2 2 ECONOMIC ENTOMOLOGY 



(b) Respiratory System. — The exchange of gases in respiration 

 is effected by means of a system of small tubes called trachea which 

 extend inward from the surface and branch to all parts of the body. 

 The external openings, called spiracles, are situated on the sides of the 

 thoracic and abdominal segments (Fig. 28). From each spiracle 

 a short tube runs inward and connects with the trunk tube running 

 along the side of the body. There are thus two main or trunk tracheal 

 tubes, one on each side of the body. Each gives off three large branches 

 to each segment, the upper, the middle, and the lower. Each of these 



vf/.f. 



Ext Tib, 



Fl.T/b. TlTjUFiTars 

 Ext.Tarij 



Fig. 27. — Muscles of hind leg of Gryllus pennsylvanicus. Ext.F., Extensor of 

 femur; Ext.Tib., extensor of tibia; Exl.Tar.i, extensor of first tarsal joint; Ext.Tar.2, 

 extensor of second tarsal joint; Exl.Tar.3, extensor of third tarsal joint; Fl.Tat.i, 

 flexor of first tarsal joint; Fl.Tar.2, flexor of third tarsal joint; PLC, flexor of claws; 

 Ext.C, extensor of claws; Fl.F., flexor of femur; Fl.Tib., flexor of tibia. 



branches subdivides frequently so that every portion of the segment 

 is entered. Moreover, these fine tubes anastomose to form a deli- 

 cate network, and allow a continuous circulation of air to take place. 



In many insects, in addition, are large sac-like dilatations of the 

 tracheae which serve as air reservoirs. 



Tracheae have a striated appearance due to the thickening of the 

 chitinous wall into a compact elastic spiral, thus preventing the collapse 

 of the tubes. Special respiratory devices are seen in aquatic insects. 

 Nymphs of May-flies and Damsel-flies have lamellate tracheal-gills, 



