Locomotion in the Coleoptera Adephaga, especially Carabidae

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Abstract

This study considers leg structure and function in the Adephaga (Caraboidea). Many ground beetles are known to be rapid runners but does this habit account for all the characteristic features of their leg structure? To answer this question, the gaits of several terrestrial Adephagan and Polyphagan beetles have been described briefly; it was concluded that they are fundamentally similar. Thus the peculiar hind legs of Adephaga (with their greatly restricted coxal angle of swing) are not specifically suited to a running habit, but are adapted for pushing. Four basic modifications for pushing have been described in the foreleg of Carabus problematicus. The particular type of pushing was apparent when the functions of its hind leg were considered. The enlarged metatrochanter contains a strong femoral rotator muscle which forces the hind tarsus vertically downwards (and the hindbody upwards). This movement is a necessary part of wedge-pushing, where the wedge-shaped head and prothorax are pushed forwards and the hindbody–the back of the wedge–is oscillated vertically to enlarge the horizontal crevice. The slightly movable metacoxa is part of the antagonistic mechanism of femoral counter-rotation, in which an ingenious lever action swings up the hind legs (and so depresses the hindbody).

The most profound locomotory changes in the Adephaga reflect swimming adaptations. These have involved changes in the pro- and mesocoxal articulations, and the immobilization of the metacoxae. Trachypachus is particularly interesting, as it is a terrestrial Caraboid with immobilized metacoxae. The terrestrial Adephaga (mainly Carabidae) can be divided into two basic groups with divergent habits (if specialist burrowers, etc. are excluded). These groups (which merge) are the strong wedge-pushers/poorer runners with relatively large metatrochanters, and the fast runners/poorer wedge-pushers with smaller trochanters. Experimental evidence for this separation includes estimates of running speeds and the vertical forces exerted by the hind legs of several species during wedge-pushing.

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