The Structure and Special Physiology of Insects 7 



both pairs may be wanting) which are expansions of the dorso-lateral 

 skin or body-wall, and are not homologous with the jointed ventral 

 appendages. The thorax usually has its first or most anterior segment, 

 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 



