PHYLUM ARTHROPOD A 



245 



within the tracheae as a chitinous internal hning of the tubes. 

 Within the tracheae the chitin is not disposed in a uniform layer 

 but assumes the form of a coiled spiral thread lining the inner wall 

 of the tracheal vessel. Small tubes lacking the spiral chitinous 

 threads form the most minute subdivisions of the tracheal system 

 and are designated as tracheoles. 

 Tracheae in some insects may 

 become modified as enlarged sacs 

 which serve as air reservoirs. 



Modifications of the Respira- 

 tory System. — Of the typical 

 respiratory system with spiracles 

 communicating directly between 

 the tracheae and the outside air, 

 there are many modifications. 

 In the CoUembola, and in many 

 aquatic larvae, there are no 

 specialized organs for respira- 

 tion, for that function is per- 

 formed directly through the skin. 

 Gills occur in many aquatic 

 nymphs and larvae. These are 

 of two distinct types, tracheal 

 and blood gills. In the former, 

 lateral or caudal evaginations of 

 the body wall are furnished with 

 numerous tracheae which are 

 continuous with the vessels of 

 the tracheal system within the 

 body and conditions suitable for 

 a respiratory exchange are thus 

 provided. Even a portion of 

 the digestive tract may be 

 appropriated as a respiratory 

 organ, as in the rectal tracheal 

 gills of the dragonfly nymphs. 

 Blood gills are usually threadlike evaginations from the body 

 wall of aquatic insects. The spaces within these gills are in 

 direct communication with the fluid-filled body cavity, and 

 through the delicate walls of the gills a respiratory exchange is 

 made possible without requiring the presence of tracheal tubes. 



Fig. 122.— Tracheal system of an 

 insect, a, antenna; 6, brain; I, leg; n, 

 nerve cord; p, palpus; s, spiracle; .si, 

 spiracular branch; t, main tracheal 

 trunk; v, ventral branch; t'.s., \dsceral 

 branch. (From Folsom's Entomology, 

 after Kolbe). 



