STRUCTURE OF INSECTS 41 



latter type we must api)ly substances which penetrate the 

 body and close up the breathing jjores, and for this, oily sub- 

 stances, particularly kerosene, tobacco extract, and various 

 other substances, are especially useful. The breathing i)()res, 

 as will be seen later, are minute openings along the sides of 

 the body, these being closed by minute quantities of an 

 oily substance, so it is easy to see how these substances 

 operate to kill the insect. 



The central region of the body, the thorax, consists of three 

 distinct segments called the pro-, meso- and metathorax. 

 The first of these next to the head bears the front pair of 

 legs; the middle segment, or mesothorax, the second i)air 

 of legs and the first pair of wings; and the hinder or meta- 

 thorax, the third pair of legs and the second pair of wings. 

 The legs are jointed appendages adapted for walking, run- 

 ning and jumping, sometimes for clinging, and consist of a 

 basal segment, the coxa; a large, strong segment, the femur; 

 a more slender segment, usually of the same length as the 

 femur, the tibia, and the terminal portion, composed of 

 from one to five small segments, called the tarsus. The 

 last segment of the tarsus usually bears a pair of strong 

 claws, and sometimes between these is located a disk-like 

 pad or brush called the pulvillus. 



The wings, ordinarily four in number, are membranous 

 expansions of the body wall and are supported by stout, 

 thickened, and rod-like portions termed ner\'es or veins, and 

 the arrangement of these throughout the wings is spoken 

 of as neuration or venation. Wings are greatly modified in 

 different groups of insects. In some cases they are thin 

 and transparent in both pairs, as in the dragon fly. The 

 front pair may be thickened or leathery, as in the grass- 

 hoppers, or still more tliickened, forming a hard, horny case 

 (elytra), as in beetles, or ])artially leathery and partly mem- 

 branous, as in Hemiptera. They are broad and covered 

 with minute scales in butterflies and moths (Lepidoptera), 

 and the number is reduced to two in flies and mosquitoes, 

 the hinder pair being aborted or modified into special organs 

 called balancers or halteres. 



