secretion between the scales, preventing an excessive flow,

as well as increase evaporation on its much enlarged

surface. When the secretions overflow the scaly zone they

can be collected by the adjacent reservoir and carried further

by the open outlet. The basiconic sensilla lined up along the

lateral edge of the gland opening are probably stimulated

when the secretion overflows the outlet and could thus

trigger reactions slowing down the flow. The muscle

inserted on the glandular epithelium could be involved in

these reactions as its contraction would dilate the reservoir

and thus reduce the outward flow of its contents.

The grille of setae lying over the opening of the gland can

be understood as a device protecting the thin hind wing

membrane, folded at rest just above the gland, from being

moistened and made sticky by its secretions.

Concerning the possible functions of the glands, Sulc

(1922) hypothesised that they are involved in sexual

attraction. They are, however, similar and equally devel-

oped in both sexes and hence are not good candidates for the

production of sexual pheromones. We would favour another

hypothesis. The species of rove beetles that are known so far

to bear prototergal glands are often encountered in diverse

decaying matter including dung and carrion. This habitat is

very rich in bacterial and fungal micro-organisms, some

species of which are possible pathogens for insects. As the

sub-elytral chamber is a space, where such pathogens could

develop and which is quite difficult to clean, prototergal

glands may have appeared in the course of evolution as a

means of defence against them. This prophylactic function

at least deserves to be tested.

4.5. Phylogenetic prospect

The distribution and morphology of prototergal glands

appear to be promising topics for the study of the still

debated systematics and phylogeny of Staphylininae which

currently includes about 300 genera and 7000 species. From

one genus to another, or even in different species of a same

genus (e.g. Quedius ) prototergal glands are either present or

lacking. When present they can also differ markedly in the

occurrence, development and morphology of such elements

as the reservoir, the gutter, the grille of setae, the calices, the

cuticular scales and basiconic sensilla. Combined with other

morphological characters these features of prototergal

glands could thus contribute toward building a cladogram

of this subfamily.

Acknowledgements

The authors are indebted to Josette Relot (CMAB), Julien

Cillis (IRSNB), Eddy Terwinghe, Christian Kumps and

Walter Dereck (ULB) for their efficient technical assistance

as well as to Claude Everaerts for his invaluable help in

TEM figure processing. They thank Prof. R.D. Kime and Dr

A.F. Newton for having kindly revised the English version

of the text.

Fig. 6. Structural variability of C1 cells found in exocrine glands of beetles.

The cuticle (in dark), the epicuticular filaments (arrowheads), the cell

cytoplasm (light stippled) and the nucleus (stippled circle) have been

considered and schematised according to data of authors. (A) Type 2 cell,

mycangium of Dendroctonus (Happ et al., 1971). (B) E3 cell, defensive

gland of Eusphalerum (Araujo, 1978). (C) Integumentary gland cell of

Semiadalia (Barbier et al., 1992). (D) Sex pheromone gland cell of

Bruchidius (Biemont et al., 1992). (E) Oviposition-deterring pheromone

gland cell of Ceutorhynchus (Ferguson et al., 1999). (F) D4 cell, defensive

gland of Drusilla (Araujo, 1978). (G) Antennal gland cell of Choleva and

Speophyes (Martin, 1977). (H) Antennal gland cell of Aleochara (Skilbeck

and Anderson, 1994). (I) C1 cell, prototergal gland of Philonthus (see text

for explanation).


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Morphology and ultrastructure of paired prototergal glands in the adult rove beetle Philonthus varians (Coleoptera, StaphylinidIntroductionMaterial and methodsAnimalsLight microscopyScanning electron microscopyTransmission electron microscopy

ResultsScanning electron microscopy and histologyTransmission electron microscopy

DiscussionPosition and name of the glandsUltrastructure and comparative anatomy of C1 cellsUltrastructure of C3 glandular unitsGeneral functioning of the glandsPhylogenetic prospect

AcknowledgementsReferences

Documents

Morphology and ultrastructure of paired prototergal glands in the adult rove beetle Philonthus varians (Coleoptera, Staphylinidae)