Comparative morphology of abdominal glands in Paederinae (Coleoptera : Staphylinidae)

Int.J. InsectMorphoL&Erabryol.,Vol.21, No. 2, pp. 117-135,1992

0020-7322/~2$5.00+ ,{}4} ~) 1992PergamonPressLtd

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C O M P A R A T I V E M O R P H O L O G Y OF A B D O M I N A L G L A N D S IN P A E D E R I N A E (COLEOPTERA • STAPHYLINIDAE)

RUPERT L . L . KELLNER a n d KONRAD DETTNER* Lehrstuhl for Tier6kologie II, Universit~it Bayreuth, Postfach 101251, D-W8580 Bayreuth, Germany

(Accepted 5 February 1992)

Abstract--Glandular structures at the sternites of 31 species of Paederinae (Coleoptera :

Staphylinidae) are described in detail for the first time. A median gland of unknown biological significance is situated at the front margin of the 4th sternite in Paederus and Paederidus (Paederus s.l.). A gland tissue at the posterior border of the antecosta opens into a reservoir situated between the antecosta and the intersegmental membrane. Each of the numerous glandular units obviously consist of 2 secretory cells that are drained by a tubule. They are thus related to dermal glands. Two types of pores, single pores and pore plates, are dispersed over the body, but are aggregated in the vicinity of the aperture of the reservoir. In Rugilus, 2 hitherto unknown glands of the same shape are located at the front margins of sternites 4 and 5. In contrast to the glands of Paederus s.l., there are big pores under the gland tissue and the opening area lies in a pit. One further species, Stilicosorna rufipes, was recently classified in the genus Rugilus, but does not have any glands, and therefore the genus Stilicosoma Casey, which was once named for S. rufipes, is re-established. Ochthephilum fracticorne does not have a reservoir, but otherwise combines features of both the glands of Paederus s.l. and those of Rugilus. This link points out the possible derivation of these glands from a common origin. All species of Lathrobiina have accumulations of tubules at the front margins of at least the 4th sternites, and thus represent a rather primitive stage in the development of the glands, while the lack of glandular peculiarities at the 4th sternites in Medonina is plesiomorphic. With respect to a special type of dermal tubules, the genera Medon, Pseudomedon, and Sunius are distinguished from Lithocharis, hence denoting a clear dividing line between the subtribes Medonina and Lithocharina. These features as well as the glands themselves, provide the basis for clarifying the largely unknown phylogeny of Paederinae. Index descriptors (in addition to those in title): Dermal glands, character evolution, phylogeny, Stilicosoma.

INTRODUCTION TrIE BEETLE f a m i l y S t a p h y l i n i d a e is u n i q u e i n h a v i n g e v o l v e d i n d e p e n d e n t l y s e v e r a l t y p e s o f a b d o m i n a l d e f e n s i v e g l a n d s w h o s e o c c u r r e n c e is o f t e n r e s t r i c t e d t o o n e s u b f a m i l y (Araujo, 1978; Dettner, 1987). Owing to the shortening of elytra, evolution of these g l a n d s is a t t r i b u t e d t o t h e n e e d f o r d e f e n s e o f t h e o t h e r w i s e u n p r o t e c t e d a b d o m e n ( D e t t n e r , 1 9 8 7 ) . I g n o r a n c e o f s u c h g l a n d s in t h e s u b f a m i l y P a e d e r i n a e d i d n o t m a t t e r because, at least in Paederus, a different kind of chemical defense system was discovered,

* Author to whom correspondence should be sent. 117

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which consists of a warning coloration attended with vesicant compounds in their h e m o l y m p h (Araujo, 1978; Dettner, 1987). Although some authors (Fish, 1979; Kanehisa et al., 1984) stated that Paederus is not provided with any abdominal gland, H e r m a n (1981) found one at the front margin of the 4th sternite. H e used this new feature in cladistic analysis, declaring it an a p o m o r p h y of 3 subtribes of the tribe Paederini, but failed to name those species where he did not find the gland. Since his figures only show reservoirs at the front margins of the sternites of several species, the morphology of that gland may be regarded as wholly unknown. In this paper, we describe morphological details of the gland in Paederus, whose biological significance is still unknown, and of glandular structures at the same location in other Paederinae and compare them with dermal glands dispersed over the sternites. Using these data, we infer the evolution of the glands and evaluate their utility to a future phylogenetic classification of Paederinae. We dwell only on species belonging to the tribe Paederini, which is no drawback as the gland in question certainly is confined to a part of that tribe.

MATERIALS AND METHODS Representatives of the staphylinid subfamily Paederinae were collected in 1990 from various sites in northeastern Bavaria (Germany) and in the south of France. Identification of 31 species (Table 1) is based on the keys of Boh~i~(1985a, b, 1986), Coffait (1982, 1984) and Lohse (1964, 1989). Since males of the genus Scopaeus were not available, the determination of these 2 species is tentative, although it is validated by comparison with the species present in the Zoologische Staatssammlung, Miinchen. The abdomina of frozen specimens were severed and dissected longitudinally along their paratergites. For examination of the cuticular parts of exocrine glands, sternites were treated with 5% potassium hydroxide (KOH) for 20 hr, dehydrated in a graded series of acetone and critical-point dried with carbon dioxide. Scanning electron microscopy (SEM) was carried out using a Cambridge Stereoscan 90. In order to determine the density of glandular structures, tubules and pores were counted within squares of 10 3 p,m 2. With these values, means and 95% confidence intervals were calculated. ,Examination of outliers was performed using Dixon's test (Dixon and Massey, 1983). Drawings were made with the aid of a drawing tube attached to an Olympus SZH dissecting microscope.

RESULTS

Gross morphology of glandular structures A median gland is situated at the anterior margin of the 4th sternite of all the available species of Paederus and Paederidus (Table 1, in the following Paederus s.l.), as indicated from an unidentified species of Paederus by H e r m a n (1981). Furthermore, 4 species of Rugilus (Table 1) have median glands in duplicate at the front edges of sternites 4 and 5, which have not yet been described. In all other Paederinae studied, sternal glands with reservoirs are missing, while different glands without reservoirs have been found mostly at the anterior margins of the 4th sternites (Table 1). As the morphological features of the various sternal glands exhibit considerable diversity, each type will be described separately. Paederidus and Paederus. In Paederus s.l. a compact gland tissue is located at the posterior border of the antecosta of the 4th sternite. The tubules that drain the gland cells penetrate the antecosta beneath the glandular tissue, opening into the basal part of

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TABLE 1. COMPILATION OF PAEDERINAE STUDIED WITH THEIR RESPECTIVE GLANDULAR CHARACTERS OF ABDOMINAL STERN1TES (NUMBERING ACCORDING TO BLACKWELDER, 1936). THIRTY-ONE SPECIES BELONGING TO 14 GENERAOF THE TRIBE PAEDERINI Subtribe Lithocharina Lithocharis ochracea (Gravenhorst) Lithocharis nigriceps Kraatz

2 tracks of pores at sternite 5

Subtribe Medonina

Pseudomedon ( Pseudomedon s. str.) obscurellus (Erichson) Sunius (Sunius s. str.) melanocephalus (Fabricius) Medon brunneus (Erichson) Medon ripicola (Kraatz)

No glandular structures

Subtribe Scopaeina

Scopaeus (Scopaeus s. str.) laevigatus (Gyllenhal) Scopaeus (Heteroscopaeus) gracilis (Sperk) Subtribe Stilicina

Ring with few ductules at sternite 4

Stilicosoma rufipes (Germar)

Accumulation of pores at sternite 4

Rugilus angustatus (Fourcroy) Rugilus geniculatus (Erichson) Rugilus orbiculatus (Paykull) Rugilus erichsoni (Fauvel)

Median glands at sternites 4 and 5

Subtribe Lathrobiina

Domene scabricollis (Erichson) Lathrobium ( Tetartopeus) terrninatum Gravenhorst Lathrobium ( Lathrobium s. str.) ripicola Czwalina Lathrobium ( Lathrobium s. str.) fulvipenne Gravenhorst Lathrobium ( Lathrobium s. str.) brunnipes (Fabricius) Lathrobium (Lathrobium s. str.) fovulum Stephens Lathrobium ( Lathrobium s. str.) longulum Gravenhorst Lathrobium (Lathrobium s. str.)pallidum Nordmann Lobrathium ( Lobrathium s. str.) multipunctum (Gravenhorst) Subtribe Cryptobiina

Ochthephilum fracticorne (Paykull)

Accumulation of ductules at sternite 4 and to a lesser extent at sternites 5 and 6

Pit and accumulation of ductules at sternite 4

Subtribe Paederina

Paederidus ruficollis (Fabricius) Paederidus rubrothoracicus (Goeze) Paederus (Harpopaederus) brevipennis Boisduval et Lacordaire

Median gland at sternite 4

Paederus (Heteropaederus) fuscipes Curtis Paederus ( Paederus s. str.) riparius (Linn6) Paederus ( Dioncopaederus) litoralis Gravenhorst Subtribe Astenina

Astenus (Astenognathus) pulchellus (Heer) Astenus (Astenus s. str.) immaculatus Stephens

Accumulation of pores at sternite 4

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FiG. 1. S c h e m a t i c d r a w i n g s of l o c a t i o n and s h a p e of the g l a n d in Paederus s.l. (a) V e n t r a l view of a m a l e Paederus riparius, with a p e r t u r e of r e s e r v o i r i n d i c a t e d by an a r r o w ; scale 5 ram. (b) V i e w of g l a n d in d i r e c t i o n of a r r o w in (a); a = a n t e c o s t a ; cl = sclerotized clasp; e = e x t e n s i o n of the r e s e r v o i r ; g = g r o o v e ; gt = g l a n d u l a r tissue; im = i n t e r s e g m e n t a l m e m b r a n e ; r = reservoir; s = s t e r n a l s u t u r e ; $3, $4 = sternite 3, 4; t = tubule.

a reservoir which is attached to the opposite side of that rim (Fig. lb). The aperture of the reservoir lies between sternites 3 and 4 (Fig. la). The reservoir itself is situated immediately inside the 3rd sternite. Its epithelium does not contain any tubule-carrying gland cells and no muscles are attached to the wall. In SEM, the surface of the cuticle is wrinkled very finely, which might be an artifact, but in light microscope fine structures are also visible, which suggests that the wall is not smooth. Both sexes of the 2 species of Paederidus and of Paederus riparius, have the reservoir extended at one side by a pouch (Fig. 2a) that usually reaches backward into the 4th segment. In Paederus fuscipes, the extension is only present in females, whereas it is lacking in males. The same incidence may be true of Paederus brevipennis and P. litoralis, but as no females of those species have been analysed, the extension might be lacking altogether. The aperture of the reservoir is constricted to a narrow gap by a sclerotized clasp that spans the antecosta from one side of the aperture to the other (Fig. 4a), which may restrict discharge of accumulated secretion. Without removal of that clasp, one cannot see the opening pores of the gland tissue lying within a groove at the base of the reservoir (Fig. 3a-d). Owing to treatment with K O H , a great many tubules with underlying pores emerge from the tissue on the other side of the antecosta (Fig. 2b, c). They are arranged in a manner corresponding to the pore area in the groove. The groove stretches inside of the clasp across the front margin of the sternite and bending, it extends along the

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FIo. 2. SEM pictures showing cuticular remains of sternal gland of Paederus s.1. treated with KOH. (a) Reservoir of Paederidus ruficollis attached to antecosta of 4th sternite, with location of gland tissue being discernible from numerous tubules at posterior border of antecosta; $3, $4 = sternite 3, 4. (b) Arrangement of tubules at posterior border of antecosta of Paederidus ruficollis and (c) Paederus litoralis. Scales: 100 p.m.

lateral corners of the aperture to the outside of the sternite. This construction renders the groove u-shaped as perceived in Paederus riparius (Fig. 3b) and P. fuscipes. In P. brevipennis (Fig. 3c) and especially in P. litoralis (Fig. 3d), the groove is modified by a median bulge thus appearing rather w-shaped. On the one hand, this implies an increase in the pore area, on the other, the aperture in P. litoralis, measured by the distance between the 2 ends of the groove, is smaller with respect to the width of the sternite, than in all other species. In Paederus, the distal rim of the groove is prominent, while in Paederidus the border is enlarged roofing over the groove (Fig. 3a). Therefore, the pores are completely covered unless the distal border is removed. The cuticle inside the groove is smooth, sometimes with shallow depressions. Only in Paederus fuscipes, is the inside creased (Fig. 30. The pores are scattered about in the groove singly or in small groups of up to 7 (Fig. 3e). In all species of Paederus, particularly in P. litoralis (Fig. 3h), small circular elevations are intermediately located. Their surface is wrinkled, but does not contain any detectable pores. The proximal rim of the groove is more or less corrugated, with certain species having spikes (Paederus fuscipes, Fig. 30 or comb-like structures (P. riparius, Fig. 3g) upon the crests. Next to that, the antecosta is connected with the reservoir, while the other side of the reservoir is linked to the sclerotized clasp. As mentioned earlier, the groove leads out of the reservoir and terminates in the antesutural area of the 4th sternite. A group of 3-5 little bristles is always situated close to the end of the groove (Fig. 4b). Such bristles are also present on the antesutural areas of the following sternites, but there they are evenly distributed without forming a group. Two types of pores are found on the outside of all sternites: single pores and pore plates (Fig. 4b, c). In the postsutural areas, pores are generally associated with hairs (Fig. 4e), but

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F16.3. SEM pictures showing groove near aperture of reservoir of Paederus s.l. (a) Shape of groove in Paederidus rubrothoracicus, (b) Paederus riparius, (c) P. brevipennis, and (d) P. litoralis. (e) Pores within groove of Paederidus ruficollis, with droplets of secretion being clung to them. (f) Lateral detail of groove in Paederus fuscipes, (g) P. riparius, and (h) P. litoralis. Scales: 100 I~m (a~l) and 10 txm (e-h). they are also located in front of the sternal suture (Fig. 4d) where such hairs are lacking. C o m p a r i s o n of the density in the antesutural areas of sternites 4--6, shows that both single pores and pore plates are aggregated on the 4th sternite (Fig. 5a, b), hence close to the aperture of the reservoir where they exhibit r e m a r k a b l e densities (Fig. 4c). Rugilus. T w o gland complexes of the same shape are situated at the front edges of sternites 4 and 5 in R u g i l u s (Fig. 6b). O n e further species of Stilicina, S t i l i c o s o m a rufipes,

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Fro. 4. SEM pictures of surface structures related to aperture of reservoir of Paederus s.l. (a) Aperture of reservoir of Paederus riparius without removal of sclerotized clasp (cl); g = end of groove. (b) Bristles, single pores and pore plates near end of groove (g) of Paederidus ruficollis. (c) Pore plates close to end of groove of Paederus litoralis. (d) Pore plates in antesutural area of 5th sternite of Paederus riparius. (e) A pore plate and a single pore near a hair in postsutural area of 4th sternite of Paederus riparius. Scales: 100 p.m (a) and 10 p.m (t>-e).

recently treated as a species of Rugilus, does not have such glands (Fig. 6a). Consequently, the following description is valid for both glands of true Rugilus only. T h e gland tissue in Rugilus, like that in Paederus s.l., is located at the posterior border of the antecosta, but is kidney-shaped and, in contrast to Paederus s.l., numerous underlying big pores are already discernible by light microscope. SEM shows that their inner diameter is 4-6 p~m, and because they are closely packed, they show a sponge-like surface structure (Fig. 6e). The n u m b e r of pores varies according to the size of the sternite (Fig. 6c, d). Variation arises from species to species and within a single specimen from the 4th sternite to the larger 5th. Correlation of the n u m b e r of pores with size of sternite, measured by its length, is highly significant (r = 0.94, P < 0.0001, n = 13). A m o n g the pores, few long tubules run into small deepenings of the surface (Fig. 6e, f). The ratio of tubules to pores is about 1 : 4. The opening area of the gland tissue is located at the hind wall of a sclerotized pit

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Paederidus rubrothoracicus Paederidus ruficollis

--F-

Paederu8 riparius

-4-

Paederus fuscipes

--t-

,>(

Paederus brevipennis X

Paederus litoralis

--F-

.-FI

number of single pores per 103#m 2

b Paederidus rubrothoracicus

.-b-I

Paederidus ruficollis Paederus riparius Paederus fuscipes Paederus brevipennis Paederus litoralis

I

"-I--.+I

-4.--+1

5 number of pore plaLes per 103#m 2

Fro. 5. Densities of pores in antesutural areas of sternites with (×) and without (I) a median gland in Paederus s.l.: mean (× or I) and 95% confidence interval ( - - ) of single pores (a) and pore plates (b) on sternites 4-6 (per species from top to bottom) of 2 individuals each (40 squares counted) except in Paederus brevipennis where only one specimen was available (20 squares counted).

under the antecosta (Fig. 7a, b). The tubules terminate in small pores, while the big pores of the inside are related to circular elevations, each surrounded by a furrow (Fig. 7c). Therein little punctures can be discerned occasionally. The surface of the elevations is wrinkled, whereas it is regularly furrowed in between elevations. This pattern might be caused by boundaries of those cells that secrete the cuticle. The other walls of the pit are more or less smooth. Laterally, the depth of the pit declines steadily, tapering off at the level of the sternite. Along the central axis, the pit is divided by a ridge (Fig. 7a). This is the reason why the gland tissue is kidney-shaped and the underlying sponge-like structure has a median depression. The spherical reservoir

Comparative Morphology of Abdominal Glands in Paederinae

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FIG. 6. SEM pictures showing cuticular remains from inside of 4th and 5th sternites of Stilicina treated with KOH. (a) Sternite 4 ($4) and anterior margin of sternite 5 ($5) of Stilicosoma rufipes and (b) of Rugulis erichsoni where 2 reservoirs are attached to antecostae. (c) Different numbers of great pores in sponge-like structure at antecosta of 5th sternite of Rugilus erichsoni and (d) R. angustatus. (e) Detail of sponge-like structure in Rugilus orbiculatus showing a depression in middle of pore area. (f) Several great pores and few tubules in R. orbiculatus. Scales: 200 ~m (a, b), 100 ~m (c, d), 50 txm (e) and 5 Ixm (f).

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FIG. 7. SEM pictures of pit at 4th or 5th sternites of Rugilus orbiculatus. (a) Exterior of pit, shreds indicating position of reservoir. (b) Hind wall with opening area of gland tissue after removing front wall of pit. (c) Detail of opening area with single pores and circular elevations. Scales: 100 v,m (a, b) and 5 v,m (c).

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FlG. 8. SEM pictures showing front margins of 4th sternites of Ochthephilum fracticorne and 2 species of Scopaeus after treatment with KOH. Views from outside (a) and inside (b) of pit in Ochthephilum fracticorne. (c) Circular elevation and tubules at posterior border of antecosta in Scopaeus gracilis and (d) S. laevigatus. Scales: 100 v,m (a, b) and 20 v,m (c, d).

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links up to the front of the pit, and is connected with the intersegmental membrane without having a sclerotized clasp. Ochthephilum. In Ochthephilum fracticorne, a pit was observed at the anterior margin only of the 4th sternite (Fig. 8a). The sclerotized walls are smooth for the most part, while there are uneven patches at the hind rim, which contains single pores. These are openings of tubules that are clustered on the inside of that area (Fig. 8b). This shape differs from the glands in Rugilus by the lack of a reservoir, and there is neither a ridge dividing the pit, nor a sponge-like structure. As Paederus s.l., Ochthephilum fracticorne has single pores and pore plates on the outside of the sternites, but these are not aggregated on the 4th sternite. Scopaeus. An amber spot, contrasting with the dark coloration of the remaining sternite, is located at the front edge of the 4th sternite in both species of Scopaeus (only females studied). Using SEM, it appears as a circular elevation of the cuticle, which is very flat in Scopaeus gracilis (Fig. 8c), whereas it is notched at the base, and has a lesser diameter in S. laevigatus (Fig. 8d). Immediately in front of that, 3--4 tubules lead into a ring-like structure at the antecosta. The openings of the tubules could not be found; they were probably covered by the intersegmental membrane. Lathrobium, Lobrathium and Domene. AI~ species studied of the subtribe Lathrobiina have accumulations of tubules at the p~sterior border of the antecosta of the 4th sternite (Fig. 9a-d). Small pores are associat6d with these tubules (Fig. 9b). The density of tubules and pores within the accumulation, is always a significant outlier with regard to the densities measured behind the suture on the same sternite (Dixon's test). For most of the species, this is also true of sternites 5 and 6, but to a lesser degree. Only sternites 5 and 6 of Lathrobium (Tetartopeus) terminatum have no accumulations of tubules at all, on the contrary there are fewer tubules and pores than in the postsutural areas. Stilicosoma and Astenus. Likewise, there are accumulations in Stilicosoma rufipes and both species of Astenus (Fig. 9e), which in contrast to those just mentioned, consist mainly of pores, with a few tubules being interspersed. These accumulations of pores are detectable almost exclusively on the 4th sternite. At the anterior margins of sternites 5 and 6, the densities of pores and tubules are also often increased, but cannot be considered as significant outliers regarding the range of densities in the respective postsutural areas (Dixon's test). Lithocharis. Lithocharis ochracea and L. nigriceps both have 2 peculiar tracks of pores (Fig. 9f) in the antesutural area of the 5th sternite extending from one side to the other. Remarkably, sternites 4 and 6 do not show any comparable structures. Medon, Pseudomedon and Sunius. The specimens of Medonina have neither an accumulation of tubules, nor a special arrangement of pores within the antesutural area of any sternite. Therefore, the maximum density of tubules and pores in that area usually does not exceed the range measured behind the suture.

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FIG. 9. SEM pictures showing cuticular remains of glandular structures at anterior margins of sternites 4 and 5, respectively, of several species of Paederinae treated with KOH. (a) Accumulations of tubules at posterior border of antecosta of 4th sternite in Lathrobium (Tetartopeus) terminatum, (b) Lobrathium multipunctum, (c) Lobrathium brunnipes and (d) L. pallidum. (e) Accumulation mainly of pores at posterior border of antecosta of 4th sternite of Astenus immaculatus. (f) Detail of 2 tracks of pores in antesutural area of 5th sternite in Lithocharis nigriceps. Scales: 50 Ixm.

Structure o f sternal tubules T u b u l e s at the interior sternite surface of various species of P a e d e r i n a e , as o b s e r v e d by S E M after t r e a t m e n t with K O H , differentiate according to their length a n d shape of receiving canal. T h e latter is particularly i m p o r t a n t since c e r t a i n types are discernible within the P a e d e r i n a e studied, each being p r e s e n t in a definite g r o u p of species only. It has to be recognized, however, that different t u b u l e s of o n e s p e c i m e n m a y have m o r e t h a n o n e type of receiving canal.

FIG. 10. SEM pictures showing sternal tubules of various species of Paederinae after treatment with KOH. (a) Entire tubule and pores from postsutural area of Paederus riparius. (b) Receiving canal of tubules from gland in Paederus litoralis and (c) P. fuscipes. (d) Peculiar type of tubule in Astenus pulchellus and (e) A. immaculatus. (f) Entire tubule from front margin of sternite 4 in Astenus immaculatus. (g) Receiving canal of tubules from glands in Rugilus angustatus and (h) R. orbiculatus. (i) Peculiar type of receiving canal in Medon ripicola, (j) Sunius melanocephalus, and (k)

Comparative Morphology of Abdominal Glands in Paederinae

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Pseudomedon obscurellus. (1) Receiving canal of tubules from accumulations at sternite 4 of Lathrobium fulvipenne, (m) L. pallidum, (o) L. brunnipes, and (p) L. (Tetartopeus) terminatum. (n) Entire tubule from postsutural area of Lathrobium pallidum. Scales: 5 i~m.

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Tubules that are dispersed over the whole sternites of Paederus s.l., are comparatively short, at most 35 ~m long (Fig. 10a). The shape of receiving canal, consisting of 2 successive dilatations separated by a constriction, has to be considered the basic pattern in Paederinae. It is also perceptible at the lengthened tubules of the median gland of Paederus s.l. (Fig. 10b, c). The terminal dilatation frequently has a ring-like widening at the base, especially in Paederus fuscipes (Fig. 10c), whereas the diameter of the intercalary dilatation sometimes hardly exceeds that of the canal. Nevertheless, the dilatation can be differentiated from the smooth canal by its rougher surface. In Rugilus, also, the dermal tubules are short (15-28 ~m), while those of the median glands are long (about 100-150 ~m). The receiving canal of the latter shows more details (Fig. 10g, h). The surface of the terminal ampulla indicates a perforation of the lining. It is basally bounded by a ring, followed by a smooth section, which includes a collar. Its intercalary dilatation is well-defined and fringed. A similar receiving canal is associated with certain dermal glands in Astenus whose tubules are short (16 ~m) and located both in the antesutural and postsutural areas of the sternites (Fig. 10f). Furthermore, species of this genus are supplied with a unique type of tubule in the postsutural area (Fig. 10d, e). Its terminal part is comparable to the type of receiving canal just described, but without a basal ring and a collar. The exceptional feature is confined to its intercalary dilatation, which is distinctly lengthened, bending back and forth so that it takes little space. The adjacent canal is also elongated. Another tubular modification only occurs in Medon, Pseudomedon, and Sunius, i.e. all Medonina studied. Apart from shorter (19 ~m) and ordinarily shaped tubules, this peculiar type of elongated tubule (50-90 Ixm) is distinguished by its bulbous terminal ampulla (Fig. 10i-k). Depending on angles of vision, a surrounding line can be detected at the base of the ampulla. The tubules of the sternal accumulations of Lathrobiina, resemble those of the basic type. There are only minor differences concerning length and diameter of the 2 successive dilations. For example, the intercalary dilatations of the tubules in Lathrobium (Tetartopeus) terminatum are short and thick (Fig. 10p), compared with those long and tapering ones in the subgenus Latrhrobium pallidum (Fig. 101, m, o). The tubules of dermal glands are shorter, and only in Lathrobium pallidum is there a modification that shows a widening of the canal near the opening, but does not have 2 distinct dilatations (Fig. 10n).

DISCUSSION The morphological investigation of the median glands of Paederinae shows novel features suitable for elucidation of the phylogeny within this subfamily. The gland of Paederus s.1. has a ground-plan similar to both the sternal gland of Omaliinae and the tergal gland of Aleocharinae (Araujo, 1978). Nevertheless, independent evolution is confirmed by different locations of the glands at sternite 4 (Paederinae), sternite 8 (Omaliinae) or tergite 7 (Aleocharinae) as well as by morphological details. Therefore, the analogous structures may be due to the median position at the anterior margin of the respective sclerite or corporeal constraints concerning their origin. As known from other Coleoptera (Dettner, 1987), the median glands of Paederinae are descended from invaginated intersegmental membranes and associated dermal glands. The sternal tubules of Paederinae are part of class III epidermal glands according to

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Noirot and Quennedey (1974, 1991). In this class, there may be one or 2 gland cells that are associated with a canal secreted by a further cell. The organization of the glandular unit can be deduced from the shape of its tubule (Pluot-Sigwalt, 1986). As reported by Hill et al. (1976), the 2 distinct dilatations of nearly all tubules in Paederinae indicate that 2 secretory cells are involved in each glandular unit. Within Staphylinidae, this type has already been found in Omaliinae and Oxytelinae (Klinger, 1979; Happ and Happ, 1973). The intercalary dilatation of the peculiar type of tubule in Astenus, may lie inside a single cell, with the convolution serving for an enlargement of the secretory surface. This convolution resembles, to a certain extent, the so-called "switchback region" in the intercalary cell of the pygidial glands of Bledius mandibularis (Happ and Happ, 1973). The bulbous terminal ampulla of the peculiar type of tubule in Medonina may act as a reservoir for the secretion of the terminal cell. A similarly extended receiving canal has been found at the tubules of the pygidial glands of Dytiscus marginalis (Kuhn et al., 1972). In Paederus s.l., the dermal tubules terminate in single pores, whereas the pore plates from the outside are visible by medium-sized pores on the inner surface of sternites treated with KOH. Likewise, gland cells with (D3) and without tubules (D4), which are concentrated near the tergal gland, have been identified in Aleocharinae (Pasteels, 1968; Araujo, 1978). In D3, an intercalary cell is lacking, hence a glandular unit consists of one canal cell and only one secretory cell (Pasteels, 1968). D 4 is a single bottle-shaped gland cell (Pasteels and Kistner, 1970) and terminates in a pore plate as found in Paederus s.l. (Steidle and Dettner, 1992). On the contrary, it is a set of tubules, which is associated with one pore plate in Pselaphidae and Scarabaeidae (Hill et al., 1976; Pluot-Sigwalt, 1986). Therefore, these pore plates may be derived from several glandular units which are crowded, while those of Aleocharinae and Paederinae belong to only one cell. Bottle-shaped cells (E3) are also situated near the aperture of the sternal gland of Omaliinae (Klinger and Maschwitz, 1977; Araujo, 1978). As for the function of all these glandular cells (D3, D4, E3), Araujo (1978) suggests the production of certain substances that shut the aperture of the respective gland by adhesion. But this does not explain the observed presence of the gland cells elsewhere on the body, and in Paederus s.l., such a task does not seem necessary, because of the sclerotized clasp. Indeed, pore plates, as found in Paederus s.l., are lacking in Rugilus where there is also no clasp at the base of the reservoir. Pasteels and Kistner (1970) point out that bottle-shaped cells (i.e. D4 and E3) are widespread in Staphylinidae, and mostly occur near articular surfaces. Therefore, they might join flexible structures like intersegmental membranes or hairs. One further possibility is the supply of protective substances as supposed for pore plates on the antenna of Catopidae (Martin, 1977). If toxic secretions are discharged from the sternal gland, such protective gland cells will be especially advantageous in the vicinity of the aperture. The ends of the groove in Paederus s.l., are reminiscent of the gutters that are located at the side of the aperture of the sternal gland in Omaliinae (Klinger and Maschwitz, 1977; Klinger, 1980). But in contrast to Paederus s.l., these have a proximal projection between the gutters where the glandular tubules open within certain areas. One further difference concerns the tubules themselves which are partly bundled together, whereas the reservoir wall, like that of Paederus s.1. and Rugilus, does not hold any gland cells (Araujo, 1978; Klinger, 1980; Araujo and Pasteels, 1987). On the contrary, there is a glandular epithelium on the reservoir wall of the tergal gland of Aleocharinae (Pasteels,

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1968; Araujo, 1978; Araujo and Pasteels, 1985; Steidle and Dettner, 1990). A division of the sac-like reservoir into lateral pouches, as described in certain Aleocharinae (Steidle and Dettner, 1990), cannot be recognized in Paederinae. According to Klinger (1979), this would be advantageous because internal organs, like the intestine, restrict an expansion of the reservoir. As seen in Paederus s.l., this may be achieved also by an unpaired extension, so that the main volume of secretion is shifted to one side of the abdomen. Since no specialized muscles are attached to the reservoir wall, discharge of secretion has to be attributed to an increase of hemolymph pressure, as in Omaliinae (Klinger, 1979). In that subfamily, the reservoir is closed by a fold, whereas in Paederus s.l. the sclerotized clasp is responsible for closure. Whether the gap is widened by a muscle, or emission is carried out via the lumen of the groove, remains uncertain. In consideration of this structural distinctiveness, it is evident that the glands of Paederus s.i. and Rugilus are not homologous to any of the various glands in Staphylinidae. Furthermore, it remains to be resolved whether these 2 glandular systems evolved independently or not. There is a great gap between the shape of the glands in Paederus s.l. and Rugilus, respectively, as long as Ochthephilum fracticorne is ignored, but the presence of a pit in combination with an accumulation of tubules is a kind of link. So the most parsimonious explanation suggests a common origin of these glandular structures at the 4th sternite, although there is no reservoir in O. fracticorne. The reservoirs may be derived independently, but in the same way, by invagination of the intersegmental membrane, as in Aleocharinae and Omaliinae (Pasteels, 1968; Klinger and Maschwitz, 1977). The duplicate at the 5th sternite of Rugilus does not complicate the situation as it might be only a matter of additional gene expression. It is Stilicosoma rufipes, which presents difficulties because it is so closely related to Rugilus that recent authors denied its own generic status. The name Stilicosoma Casey was introduced for S. rufipes, emphasizing its divergence in general habitus and the presence of 4 labral denticles in contrast to 2 in Rugilus (Casey, 1905). After that, Bernhauer and Schubert (1912) lowered the rank of Stilicosoma, including it as a subgenus in Rugilus (there called Stilicus), while Blackwelder (1939) dismissed the name, whereupon it vanished from the literature. Only Frania (1986a) took independence of Stilicosoma into consideration again. Unlike all other Rugilus species studied, sternal glands are missing in S. rufipes, which coincides with the differences just mentioned. Based on this set of disparities, here we re-establish the genus Stilicosoma Casey to include S. rufipes (Germar). If paederine glands are descended from a common origin, as assumed above, the lack in S. rufipes implies reduction of that character. In contrast to this view, treating S. rufipes as plesiomorphic would either disregard the relationship to Rugilus or connote independent evolution of the glands in Paederus s.l. and Rugilus. Investigation of further genera of the subtribe Stilicina could furnish evidence in favor of this alternative. In order to evaluate the evolution of these characters, information on chemical composition and biological significance of paederine secretions is desirable. These are under investigation. Using the morphological glandular features of the available specimens, an evolutionary pathway cannot be recognized, and some outlines may be suggested as follows. The staphylinid subfamilies Xantholininae and Staphylininae are closely related to Paederinae, as is confirmed by 4 apomorphies (Newton and Thayer, 1988), but none of them has glandular structures at the 4th sternite (Araujo, 1978; Fish, 1979; Kanehisa et al., 1984; own observations on Xantholininae: Xantholinus and Gyrohypnus, and

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O

t ? i

t

M

t

F1G. 11. Hypothetical series of transformation of glandular structures at 4th sternites of Paederinae (below: plesiomorphic, above: apomorphic condition). Schematic longitudinal sections of sternites 3 (on the left) and 4 (on the right) show sclerotized cuticle (thick lines), membranes or reservoirs (thin lines) and locations of glandular tubules (short, curved lines) of Medon (M), Lathrobium (L), Paederus (P; broken line: unpaired extension of the reservoir), Ochthephilum (O), and Rugilus (R).

Staphylininae: Gabrius). Therefore, and as many genera within Paederinae lack such glands too, their evolution has certainly taken place within that subfamily. Consequently, the lack of glandular structures at the front margins of the sternites of Medonina is the plesiomorphic condition. Since the accumulations of tubules in Lathrobiina are most distinct at the 4th sternite and arranged like those of the gland tissue in Paederus s.l., they represent a further stage in the development of such a gland. The exact branching pattern among Paederus s.l., Rugilus and Ochthephilum, however, is not revealed by the species studied (Fig. 11). Based on the presence of a gland at the 4th sternite, Herman (1981) separated the subtribes Cryptobiina, Paederina and Dolicaonina from all other Paederinae. But, on the one hand, Ochthephilum fracticorne shows that at least some Cryptobiina do not have a gland with a reservoir, on the other, such a gland is actually present in all species of Rugilus. Accordingly, the set of subtribes separated in terms of the presence or absence of that character, has to be amplified, or if independent evolution should be proved, the character states will be delimited more precisely in order to place Rugilus apart. For that purpose, examination of various genera, now classified within Stilicina, is badly needed, as for example, Eustilicus is related to certain Medonina, while the sister group of Rugilus is unknown but may be searched for in some other Medonina (Frania, 1986a, b). Current knowledge does not permit establishment of a new cladogram, but glandular structures promise further insight into the largely unknown phylogeny of Paederinae. This is also true of dermal tubules as already shown in Scarabaeidae (PluotSigwalt, 1986). Tribal as well as subtribal classification of Paederinae is unfortunately based on characters, which have not been assessed with regard to their phylogenetic polarity (Campbell and Peck, 1989). Therefore, the division and accurate placement of genera is

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controversial, h e n c e f u r t h e r characters are n e e d e d in o r d e r to clarify the situation. T h e g e n u s Lithocharis is often i n c l u d e d in M e d o n i n a (e.g. Coiffait, 1984), a n d B o h ~ (1985b) e m p h a s i z e s a close r e l a t i o n s h i p to M e d o n , b u t the o c c u r r e n c e of 2 tracks of pores in the a n t e s u t u r a l area of the 5th sternite supports the s e p a r a t i o n of Lithocharis from M e d o n i n a in a separate s u b t r i b e called L i t h o c h a r i n a (Casey, 1905; B o r d o n i , 1975). T h e inclusion of P s e u d o m e d o n as a s u b g e n u s of Lithocharis ( B l a c k w e l d e r , 1939) or as a g e n u s within L i t h o c h a r i n a ( F r a n i a , 1986b), h o w e v e r , is clearly c o n t r a d i c t e d n o t only by the a b s e n c e of the 2 tracks of pores, b u t also by the d i s t r i b u t i o n of the p e c u l i a r type of gland t u b u l e occurring only in Medon, P s e u d o m e d o n a n d Sunius. So at least this type of t u b u l e is a v a l u a b l e a p o m o r p h y . A n o t h e r type is c o n f i n e d to A s t e n u s , a n d h e n c e does n o t reveal any r e l a t i o n s h i p to the o t h e r species studied, b u t m a y be useful w h e n s p e c i m e n s are e x a m i n e d out of those g e n e r a that are m o r e closely allied to Astenus. C o n s e q u e n t l y , w i t h o u t restricting to g l a n d u l a r features, these are an a p p r e c i a b l e aid in the r e c o n s t r u c t i o n of p a e d e r i n e phylogeny. Acknowledgements--We thank Dr W. Arens (Tier6kologie I, Bayreuth) for putting additional species at our disposal and Dr G. Scherer (Zoologische Staatssammlung, Miinchen) for access to the Scopaeus under his care.

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FRAN1A, H. E. 1986a. Status of Eustilicus Sharp, Trochoderus Sharp, Deroderus Sharp, and Stilocharis Sharp (Coleoptera : Staphylinidae : Paederinae : Paederini) with implications for classification of the Medonina and Stilicina. Can. J. Zool. 64- 467-80. FRANIA, H. E. 1986b. Larvae of Eustilicus Sharp, Rugilus Leach, Deroderus Sharp, Stilocharis Sharp, and Medon Stephens (Coleoptera : Staphylinidae : Paederinae : Paederini), and their phylogenetic significance. Can. J. Zool. 64: 2543-57. HAPP, G. M. and C. M. HAPP. 1973. Fine structure of the pygidial glands of Bledius mandibularis (Coleoptera : Staphylinidae). Tissue Cell 5: 215-31. HERMAN, L. H. 1981. Revision of the subtribe Dolicaonina of the New World, with discussions of phylogeny and the Old World genera (Staphylinidae, Paederinae). Bull. Amer. Mus. Nat. Hist. 167: 327-520. HILL, W. B., R. D. AKRE and J. D. HUBER. 1976. Structure of some epidermal glands in the myrmecophilous beetle Adranes taylori (Coleoptera : Pselaphidae). J. Kansas Entomol. Soc. 49: 367-84. KANEHISA, K., T. SHIRAGA and K. KAWAZU. 1984. Defensive secretory organs of the rove beetles (Coleoptera : Staphylinidae). Nogaku Kenkyu 60: 111-21. KLINGER, R. 1979. Line Sternaldriise bei Kurzfliigelkilfern. Systematische Verbreitung sowie Bau, Inhaltsstoffe und Funktion bei Eusphalerum minutum (L.) (Coleoptera : Staphylinidae). Inaugural-Dissertation, Johann Wolfgang Goethe Universitht, Frankfurt am Main. KLINGER, R. 1980. The defensive gland of Omaliinae (Coleoptera : Staphylinidae). II. Comparative gross morphology and revision of the classification within the genus Eusphalerum Kraatz, Entomol. Scand. 11: 454-7. KLINGER, R. and U. MASCHWITZ. 1977. The defensive gland of Omaliinae (Coleoptera : Staphylinidae). I. Gross morphology of the gland and identification of the scent of Eusphalerum longipenne Erichson. J. Chem. Ecol. 3: 401-10. KUHN, C., E. SCHNEPFand H. SCHILDKNECHT. 1972. Uber Arthropoden-Abwehrstoffe. LVIII Zur Feinstruk:ur der Pygidialdriisen des Gelbrandk/ifers (Dytiscus marginalis L., Dytiscidae, Coleoptera). Z. Zellforsch. 132: 563-76. LOHSE, G. A. 1964. Staphylinidae I, pp. 1-264. In H. FREUDE, K. W. HARDE and G. A. LOHSE (eds) Die Kiifer Mitteleuropas, Vol. 4. Goecke and Evers, Krefeld. LOHSE, G. A. 1989. Erganzungen und Berichtigungen zu Freude-Harde-Lohse "Die K~ifer Mitteleuropas" Band 4 (1964), pp. 121-84. In G. A. LOHSE and W. H. LUCHT (eds) Die Kiifer Mitteleuropas, 1. Supplementband mit Katalogteil. Goecke and Evers, Krefeld. MARTIN, N. 1977. Glandes dermiques chez deux Col6opt6res cavernicoles (Catopidae). Int. J. Insect Morphol. Embryol. 6: 179-89. NEWTON, A. F., JR. and M. K. THAYER. 1988. A critique on Naomi's phylogeny and higher classification of Staphylinidae and allies (Coleoptera). Entomol. Gen. 14: 63-72. NOIROT, C. and A. OUENNEDEY. 1974. Fine structure of insect epidermal glands. Annu. Rev. Entomol. 19: 61-80. NOIROT, C. and A. QUENNEDEY. 1991. Glands, gland cells, glandular units: Some comments on terminology and classification. Ann. Soc. Entomol. Ft. (N.S.) 27: 123-8. PASTEELS, J. M. 1968. Le syst~me glandulaire t6gumentaire des Aleocharinae (Coleoptera, Staphylinidae) et son 6volution chez les esp/:ces termitophiles du genre Term#ella. Arch. Biol. 79: 381-469. PASTEELS, J. M. and D. H. KISTNER. 1970. A taxonomic revision of the termitophilous subtribe Perinthina (Coleoptera : Staphylinidae). I. The genera Paraperinthus, Perinthodes and Physoperinthus with a discussion of their integumentary glands and their relationships. Ann. Entomol. Soc. Amer. 63: 546-62. PLUOT-SJGWALT, D. 1986. Les glandes t6gumentaires des col6opt6res Scarabaeidae: Structure et diversit6 des canalicules. Ann. Soc. Entornol. Fr. (N.S.) 22: 163-82. STEIDLE, J. L. M. and K. DErrNER. 1990. Die Tergaldriise der Aleocharinae (Staphylinidae, Coleoptera): Chemie, Morphologie und phylogenetische Bedeutung. Mitt. Dtsch. Ges. Allg. Angew. Entomol. 7: 541-5. STEIDLE, J. L. M. and K. DETrNER. 1992. Chemistry and morphology of the tergal gland of freeliving Aleocharinae (Staphylinidae, Coleoptera) and its phylogenetic significance. Syst. Entomol. (in press).