Beetles, with about 170 families and 350 000 named species, is the largest order of insects. Larvae are much more poorly known than adults with descriptions amounting only about 3% of the total number of named species. Phylogenetic hypotheses are predominantly based on adult morphological characters. One of the reasons for this is an insufficient number of reliable morphological characters normally obtained from poorly sclerotized beetle larvae. However, chaetotaxy (sensilla pattern) provides numerous reliable characters for morphological study and comparison of beetle larvae. It is believed that all present-day diversity of setal patterns in beetle larvae is derived from one ancestral chaetotaxy with the evolutionary process resulting mainly in reduction of primary sensillae. The chaetotaxy of predatory and saprophagous larvae of Adephaga and some Polyphaga (Staphylinoidea) are considered the closest present day example to the ancestral pattern. The number of elements of chaetotaxy for these groups (different sensillae, mainly setae=trichoid sensillae) totals about 200. Minute Ptinella larvae (Polyphaga: Ptiliidae) with body length less than 1 mm possess about 160 elements of chaetotaxy. Larvae of beetles that inhabit the substratum upon which they feed, have more modified and normally reduced chaetotaxy (Buprestidae, Bruchidae, Cerambycidae, Curculionidae), often accompanied by a reduction of legs and stemmata. Extreme cases of modification of primary chaetotaxy pattern is found in larvae of stag beetles (Lucanidae) and incapsulated larvae of dung beetles (Scarabaeidae: Scarabaeinae). It is hoped that it will be possible to produce a chaetotaxy system applicable for all beetle larvae and to homologize sensory cuticular structures throughout the order.
Keywords: morphology, phylogeny
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