On the head morphology of Lepiceridae ( Coleoptera : Myxophaga ) and the systematic position of the family and suborder

Adult head structures of Lepicerus inaequalis were examined in detail and interpreted functionally and phylogenetically. The monogeneric family clearly belongs to Myxophaga. A moveable process on the left mandible is an autapomorphy of the suborder. Even though Lepiceridae is the “basal” sistergroup of the remaining three myxophagan families, it is likely the group which has accumulated most autapomorphic features, e.g. tuberculate surface structure, internalised antennal insertion, and a specific entognathous condition. Adults of Lepiceridae and other myxophagan groups possess several features which are also present in larvae (e.g., premental papillae, semimembranous mandibular lobe). This is probably related to a very similar life style and has nothing to do with “desembryonisation”. Lepiceridae and other myxophagans share a complex and, likely, derived character of the feeding apparatus with many polyphagan groups (e.g., Staphyliniformia). The mandibles are equipped with large molae and setal brushes. The latter interact with hairy processes or lobes of the epiand hypopharynx. This supports a sistergroup relationship between both sub-


INTRODUCTION
Lepicerus is a rather enigmatic and highly unusual genus of Coleoptera.It was pointed out by Sharp (1882) that the beetle "...shows no relationship of an evident character to any hitherto described Coleopterous families...", and affinities with Georissidae (Sharp, 1882), Dryopoidea (Blackwelder, 1944;Costa Lima, 1953), Cucujoidea (especially Trogossitidae; Sharp & Muir, 1912), and Hydradephaga (Forbes, 1926) were discussed.Whereas the systematic position appears to be largely clarified today (e.g., Crowson, 1955;Reichardt, 1973;Beutel, 1999; see below), up to now very little is known about the biology and morphology (especially internal features), and the larvae are still unknown.
Lepicerus is the only genus of Lepiceridae (Reichardt, 1973(Reichardt, , 1976) ) and is presently represented by two species.Together with Sphaeriusidae (= Microsporidae), Torridincolidae and Hydroscaphidae, Lepiceridae form the small coleopteran suborder Myxophaga, which is the sistergroup of the extremely species-rich Polyphaga according to Beutel & Haas (2000), but the sistergroup of Adephaga according to Kukalová-Peck & Lawrence (1993, 2004).Members of Lepiceridae can be very easily distinguished from all other known beetles by the fused tarsomeres with a single claw and the four-segmented, highly unusual antennae.Like all myxophagan beetles, Lepicerus spp.are small in size (< 2 mm) and live in sandy substrates close to river edges (Navarrete-Heredia et al., 2005).
Even though the proposed phylogenetic position of Lepiceridae as sistergroup to the remaining three myxophagan families (Beutel, 1999) appears plausible, important questions remained open (e.g., proximal segments of antennae, mandibular tooth, endoskeleton), and the entire morphology of these exceptional and enigmatic beetles is still very insufficiently known.A detailed description of the head appeared appropriate as several important functions are combined in this part of the body, and the phylogenetic value of characters of different parts of the head (e.g., antennae, mouthparts, sense organs, endoskeleton) is undisputed (see e.g., Anton & Beutel, 2004).
The main purpose of this study is to provide detailed morphological information on external and internal head structures.A functional interpretation of some structural features is attempted based on the morphological data, and the characters are evaluated phylogenetically, in order to test the earlier proposed placement of the family.
Specimens of Lepicerus inaequalis were stored in ethanol (70%).Euparal was used as embedding medium for slide preparations of mouthparts and antennae.Parts of head capsules were macerated in KOH for skeletal preparations.Entire heads were embedded in Historesin and cut at 3 µm with a Microm Microtome (HM 360) (cross section and longitudinal section).The sections were stained with methylene blue and acid fuchsine.SEM micrographs were made with an FEI (Philips) XL 30 ESEM TMP.Drawings were made with a camera lucida.The muscle terminology is based on v. Kéler (1963).

Head capsule
External structures (Figs 1-3).Head short and relatively broad (length ~400 µm, width ~500 µm), anteriorly bluntly rounded, due to protracted position of mentum (see below).Surface strongly uneven, rugulose.Mouthparts largely concealed.Labrum and mentum exposed, together forming a lid-mechanism enclosing the mandibles, maxillae and prementum.Protracted mentum kept in a nearly perpendicular position to clypeus, so anterior head appears snout-like in lateral view (Fig. 2).Short neck region posterior to compound eyes dorsally delimited by a transverse edge.Compound eyes strongly protruding, of hemisphaerical shape, placed on short, tubelike posterolaterally directed projections of the head capsule.Clypeus enlarged, dorsally bulging, with straight and anteriorly converging lateral margins.Frons raised above level of clypeus, with some small dorsal bulges, strongly uneven.Distinct frontoclypeal suture not recognisable due to deep transverse furrows between frons and clypeus, which enclose dorsal tubercles and reach lateral part of head capsule.Lateral part of frons forming duplicature around antennal insertion, covering scapus and pedicellus (proximal antennomeres concealed), with a narrow opening antero-mesad of compound eyes.Sides of head capsule with numerous irregular furrows, ridges and large tubercles, forming a relief for reception of antennae, anterior margin of prothorax and fore-legs.Occipital region around wide and roughly oval foramen occipitale very large, circular, distinctly separated from the rest of the head capsule by a sharp, nearly rectangular edge (Figs 2,3).Gula very short, transverse, strip-like, externally not distinctly separated from submentum.Laterally delimited by short, anteriorly converging gular carinae.

Appendages
Labrum (Figs 5,6).Nearly semicircular, c. 2× broader than long.Dorsal surface strongly uneven, irregularly and coarsely punctured.Anterior margin smooth, without bristles, spines or sensilla.Lateral margin with rather indistinct sinuation with some frontomesally directed bristles.Labrum moveably attached to anterior clypeal margin by a hinge (Figs 16,26), its outline fitting in a concavity of the anterior mentum, thus forming a lidmechanism.Ventral side of labrum largely smooth, but median line with anterior and posterior fields of microtrichia, followed posteriorly by densely setose longitudinal epipharyngeal process (LEP).Hind margin of labrum slightly concave, connected with a strongly developed, anteriorly forked suspensorium.
Two additional muscles with unknown homology are associated with the labio-hypopharyngeal complex.The first muscle (Mx,Figs 19,20,26) originates on the posterior end of the strut-like premental sclerite, anterior to the insertion of M30.The precise point of insertion could not be clarified.It is possibly homologous with M33 (M.praementopalpalis internus).The second muscle (Mxx, Fig. 26) originates ventrally on the plate-like sclerite connecting the median oral glands (see below).The muscle is short and splits into two subunits, which insert dorsally on the thickened posterior end of the strut-like premental sclerite, laterad of the insertion of M30 and the origin of Mx (Fig. 26).
Glands 26).Head capsule with two pairs of large tubular glands.Glands circular in cross-section; cell layer simple, surrounding sclerotised glandular duct.First pair of glands (lateral oral glands) positioned between M11 and M12; glandular duct strongly widening anteriorly, glands thus appearing cone-shaped; open into preoral cavity dorsad of maxillae at the level of the stipes.Second pair of glands (median oral glands) constant in diameter, positioned close to each other and above labial musculature (M28-M30); posterior margin in front of suboesophageal ganglion; open into preoral cavity in a fold of the posteriormost dorsal hypopharynx, ventrad of the posterior molae.glandular ducts connected with each other by sclerotised plate (Fig. 26) posterad of gland opening; plate anteriorly connected with hypopharyngeal sclerite; musculature attached to plate (Mxx, see above) possibly responsible for release of secretions.

Functional interpretations
Among other autapomorphies (see below), Lepicerus is characterised by two unusual and complex characters, a specific entognathous condition and a strongly modified antenna and antennal insertion.The mandibles, maxillae and the prementum are completely or almost completely enclosed in a preoral chamber formed by the mentum, labrum, clypeus and the genae.The labrum and mentum are moveably connected with the clypeus and submentum, respectively, and form a lid-like closing mechanism.No muscles are directly attached to the mentum.Nevertheless, it can be lifted by contraction of the premental muscles (M.submentopraementalis and Mm.tentoriopraementales, M28-30) and, indirectly, by con-traction of M. frontohypopharyngalis (see below).Closing the opening of the functional mouth is apparently mainly achieved by the labrum.Its posterior edges are connected with the head capsule by a hinge (Figs 16, 26; "cndcl").The labral retractor M. frontoepipharyngalis (M9) is absent.The labrum is moved by contraction of the strongly developed M. frontohypopharyngalis (M41).The apices of the long and thin processes of the labral suspensorium serve as attachment area for this muscle (= "M.retractor angulum oris"; Snodgrass, 1935).Posterior extensions of the hypopharyngeal sclerite, which is firmly joined with the mentum, are directly connected with the posterior labral suspensorium (Figs 20,21;lsps,hpscl) and also serve as attachment area of M41.Therefore, in addition to its usual function, i.e. the elevation of the hypopharynx and opening of the anatomical mouth ("mouth angles"), M41 serves as flexor of the labrum (and, indirectly, of the mentum to a minor degree), together with the hinge at the labral base.Closing the functional mouth is mainly caused by muscle contraction, whereas the opening is mainly achieved by the elasticity of the associated sclerotised and semimembranous structures.
The enclosed mouthparts are relatively small, but otherwise fully developed except for the loss of the galea.The preferred food and feeding behaviour of Lepicerus is unknown, but is is likely that they largely rely on algae as do other myxophagans and Hydraenidae, which live in similar habitats.As in other groups with weakly developed mandibular apices (Hydraenidae, Ptiliidae; Betz et al., 2003), the strongly sclerotised and toothed lacinia is used to collect food, whereas the mechanical processing takes place between the mandibular molae (see also Beutel et al., 2001).
A condition not described for any other group of beetles is the antennal articulation, which is deeply sunk into a socket formed by the frontal parts of the head capsule.In addition to the unusually large cavity enclosing the large and unsclerotised base of the scapus ("periscapal 92 Fig. 27.Two hypotheses about the relationships of the four coleopteran suborders.a -[after Kukalová-Peck & Lawrence (1993, 2004)] this branching pattern suggests that the complex oral food processing and cleaning devices (mandibular molae, mesal mandibular brushes, longitudinal epipharyngeal process, longitudinal hypopharyngeal process) belong to the groundplan of Coleoptera, and were secondarily lost twice in Adephaga and Archostemata (further losses within Polyphaga); b -[after Beutel & Haas (2000); Crowson (1955)] the complex feeding apparatus originated once in the stem-lineage of a Myxophaga + Polyphaga clade (with secondary modifications in some polyphagan groups [e.g.Staphylininae, Curculionoidea]).
bulb"), the duplicature of the frons encloses also the first three antennomeres.It is somewhat surprising that the extrinsic antennal musculature (M1/2/4) is very strongly developed despite of the limited movability and the shortness of the antenna as a whole.The reduced number of antennomeres and the shape of these segments strongly suggest that the intraantennal movability is also restricted.Nevertheless, Mm. scapopedicellares anterior and posterior (M5/6) are very large.The functional background of these transformations is rather unclear and interpretations remain speculative as long as direct observations are not available.What can be said with reasonable certainty, however, is that the main movement carried out by the antenna is retraction into the socket from a position with the apical part resting in the prothoracic antennal groove.Shortened antennae with prothoracic antennal grooves are also present in Hydraenidae and Hydrophiloidea.However, the antennae of most adults of these taxa have a specialised function as accessory breathing organs, which can be ruled out for Lepiceridae.A dense field of hairs along the distal edge of the large apical antennomere has probably a sensorial function, but is certainly unsuitable for holding an air film, as it is the case in most hydraenids and hydrophiloids.It cannot be excluded that the antennal modifications are related to burrowing in substrate, but a similar condition is not found in Sphaeriusidae, which live in similar conditions, and the antennae are largely unmodified in other adult beetles with burrowing habits such as Omophron or Clivinia (Carabidae).

DISCUSSION
Adult myxophagan beetles share two presumptive apomorphies of the head.A moveable "prostheca" is present on the left mandible.This character was verified for Lepiceridae for the first time in the present study.The galea is completely reduced or fused with the lacinia.The former interpretation appears more likely, as the "mala" of Lepiceridae does not show any structural differences to the lacinia of other beetles.Another common feature of myxophagan adults (and larvae) is a tendency towards entognathism (see Beutel & Haas, 1998: larvae).The mandibles and the distal parts of the maxillae are enclosed to a varying degree in a working chamber between the mentum and labrum.The highest degree of closure of the preoral cavity is reached in adults of Lepiceridae.This tendency is likely related to small size and microphagous habits.Similar conditions occur in small sized polyphagans, e.g., in Ptiliidae.Another trend in Myxophaga goes towards the reduction of the number of antennomeres.However, this has taken place several times independently, and the basal coleopteran condition of 11 antennomeres (Coleoptera s. str.; see Beutel, 1997) belongs to the groundplan of Myxophaga (Sphaeriusidae).Nine antennomeres are present in Torridincolidae, eight or five in Hydroscaphidae, and only four in Lepiceridae (Reichardt, 1973).
Lepiceridae were placed as sister-group of the remaining myxophagan families in Beutel (1999).As the larvae of Lepicerus are unknown, this hypothesis is based only on adult synapomorpies of Torridincolidae, Hydroscaphidae and Sphaeriusidae, i.e. the small, peglike apical maxillary palpomere, the complete fringe of hairs at the hind wing margin, a distinctly curved subcosta which is connected with the radius anterior after a short distance, and the absence of the transverse ridge of the metaventrite (Beutel, 1999).New synapomorphic characters of Myxophaga excluding Lepiceridae were not discovered during this study, but neither characters which would suggest an alternative placement of the group.
Despite the position of Lepiceridae as "basal" sistergroup of the remaining Myxophaga, they are probably the "most derived family", i.e. the group which has accumulated the highest number of autapomorphies.Lepiceridae are characterised by several unusual characters of the head such as the ventral orientation of the mouthparts, the four-segmented, clubbed antennae with a unique type of insertion, the specific entognathous condition (see above) and the strongly sculptured surface of the dorsal side.However, there are also unusual autapomorphic features of other body parts such as the pronotal antennal groove, the fused tarsomeres, the presence of elytral grooves for reception of the middle and hind legs, and the strongly sculptured surface of the pronotum and elytra.Besides the specific habitat and feeding-habits it appears plausible that at least some of these apomorphies are related to specific defense mechanisms.The body of Lepiceridae adults is not only camouflaged with substrate particles, but the strongly sclerotised beetles are obvioulsly able to assume a compact defensive position with retracted antennae, mouthparts and legs.This makes them very inconspicuous and unattractive for potential predators.Some features resemble morphological specialisations of paussine beetles (e.g., enlarged labium, large and compact antennal club formed by fused flagellomeres; see Darlington, 1950: p. 64;Nagel, 1979).However, to assume that Lepiceridae are myrmecophilous at least in one stage of their life would be speculative at the present stage of knowledge, and this is apparently not the case in adults (Navarrete-Heredia et al., 2005).
It is interesting that, in contrast to other groups of beetles (or Endopterygota), adults of myxophagan groups show some characteristic features which do also occur in larvae: semientognathous mouthparts, the presence of a semimembranous mandibular lobe, the absence of the galea, and the presence of ligular papillae.It is plausible to assume that this is due to very similar microhabitats and feeding habits in the larval and adult stages.To consider myxophagan larvae as "desembryonized" (like polyphagan larvae) as suggested by Kirejtshuk (1991) would be misleading.Larvae of Myxophaga show the typical features of immature holometabolous insects such as e.g., the absence of a sclerotised gula, an antenna with few segments, short legs without subdivided tarsi, and a structurally simple condition of the thorax and abdomen.
A sistergroup relationship between Myxophaga and Polyphaga was postulated by Beutel & Haas (2000) based on a cladistic analysis of a broad set of morphological caracters of larvae and adults, in contrast to Kukalová-Peck & Lawrence (1993, 2004), who proposed a sistergroup relationship between Myxophaga and Adephaga based on characters of the hind wing.The former hypothesis, especially the sistergroup relationship between Myxophaga and Polyphaga, seems to be supported by features of the head of adults (Fig. 31).The feeding apparatus of adult myxophagans and many polyphagan groups (e.g., Hydrophiloidea, Hydraenidae, Leiodidae; Anton & Beutel, 2004;Lathridiidae, Phalacridae, etc., Anton, pers. obs.) are complex and structurally very similar.The mandibles are equipped with large, asymmetric molae and brush-like structures on the inner surface (Anton & Beutel, 2004).These brushes interact with a hairy median lobe or process of the epipharynx, and a similar lobe of the hypopharynx.This condition is probably correlated with microphagous feeding habits (algae, hyphae or spores of fungi; e.g., Leschen, 1993;Hansen, 1997).A mola and the specific brushes are absent in Archostemata (Hörnschemeyer et al., 2002) and also in the groundplan of Adephaga (Acorn & Ball, 1991), even though different brush-like structures have evolved within Carabidae and other adephagan groups (e.g., Noteridae).That this is a plesiomorphic condition within Coleoptera is suggested by the fact that molae and brushes are also missing in adults of potentially related groups such as Neuropterida (e.g., Röber, 1942) or Strepsiptera (e.g., Protoxenos;Pohl et al., 2005;Beutel & Pohl, 2004).A sistergroup relationship between Polyphaga and the remaining Coleoptera as suggested by Kukalová-Peck & Lawrence (1993, 2004) would imply that this type of feeding apparatus has evolved once (groundplan of Coleoptera) and was lost twice (Archostemata, Adephaga) (Fig. 31), or has evolved two times independently, which appears rather unlikely considering the complexity of the involved structures.The alternative hypothesis proposed by Crowson (1955) suggests that this condition has evolved once in the myxophaganpolyphagan lineage.Both hypotheses require the additional assumption that other types of feeding apparatuses have evolved several times within Polyphaga in correlation with carnivorous or specialised phytophagous feeding habits.
pars genalis); both parts forming an almost right angle in cross-section, enclosing the lateral parts of the extrinsic maxillary musculature (see below), I: apodeme attached with short tendon on posteromedian edge of molar part of mandible; M12 -M.craniomandibularis externus (Figs 19-23), O: ventrolateral part of head capsule; very small and flat, located distinctly below M11; I: on ventral side of a plate-like apodeme; long, thin tendon connects apodeme with external edge of mandibular base close to the ventrolateral wall of the head capsule (Fig. 19, maba).
Palp 2-segmented, sclerotised; palpomere I stout, longer than wide, with slightly convex outer margin; palpomere II nearly quadrangular in lateral view, with blunt and semimembanous apex.Several long setae insert laterally on the ligula and palpomeres.Musculature: M28 -M.submentopraementalis (Figs 21, 26): unpaired, short, very close to dorsal surface of mentum, O: together with M29 and M30 on frontal side of a transverse submental carina, I: with long thin tendon