IV. 1NSECTA. 459 



man only in that they are tubes filled with air, and kept from 

 collapse by firm walls. They open to the exterior by openings 

 (spiracles, stigmata) on the sides of the body. They are inpushings 

 of the skin and consequently have the same structure, an epithe- 

 lium and an outer chitinous layer. The latter lines the lumen 

 of the tubes, and since it must be thin to permit the passage 

 of gases (oxygen, carbon dioxide), and at the same time firm, 

 to keep the tubes open, it is thrown into folds which usually 

 pursue a spiral course. The turns of the spiral are so close 

 that it gives the tubes a ringed appearance. Inside the spiracles 

 the tracheae branch repeatedly until they end in the tissues in 

 fine tracheal capillaries. In general it may be said that each 

 segment has a right and a left spiracle and corresponding tracheal 

 systems (fig. 59), but this scheme is complete in no known species, 

 for there are always some segments (especially in the head) which 

 lack these organs and are supplied from adjacent segments (fig. 

 479). Again, the tracheae may be connected by longitudinal trunks 

 (fig. 494, ib), so that spiracles occur in only a part of the segments, 

 these supplying the whole system. Although the tracheae are for 

 aerial respiration, there are aquatic insects, but these also breathe 

 air, since they carry air about with them entangled among the 

 hairs which surround the spiracles. Then, too, aquatic larvae often 

 have tracheal gills, thin-walled processes of the integument which 

 project into the water and are penetrated by numerous tracheal 

 twigs (fig. 495). 



The alimentary tract always has excretory organs, the Mal- 

 pighian tubules, connected with it. These vary in number be- 

 tween wide limits, but are always placed at the junction of the 

 rectum with the rest of the track They diifer from the physiolog- 

 ically similar tubes of the Arachnida in being of ectodermal origin, 

 so that no homology can be traced between them. The gonads 

 are always paired and placed dorsal to the intestine, while the 

 ducts (at least in some cases modified nephridia) open ventrally 

 at the hinder end of the body. The spermatozoa are motile. 



In the subdivision of the ' tracbeate ' arthropods a group of Myriapoda 

 is usually recognized, containing forms known as centipedes and ' galley 

 worms.' These two types are in reality very different. The centipedes 

 (Chilopoda) show in all structural features close relationships to the Hex- 

 apoda, while the other group, Diplopoda, differ in almost every respect, 

 except the presence of numerous walking legs, from the Chilopoda. 

 Hence, since the object of classification is to show resemblances and dif- 

 ferences, the group of Myriapoda has been dismembered, the Chilopoda 



