The insect abdomen is more obviously segmental in origin than either the head or the thorax, consisting of a series of similar segments, but with the posterior segments modified for mating and oviposition. In general, the abdominal segments of adult insects are without appendages except for those concerned with reproduction and a pair of terminal, usually sensory, cerci. Pregenital appendages are, however, present in Apterygota and in many larval insects as well as in non-insectan hexapods. Aquatic larvae often have segmental gills, while many holometabolous larvae, especially amongst the Diptera and Lepidoptera, have lobe-like abdominal legs called prolegs.

Reviews: Bitsch, 1979; Matsuda, 1976; Snodgrass, 1935

SEGMENTATION

Number of segments

The basic number of segments in the abdomen is eleven plus the postsegmental telson which bears the anus, although Matsuda (1976) regards the telson as a twelfth segment. Only in adult Protura and the embryos of some hemimetabolous insects is the full complement visible. In all other instances there is some degree of reduction. The telson, if it is present at all, is generally represented only by the circumanal membrane, but in larval Odonata three small sclerites surrounding the anus may represent the telson.

In general, more segments are visible in the more generalized hemimetabolous orders than in the more specialized holometabolous insects. Thus in Acrididae all eleven segments are visible (Fig. 11. 1a) whereas in adult Muscidae only segments 2–5 are visible and segments 6–9 are normally telescoped within the others (Fig. 11.1b).