346 PHYLUM ARTHROPODA. 



expiratory muscles. We see their action when we watch a 

 drone-fly panting on a flower. Inspiration is passive, as in 

 birds, and depends on the elasticity of the skin and of the 

 trachea! walls ; expiration is active, and depends upon these 

 muscles. They are chiefly situated in the abdomen, but in 

 some beetles (at least) they are also present in the metathorax. 

 The tracheae seem to arise as tubular ingrowths of skin, 

 and, primitively, each segment probably contained a distinct 

 pair ; but their number has been reduced, and they are often 

 in part connected into a system. With the doubtful excep- 

 tion of one of the primitive Collembola, and the certain 

 exception of caterpillars, no insects have any tracheal 

 openings in the head region. There are rarely more than 

 two pairs in the thorax ; there are often six to eight pairs in 

 the abdomen ; the maximum total is ten pairs. Each trachea 

 is kept tense throughout the greater part of its course by 

 internal chitinous thickenings, which apparently have a 

 spiral course. The branches of the tracheae penetrate 

 into all the organs of the body, carrying oxygen to every 

 part. The very efficient respiration of insects must be kept 

 in mind in an appreciation of the general activity of their life 



As the conditions of larval life are often different from those of the 

 adult insects, the mode of respiration may also differ in details. 



In insects without marked metamorphosis, and even in some beetles 

 in which the metamorphosis is complete, the young insect and the adult 

 both breathe by tracheae with open stigmata. Both are said to be 

 " holopneustic." 



When the larvae live in water, the tracheal system is closed, other- 

 wise the creatures would drown. This closed condition is termed 

 " apneustic." These larvae (of dragon -flies, may-flies, and some others) 

 breathe by "tracheal gills" (see Fig. 183) little wing-like outgrowths 

 from the sides of the abdomen, rich in tracheae or by tracheal folds 

 within the rectum, in and out of which water flows. In either case, 

 an interchange of gases between the tracheae and the water takes place. 

 In adult aerial life the tracheae of the body acquire stigmata, and the 

 insect becomes "holopneustic." 



In most insects with complete metamorphosis, the larva (e.g. cater- 

 pillar or grub) has closed stigmata on the last two segments of the 

 thorax (those which will bear wings), but there is a pair of open 

 stigmata on the prothorax. In the adult the reverse is the case. 



There are some other modifications for instance, what obtains in the 

 parasitic larvae of some flies, e.g. gadflies. In these the stigmata are 

 open only at the end of the body. In all cases, however, the stigmata 

 of the adult are already present as rudiments in the larva, though they 

 may not open till adolescence is over. 



