Description of the larva of Rhipsideigma raffrayi ( Coleóptera : Archostemata ) , with phylogenetic and functional implications

Larvae of Rhipsideigma raffrayi are described in detail and those of Distocupes varians are re-examined. Their morpho­ logical structures are evaluated with respect to their functional and phylogenetic significance. Larvae of Rhipsideigma are woodborers with a straight body and a wedge-shaped head capsule. Most of their apomorphic features are correlated with their xylobiontic habits. The strong mandibles, the sclerotized ligula and the wedge-shaped head enable the larvae to penetrate rotting wood. The broadened prothorax, prosternal asperities, tergal ampullae, the short legs, and eversible lobes of segment IX play an important role in locomotion in galleries within rotting wood. Leg muscles are weakly developed, whereas the dorsal, pleural and ventral muscula­ ture is complex. The larval features allow Rhipsideigma to be placed in the clades Archostemata, Cupedidae + Micromalthidae, Cupedidae, Cupedidae excl. Priacma, and Cupedidae excl. Priacma and Distocupes. The monophyly of Cupedidae and Cupedidae, excluding Priacma, so far is only supported by apomorphies of the adults. However, the presence of glabrous patches on the pro­ sternum and of a medially divided field of asperities may be larval apomorphies of the family. A clade, which comprises Rhip­ sideigma, Tenomerga and probably other genera of Cupedidae with hitherto unknown larvae, is well supported by larval apomorphies such as the broadened prothorax, the presence of coxal asperities and the presence of a distinct lateral longitudinal bulge. Increased numbers of antennomeres and labial palpomeres are apomorphies only found in larvae of Distocupes.


INTRODUCTION
The cupedid genus Rhipsideigma was designated by Neboiss (1984).It is represented by one species in East Africa, R. cretaceocincta (Kolbe, 1897) and four species in Madagascar (Neboiss, 1984(Neboiss, , 1989)).The distribution of Rhipsideigma raffrayi (Fairmaire, 1884) (Fig. 1) is Northern Madagascar according to Neboiss (1984).How ever, the hitherto undescribed larvae were collected, together with adults, in the central part of the island (21.-22.11.1996, Madagascar, Manankazo env., Ambohitantely Nat. Res., Petr Svacha lgt.), which considerably extends the range of this species.Little is known about the biology of Rhipsideigma.However, the fact that the larvae of R. raffrayi were collected in dark rotting logs on the ground (Svacha, pers. comm.)strongly suggests that they develop in fungus-infested wood like other species of Cupedidae (Lawrence, 1991).
Little is known about the immature stages of Archoste mata.Only the larvae of Micromalthus debilis LeConte, 1878 are described in detail (Beutel & Hornschemeyer, 2002).Descriptions of larvae of Omma Newman, 1839 (Ommatidae; Lawrence, 1999), Priacma serrata LeConte, 1861 (first instar; Ross & Pothecary, 1970), Tenomerga spp.(Boving & Craighead, 1931;Fukuda, 1938) and Distocupes varians Lea, 1902(Neboiss, 1968) only cover external features and some are very short and not well illustrated.Therefore, the main purpose of this study is to provide comprehensive information on the larval morphology of Cupedidae.The larva of the madegassan species R. raffrayi Neboiss, 1984 is described in detail and the larva of Distocupes varians is re-examined.
The morphological data are interpreted with respect to their function, and the phylogenetic position of Rhip sideigma is discussed.

MATERIAL AND TECHNIQUES
Larvae of R. raffrayi along with adults in the same habitat were collected from rotting logs on the ground.Three specimens, probably one penultimate instar and 2 ultimate instars, were fixed in boiling water and then transferred to etha nol, one slightly damaged larva (probably ultimate instar) was directly transferred to Pampel's fluid.The latter specimen was used for dissection and microtome sections.
For sectioning, selected body parts were embedded in Historesin.Sections were cut at 5 pm with a Microm microtome and stained with methylene blue and acid fuchsin.Drawings were made using an ocular grid or a camera lucida (cross sec tions).
V. Keler's (1963) nomenclature is used for muscles of the head and Larsen's (1966) nomenclature for thoracic muscles.

RESULTS
The description of internal structures is based on a larva slightly damaged in the posterior thoracic region.

General appearance (Fig. 2)
Length of larvae up to 38 mm.Most parts of elongate body unpigmented or of a light brown colour.Thorax short in relation to rest of body.Prothorax broader and longer than meso-and metathorax.Legs very short.Abdomen elongate, slightly widening posteriorly, very slightly flattened, with tergal ampullae and a longitudinal semi-membranous lateral bulge.Segment IX sclerotized and pointed apically.

Head capsule (Figs 3-5)
Maximum width 3.78 mm.Prognathous, posterior part retracted into voluminous semi-membranous collar region of prothorax (Fig. 3).Most parts cream-coloured or testa ceous.Broad anterior margin of frons and lateral area posterior to antennal articulation dark brown.Head almost 2x as broad as long, strongly rounded laterally and

Internal skeletal structures (Figs 6, 7, 9)
Postoccipital ridge very broad laterally.Hypostomal rods internally represented by strong longitudinal sulci.High ridges enclose gula and submentomentum, anteri orly continuous with short and flat posterior tentorial arms.Apical part of posterior arm laterally attached to anatomical mouth.Tentorial bridge, dorsal-and anterior tentorial arms absent.

Antenna (Fig. 4)
4-segmented, short, inserted on antennomere-like, membranous structure on dark brown, strongly sclerotized craniolateral margin of head capsule.Antennomere I about as long as broad, antennomeres II-IV slightly longer.Width of antennomeres decreasing towards apex.Sensorial appendage of antennomere III present but small, inserted on ventral side.Apex of distal antenno mere with minute sensorial spines.

Mandibles (Figs 10, 11)
Very strongly sclerotized and pigmented, almost black.Short and compact, roughly triangular, with three strong, shovel-like apical teeth and a distinctly delimited, quad rangular smooth mola.Retinaculum or moveable appendage absent.One thin seta inserted on distinct dor solateral bulge.

Labium (Figs 5, 9,13)
Submentum and mentum fused, very short, slightly nar rowed between maxillary grooves, posteriorly continuous with gular region.Pair of short setae present on anterior mental region and pair of long setae on posterior sub mental part.Prementum markedly developed, larger than submento-mentum.With one long and one short seta close to the median line and one long seta on large articu latory membrane of palp.Palpomere I about 2 x as long as broad.Palpomere II small, subulate.Ligula large, wedge-shaped, almost black and strongly sclerotized, with transverse subapical edge and slightly emarginate cranial edge.
Transverse epipharyngeal muscles are absent.

Hypopharynx (Figs 6, 9,13)
Hypopharynx fused with anterior part of labium, anteri orly continuous with upper surface of prementum.Short, slightly concave, enclosed by distinct lateral edges.Hairs, bristles, or densely pubescent areas absent.Posterior part laterally not fused with posterior part of epipharynx, pre pharyngeal tube absent.
Musculature (Fig. 9): M. 41 (M.frontohypopharyngalis): unusually large muscle, represented by 2 subcomponents, M. 41a, O: posterior part of frons and median endocarina, M. 41b, O: dorsomesally from post occipital ridge, immediately close to posterior component of M 9; I: both subcomponents attached to posterolateral edge of hypopharynx by means of a tendon.M. 42 (M.tentoriohypopharyngalis): thin bundle, O: gular ridge, I: ventrolaterally at anatomical mouth; it cannot be fully excluded that this is a ventral dilator of the anterior pha rynx.

Cerebrum and suboesophageal ganglion (Figs 9,14)
Small in relation to head size.Located in posterior head region.
Musculature (Figs 9,16,17; cervical muscles see above): With complex system of dorsal, ventral and dorsoven tral muscles (dis, vis, dvis).Flat and broad longitudinal-(dlm) and oblique (otm) muscles arise from intratergal folds and insert on posterior tergal margin.Two slender bundles originate from lateral tergal margin (nst) and insert on precoxal rim.Broad and flat muscle with same insertion originates from the ventral fold separating the anterior and posterior part of the prothorax (vlm).Pleurosternal muscle (plst) originates from posterodorsal pleural area.Two muscles with origin on anterior and dorsal pleura insert laterally on intrapleural fold (plm).Two thin extrinsic leg muscles, probably M. nototrochantinalis (ntr) and M. notocoxalis (ncx), originate from lat eral tergal area.One short muscle, probably M. furcacoxalis, originates from small apodeme attached to precoxal rim.

Mesothorax (Figs 2, 3, 18)
Shorter and narrower than prothorax, but also divided into longer anterior-and shorter, ring-like posterior sec tion.Tergum with distinct shoulder region, but less strongly sclerotized than pronotum and without posterior pseudo-scutellar region.Anterior part of pleura with slit like depression superficially resembling a compressed, longitudinal spiracle.Postpleura distinctly bulging.Sternal field of asperities absent.Legs like those on pro thorax.
Musculature (Fig. 18): longitudinal and oblique dorsal muscles (dlm), dorsolateral and lateral muscles well developed, attached to hind margin of prothorax.Large ventral longitudinal muscles (vlm) and several oblique, ventral muscles (ovm) attached to hind margin of ante rior prosternal section.Leg muscles similar to those of prothorax.

Abdominal segments I-VII (Figs 2, 18)
Distinctly longer than thoracic segments, I slightly shorter than segments II-VI.Terga with indistinct anterior transverse ampulla and distinct posterior ampulla.Pleural membranes form a distinct lateral longitudinal bulge.Longitudinal impressions on the lateral area of the tergites indicate attachment of strong tergopleural muscles (tpm).Sternites with anterior-, intermediate-and posterior transverse line and posterior transverse bulge.Sternal asperities absent.
Musculature: with complex system of dorsal, lateral and ventral muscles (Fig. 18: dlm, llm, vlm).Dorsoventral muscles (dvm) less strongly developed.Compact, regular series of muscles extend between the lateral tergal impressions and the ventral side of the lateral longitudinal bulges (see above; tpm).Oblique, thin muscles, with origin on mesal and lateral parts of sternites, insert imme diately close to the ventral attachment area of the tergopleural muscles.
Musculature: with several dorsoventral muscle bands and very strong dorsal, lateral and ventral muscles.Tergo-pleural muscle series absent.

Abdominal segment IX (Figs 19-21)
Tergite sclerotized, extends to ventral side of segment.With apically truncate, narrow sclerotized apex with sev eral subapical teeth.Dorsal side with minute spines, more distinct pointed tubercles present posterolaterally.Lateral margin with a row of thin hairs.Sternum well developed anteriorly, unsclerotized, indistinctly separated from tergum.Urogomphi absent.Large eversible lobes present posterior to sternum.
Musculature (Figs 19,20): Large dorsoventral muscle originates from anterior tergal margin and inserts on hind margin of sternite VIII (dvml).A broad and a narrow dorsoventral muscle insert immediately lateral to the eversible lobes (dvm2+3) and an additional dorsoventral muscle connects the posterior part of tergite IX with the postero ventral wall of the segment (dvm4).Two dorso lateral muscles insert on the eversible lobe (revll+2) and an additional thin muscle arises between them.Rectum suspended by series of thin dorsal, ventral and lateral dilators (dmr).

Abdominal segment X (Fig. 21)
Not visible externally as a distinct segment or proleg like structure.Possibly represented by eversible lobes on ventral side of segment IX.

Postcephalic glands (Fig. 24)
Glands covered by the strong intrinsic muscle bundles of the lateral longitudinal bulge present in abdominal segments I-VII.

DISCUSSION
Most apomorphic features found in larvae of Rhipsideigma raffrayi are probably associated with its xylobi-ontic habits.The functional interpretations that follow should be treated tentatively as they are not based on experiments or direct observation.The mandibles of larvae of R. raffrayi (Figs 10,11) are stout and tridentate like in other cupedid and micromalthid larvae and some of the larvae of other groups with wood-boring habits, such as Callirhipidae or Cerambycidae (Lawrence, 1991).The shovel-like apices remove material, thus enabling the larvae to bore into rotting wood.The sclerotized, wedge shaped ligula (Figs 5, 9) may have an additional role.Contraction and relaxation of M. frontohypopharyngalis (M.41) results in retraction and protraction of the labiohypopharyngeal complex, and therefore in forward backward movements of the ligula.This may facilitate the penetration of the substrate.The labium as a whole is shortened, the mentum and submentum are fused and retractor muscles are absent (Figs 5,9).Other modifica tions of head structures probably correlated with wood boring are short antennae, the absence of stemmata and the presence of an extensive median endocarina (Figs 3,4).Similar derived features are also characteristic of the xylobiontic larvae of other groups of Coleoptera not closely related to Cupedidae (e.g.Buprestidae, Callirhipidae, Cerambycidae).The endocarina of cupedid larvae strengthens the head capsule and provides an addi tional area of attachment for the unusually large M. fron tohypopharyngalis (Fig. 9).The head capsule of archostematan larvae is wedge-shaped as in larvae of Rhysodidae (Beutel, 1992), Buprestidae (Lawrence, 1991) and Cerambycidae (Lawrence, 1991;Svácha et al., 1997).In contrast, a globular head capsule is found in other larvae associated with wood such as Lymexylidae and Melandryidae (part.)(Wheeler, 1991;pers. obs. Beu tel), and a strongly flattened head in larvae of Cucujidae, Pythidae (part.),Pyrochroidae and Prostomidae (pers.obs.Beutel).Larvae with the latter type of head seem to be restricted to subcortical habitats, whereas a globular head is at least present in some wood-boring forms.
External structures and the musculature of the body segments of larvae of R. raffrayi are also clearly modified compared to those features of larvae of other groups, e.g.Dytiscus marginalis Linnaeus, 1758 (Speyer, 1922) or Tenebrio molitor Linnaeus, 1758(Josting, 1942;Doyen, 1966).Larvae of R. raffrayi clearly belong to the straight wood-boring type as defined by Crowson (1981) and most derived features of their thorax and abdomen are probably adaptations for locomotion in galleries.The leg muscles are only feebly developed (Fig. 16).In contrast, the intersegmental muscles are unusually complex.It is plausible to assume, that they play an important role in locomotion together with tergal and sternal modifications and hemolymph pressure.The prothorax of larvae of R. raffrayi is broadened (Fig. 2) as in some buprestid and cerambycid larvae.Its broad shoulders and ventral asperi ties are used to anchor the anterior part of their body in a gallery.Then, the following segments are drawn forwards by contraction of the intersegmental muscles.It is likely that the tergal ampullae (Figs 2, 3) are used to hold the body segments in a fixed position for short intervals during the forward movement.The ventral eversible lobes, together with asperities of segment IX (Fig. 21) may then be used to fix the abdominal apex.The fol lowing anterior movement of the head and prothorax is achieved by the combined activity of the mandibles, sclerotized ligula, the short legs, and possibly hemolymph pressure.
The diet of larvae of R. raffrayi is unknown, but it is likely they feed on rotting wood infested with fungus and micro-organisms as do other larvae of Cupedidae and larvae of Micromalthidae (Lawrence, 1991).The preoral chamber, pharynx and the pharyngeal musculature is very similar to that found in larvae of Micromalthus.The dense brush of hairs on the lacinia is used to sweep mate rial removed by the mandibular apices into the preoral chamber.The smooth mandibular mola (Figs 10,11) is not suited for grinding, but rather for compacting material.No openings of glands in the anteroventral head region (Fig. 9) were found, but it is likely that the food is mixed with secretions in the preoral chamber.Transport to the posterior region of the digestive tract is achieved by alternating contractions of the muscles of the wide and straight pharynx (Fig. 9).A pumping apparatus is formed by the pharyngeal ring musculature and 2 corresponding sets of dorsal and ventral dilators, M. tentoriohypopharyngalis (M 42) and M. frontobuccalis anterior (M 45), and M. frontobuccalis posterior (M 46) and M. tentoriopharyngalis (M 52), respectively.The latter 2 muscles are composed of corresponding series ofbundles (Fig. 9).
The postcephalic digestive tract is also similar to that of Micromalthus debilis.A proventriculus with cuticular teeth and well developed ring muscles is present (Fig. 25).It is likely that a compact cylindrical food mass is formed here.The mid gut (Figs 22,26) is devoid of caeca and regenerative crypts and unusually long.Another fea ture shared by larvae of both taxa is the presence of a loop in the hind gut (Fig. 26), with well developed ring muscles.A similar condition is described for some wood boring larvae of Cerambycidae (Svacha et al., 1997: fig. 53).The posterior hind gut oflarvae of R. raffrayi is char acterised by a regular and unusually strong longitudinal musculature (Fig. 26).Another characteristic feature is the presence of ventral, lateral and dorsal dilators of the rectum (Figs 19,20).
Several internal features of the head are either autapomorphies of Archostemata or synapomorphies of Micromalthidae and Cupedidae.The internal structures of larvae of Ommatidae are unknown.Elongate gular ridges and a strongly reduced tentorium, the posterior arms of which are attached to the anatomical mouth (Figs 7, 9) have not been described for larvae of other groups of Coleoptera.The gular ridges are the attachment area of a series of ventral pharyngeal dilators (Fig. 9).The unusual connection between the tentorium and the anterior pharynx indicates a double function for the M. tentoriocardinalis (M 17).It functions as a levator of the maxilla and a dilator of the anatomical mouth, together with M. frontopharyngalis anterior (M 45).Another unusual feature is the strongly enlarged M. frontohypopharyngalis (M 41), which has an extensive area of attachment on the median endocarina.Complete absence of labial muscles is another feature that has not been described for other beetle larvae.
Archostemata is the earliest group of Coleoptera repre sented in the fossil record and the sister group of the other 3 suborders (Beutel, 1997;Lawrence, 1999;Beutel & Haas, 2000).Nevertheless, comparatively few groundplan features are preserved in the larvae.The large number of derived features found in larvae of Cupedidae and other archostematan families is apparently due to their highly specialised wood-boring habits, which may have been acquired in the early Mesozoic.The earliest traces of insect-created galleries in wood date back to the Triassic (Crowson, 1975) and it is plausible to assume that they were made by cupedid larvae.
widened posteriorly.Greatest width posterior to mid length, distinctly narrowed in occipital region.Well defined neck region absent.Dorsal side longer than ven tral side.Hind margin with deep dorsal and ventral trian gular emargination.Setae thin, distribution as shown in Figs 3-5.Stemmata absent.Labrum separated from clypeus by a distinct sclerotized fold (Fig. 9).Clypeus large and trapezoid, unpigmented and transparent, sepa rated from dark and strongly sclerotized anterior margin of frons by internal transverse sulcus.Frons completely fused with adjacent parts of head capsule.Frontal suture absent or vestigial, posterior part possibly represented by very faint diverging lines.Coronal suture absent.Median endocarina present, unforked, almost reaching anterior frontal margin.Internally represented by very extensive median apodeme.Maxillary grooves deep, separated from each other by slightly narrowed posterior part of labium.Hypostomal rods distinct, diverging posteriorly.Gula rep resented by semi-circular, semi-membranous area, poste riorly fused to submento-mentum, covered by unsclerotized, semi-circular fold of anterior prosternal margin.