Morphology of the first instar larva of Tenomerga mucida (Chevrolat, 1829) (Coleoptera: Archostemata: Cupedidae)

Abstract

External and internal structures of 1st instar larvae of Tenomerga mucida are described and illustrated in detail. The larvae display previously identified archostematan autapomorphies such as the posterodorsal and posteroventral cephalic emarginations, the median endocarina, a distinctly reduced epicranial suture, the lateral cardinal sclerite, the loss of M. craniocardinalis, a sclerotized prominent ligula, a distinctly reduced tentorium, the transverse muscle of the posterior head capsule, tergal and sternal ampullae, and the strongly muscularized loop of the hind gut. Unusual plesiomorphies are the presence of a well-developed M. frontolabralis (M8) and a tentoriomandibular muscle. The presence of two extrinsic premental muscles and a subdivided postlabium are also plesiomorphic features. A derived character state described for the first time in an archostematan larva is the presence of glands in the anterior and posterior abdomen. The postcephalic musculature is similar to the condition found in Micromalthus and Rhipsideigma. The neck musculature is strongly developed, and also the dorsal and ventral longitudinal muscles, probably in correlation with wood-boring habits. The leg muscles are only moderately sized. Well-developed muscles of the eversibles lobes of segment IX are an apomorphy of Cupedidae. The cephalic central nervous system of the 1st instars is affected by the small size: the brain and suboesophageal ganglion are shifted to the prothorax. The postcephalic part is plesiomorphic, with three and eight distinctly separated ganglia in the thorax and abdomen, respectively. The phylogenetic results tentatively suggest a placement of Archostemata as sistertaxon of the other three beetle suborders. The monophyly of Archostemata is strongly supported but the interrelationships of the families are largely unresolved. Micromalthidae are placed as sistergroup of Crowsoniellidae in analyses of larval and adult morphological characters. Potential larval synapomorphies of Micromalthidae and Cupedidae are the transverse and laterally rounded head, the absence of stemmata, the shortened distal part of the mandibles, the presence of sternal asperities, the eversible lobes of segment IX and the caudal process of tergum IX. Cupedidae excl. Priacma and Paracupes are monophyletic. A cupedid subgroup well supported by larval features comprises the genera Tenomerga and Rhipsideigma, and possibly also Cupes (larval features unknown). The reconstruction of the phylogeny and character evolution of Archostemata is still greatly impeded by the scarcity of data, especially the lacking information on internal features and larval characters.