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General body structure of beetles

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Body normally oval, elongated oval or elongated, relatively convex from above with strongly sclerotized integuments and appendages, including elytra covering membranized tergites of the abdomen. Firm exoskeleton permits analogies with some reptiles (in particular with tortoises) or mammals (e.g. armadillos), but particularly many parallels are observed among different arthropods in particular in the orders of earwigs, cockroachers, bugs, Homoptera, etc. Hard wing syndrome in insects has its basis similar t o that in tortoises and armadillos and is defined by passive defence. In vertebrates the function of protection against predators prevails. In insects, however, this is a form of body protection (particularly of soft tergites of the abdomen) against action of substratum when insects move inside it. Body is divided into 3 pronounced parts: head, thorax and abdomen, appendages of the first two parts usually protruded outwards, whereas appendages of abdomen (pregenital and genital sclerites) are most frequent ly drawn inside to a certain extent.


General view of a beetle (Carabidae: Carabus sp.) from a side (Krivolutskaya, 1989).


Body shape reveals a wide range of variation so that in some cases adult beetles with most reduced elytra are not even recognized as such and are placed in the order of sylopids. Mode of life of beetles influences tendencies that are manifested in the general structure and in the structure of it separate parts (see part «Life forms»). The most frequent modification of the general structure is related to shorteninig of elytra and corresponding transformations in the structure of other organs (most pronounced in the structure of wings and abdomen). This modification is revealed in all recent suborders and attains a particularly high degree in many groups of the family of rove beetles [Staphylinidae] (see also the paper by A. G. Kirejtshuk on parallelisms). Many taxa, particularly in the suborder Polyphaga are characterized by pedomorphic transformations in the structure of separate organs, which is particularly pronounced in groups where females became larviform [Phengodidae, Lampyridae and Pachypodinae (Scarabaeidae) of the infraorder Elateriformia, and also Dermestidae (Thyladrias), Stylopidae and Rhipiphoridae of the infraorder Cucujiformia, in some (Stylopidae) females turned into sack without appendages consisting of narrower sclerotized cepahlothorax and a large membranized abdomen. In a certain sense pedogenesis of larvae of Micromalthus may be regarded as terminative manifestation of that tendency towards pedomorphosis of the definitive developmental stage.


General view of a beetle (Cerambycidae: Prionus sp.) from above and below (Villiers, 1978)


The mode of life affects body shape of large beetles. However beetles of average sizes depend on the mode of life much less and small beetles display no such relationship. Nevertheless inhabitants of aquatic environment, particularly actively swimming forms always acquire streamlined and compact shape (whirligig beetles, predaceous diving beetles and crawling water beetles, etc.). Actively moving over substratum surface predators have a relatively compact body with well-developed and relatively long limbs. Inhabitants of interstitial substrata are frequently elongated and openly living forms frequently become strongly convex. Burrowers and other inhabitants of interstitial substrata acquire a more extended anterior body edge and thicker pronotum, than those in openly living species. For more detail see part «Life forms».


Body integuments and appendages are normally relatively distinctly punctured, in the spaces between dots they frequently bear pronounced microsculpture and are covered with most frequently by relatively dense or rare short pubescence. Sometimes as a result of reduction of sculptural formations sclerites become absolutely smooth. Sizes of dots vary strongly, but they may be homogenous over the entire surface of a separate sclerite or over the entire body, or along with large dots there may be small dots or dots may form rows or accumulations. Normally such formations are associated with the sensory and gladular functions. Sculpture of the surface between dots is seldom smooth entirely and surface usually has relatively regular microsculpture, frequently revealing a relative specificity. Pubescence is strongly variable and sometimes all hairs or a part of them may be transformed into scales or setae, or be arranged in bundles.


On elytra primary venation in not infrequently retained in extinct Paleo- and Mesozoic groups of the suborders Protocoleoptera and Archostemata and also in recent representatives of the families Cupedidae and Ommatidae from the last suborder. In the majority of other beetles venation of elytra replaces cells in such a way that sculpture on the upper surface is similar to sculpture on other sclerites. At the same time longitudinal rows of dots, ridges or keels develop on elytra; on other sclerites such form ations occur much less frequently. Moreover, on elytra punturation may be double or tripple and consist of dots of different sizes diffuses spread or structure in a certain order.


Body colouration is highly variable and diverse. Not infrequently large beetles are brightly coloured and often bear varied and sometimes quite bright pattern on different parts of the body. Particularly brightly coloured are large beetles with diurnal activity and associted with open vegetative substrata, such as flowers or leaves (buprestids or cetonine scarabs). At the same time many beetles moving over soil surface particularly preferring nocturnal activity and also many beetles inhabiting in water are characterized by more or less monotonous and often dark colouration, although not infrequently with pronounced metallic shades (ground beetles and darkling beetles). Small and average size beetles and also beetles with relatively cryptic mode of life are usually unicolourous; cave beetles and other beetles not emerging on soil surface normally pale brown. In some groups bright colouration and loud pattern are warning for predators about their toxicity (lady beetles, blister beetles and so on). Some beetles have luminescent organs (fireflies and click beetles), others may change colouration. For examples, Dynastes hercules may change colouration of elytra from yellowish to black in a few minutes, and tortoise beetle Aspidimorpha tecta changes green colouration of pronotum and elytra for red brassy in a few minutes. And eventually some forest beetles can fluorescelight by separate parts of body. Colouration and pattern are not infrequently characteristic of separate species or groups of different taxa. They provide good characters for diagnosis and also for recognition of beetles of individuals of their own file. Nevertheless young beetles usually have relatively weakly sclerotized covers, paler colouration and less pronounced pattern. With age integuments become stronger, colour becomes more intensive and pattern – more loud.


A.G. Kirejtshuk
October 2002



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