426 



GENERAL ZOOLOGY 



for the posterior pair when the insect is not flying. In butter- 

 flies and moths the wings are 

 covered with small scales. There 

 are two general views as to the 

 origin of insect wings: (1) that 

 they are modified gills, or (2) 

 that they have developed from 

 chitinous thoracic plates. 



The tracheal system of respira- 

 tion, though present in certain 

 other arthropods, is best devel- 

 oped in insects. It consists of 

 tubes called tracheal tubes, which 

 beginning at openings (spiracles) 

 arranged in a row on each side of 

 the body penetrate to all parts of 

 the body, branching as they go 

 and terminating in trachioles in 

 the tissues. The wall of the 

 tracheal tube consists of a single 

 layer of cells lined with a chitinous 

 membrane. Under the micro- 

 scope the larger tubes have a 

 ringed appearance caused by 

 transverse folds in the chitinous 

 lining arranged in a close spiral that prevents collapse of the 

 tubes (Fig. 249). In flying insects, the tracheal tubes near the 



oc. o 



Fig. 249. — A, portion of trachea 

 of caterpillar with its branches, B, 

 C, D. a, peritracheal membrane; 

 b, nucleus. (From Packard, " Text 

 book of Entomology, 1 ' The Macmil- 

 lan Company. By permission.) 



s v 



Fig. 250. — The distribution of tracheae and air sacs in a grasshopper. D, left 

 dorsal trachea; 0, left cephalic trachea; OC, ocular trachea; S, left stigmatal 

 trachea with stigmata (spiracles) ; V, ventral trachea. (After Packard, Textbook 

 of Entomology.) 



spiracles are expanded into thin-walled air sacs, which are com- 

 pressed and expanded by contractions and expansions of the 



