The Structure and Special Physiology of Insects 7 
both pairs may be wanting) which are expansions of the dorso-laterai 
skin or body-wall, and are not homologous with the jointed' ventra 
appendages. The thorax usually has its first or most anterior segmentl 
the prothorax, distinct from the other two and freely movable, while 
the hinder two,» called meso- and meta-thoracic segments, are usually ? 
enlarged and firmly fused to form a box for holding and giving attachment 
to the numerous strong muscles which move the wings and legs. The 
abdomen usually includes ten or eleven segments without appendages or 
projecting processes except in the case of the last two or three, which bear 
in the female the parts composing the egg-laying organ or ovipositor, or 
Fig. 9. Fig. 10. 
Fig. 9. —Head, much enlarged, of mosquito, Culex sp., showing piercing and sucking 
mouth-parts. (After Jordan and Kellogg.) 
Fig. 10.—Head and mouth-parts of honey-bee, much enlarged. Note the short, trowel¬ 
like mandibles for moulding wax when building comb, and the extended proboscis 
for sucking flower-nectar. (Much enlarged.) 
in certain insects the sting, and in the male the parts called claspers, cerci, 
etc., which are used in mating. On the abdomen are usually specially notice¬ 
able, as minute paired openings on the lateral aspects of the segments, the 
breathing-pores or spiracles, which admit air into the elaborate system of 
tracheae or air-tubes, which ramify the whole internal body (see p. 19). 
Of all these external parts two groups are particularly used in schemes 
of classification because of their structural and physiological importance 
in connection with the special habits and functions of insect life, and because 
