ANATOMY AND PHYSIOLOGY 



107 



9. RESPIRATORY SYSTEM 



In insects, as contrasted with vertebrates, the air itself is conveyed to 

 the remotest tissues by means of an elaborate system of branching air- 

 tubes, or trachea, which receive air through paired segmentally-arranged 

 spiracles. Each spiracle is commonly the mouth of a short tube which 

 opens into a main tracheal trunk (Fig. 

 1 68) extending along the side of the 

 body. From the two main trunks 

 branches are sent which divide and 

 subdivide until they become extremely 

 delicate tubes, which penetrate even 

 between muscle fibers, between the 

 ommatidia of the compound eyes and 

 possibly enter cells. In most cases 

 each main longitudinal trunk gives off 

 in each segment (Fig. 169) three large 

 branches: (i) an upper, or dorsal, 

 branch, which goes to the dorsal mus- 

 cles; (2) a middle, or visceral, branch, 

 which supplies the alimentary tract and 

 the reproductive organs; (3) a lower, or 

 ventral, branch, which pertains to the 

 ventral ganglia and muscles. 



In many swiftly flying insects (dra- 

 gon flies, beetles, moths, flies and bees) 

 there occur tracheal pockets, or air-sacs, 

 which were formerly and erroneously 

 supposed to diminish the weight of the 

 insect, but are now regarded as simply 

 air-reservoirs. 



Types of Tracheation. T wo 

 types of tracheal system are distin- 

 guished for convenience: (i) The pri- 

 mary, open, or holopneustic type described above, in which the spiracles 

 are functional; (2) the secondary, closed, or apneustic type, in which the 

 spiracles are either functionless or absent. This type is illustrated in 

 Collembola and such aquatic nymphs and larvae as breathe either directly 

 through the skin or else by means of gills. The two types, however, are 

 connected by all sorts of intermediate stages. 



FIG. 168. Tracheal system of an 

 insect. a, antenna; b, brain; I, leg; 

 n, nerve cord; p, palpus; s, spiracle; 

 st, spiracular, or stigmatal, branch; /, 

 main tracheal trunk; v, ventral branch; 

 vs. visceral branch. After KOLBE. 



