10 



GUIDE TO INSECTS. 



The thorax is composed of three segments called respectively pro- 

 thorax, inesothorax and metathorax. The relative sizes of these three 

 parts vary greatly, and furnish important characters for purposes 

 of classification. In some it is the prothorax that is greatly 

 developed (as in beetles), but in others it is the mesothorax 

 that is the largest (as in flies, bees, etc.). The prothorax bears 

 the front pair of legs. The mesothorax bears the front pair 

 of wings and the second pair of legs, and the metathorax bears 

 the second pair of wings and the hind pair of legs. Speci- 



11 



9 



Theoretic diagram of wing. 



mens dissected (1015, 1016) to show this are exhibited in Table- 

 case No. 29. 



The abdomen generally consists of nine visible rings or segments, 

 but occasionally there are ten, and the number is often less ; the 

 reduction in the number is due, either to the basal segments becoming 

 membranous and so disappearing, or to the apical segments having 

 been modified and withdrawn into the body. The last segment often 

 bears a pair of jointed organs called cerci. 



In the wings are seen a number of horny rib-like lines, usually 

 called veins, in which the tracheal tubes run. There are nine 

 principal veins, but these often send off branches, especially towards 



