152 CIVIC BIOLOGY 



poisons upon their food plants or by mixing poisons with foods 

 which attract them. Those whose mouth parts have been 

 modified into an apparatus for piercing and sucking can be 

 reached only by insecticides which kill by contact. 



The thorax is divided by rather conspicuous sutures into 

 three parts named prothorax, mesothorax, and metathorax. 

 Each carries a pair of legs ; that is, all insects have three pairs 

 of .legs. Wmgs may be present or absent. If two pairs, they 

 are attached to the meso- and meta-thorax, and a single pair 

 is usually attached to the meso-thorax. The insect wing is the 

 most perfect flying mechanism in existence, and until man can 

 match its structure for lightness and strength, he can hardly 

 hope to solve completely the problem of flight. 



Note that insect wings vary in texture from the hard, shell- 

 like structures, as in the fore wings of beetles and the leathery 

 or parchment-like wings of grasshoppers and many bugs, to 

 the transparent membranous wings of bees and flies. 



The abdomen is made up of a series of similar rings termi- 

 nated by various organs concerned with reproduction, ovipos- 

 itors, etc., sometimes modified into sharp stings. 



The life history of an insect also gives characters for classi- 

 fication. With many insects the egg hatches into a worm-like 

 maggot, grub, or caterpillar wholly unlike the parent, and 

 later passes through a quiet stage (pupa or chrysalis) before 

 becoming like the parent. In these cases the insect is said 

 to show a complete metamorphosis Qnefa, " over " ; morplie^ 

 '' form" — '' change of form*'^. Name insects that you know% 

 of which this is true. 



In other insects the egg hatches into something like the 

 parent. To study this point, watch a nest of grasshopper or 

 squash-bug eggs hatch. These insects are said to have an 

 incomplete metamorphosis. In the case of a few insects — 

 the San Jose scale, aphides, and some of the flies — the eggs 

 hatch within the body and the young are born alive. 



