ANATOMY AND PHYSIOLOGY 



135 



larvae of aquatic habit, as Gyrinus and FIG. 171. 



Cncinidotus, possess tracheal gills, as do 

 also caterpillars of the genus Paraponyx 

 (Fig. 171), which feed on the leaves of 

 several kinds of water plants. 



Though manifold in form, tracheal 

 gills are generally more or less foliaceous 

 or filamentous, presenting always an ex- 

 tensive respiratory surface; their integu- 

 ment is thin and the tracheae spread 

 closely beneath it. These adaptations 

 are often supplemented by waving move- 

 ments of the gills, as in May fly nymphs, 

 and by frequent movements of the insect 

 from one place to another. 



Especially noteworthy are the rectal 

 tracheal gills of odonate nymphs. In 

 these insects the lining of the rectum 

 forms numerous papillae or lamellae, which 



contain a profusion of delicate tracheal 

 branches; these are bathed by water 

 drawn into the rectum and then expelled, 

 at rather irregular intervals. A similar 

 rectal respiration occurs also in ephemerid 

 nymphs and mosquito larvae. 



A few forms, chiefly Perlidae, are 

 exceptional in retaining tracheal gills 

 in the adult stage; in some imagines 

 they are merely vestiges of the nymphal 

 gills, but in others, such as Pteronarcys 

 (Fig. 1 8), which habitually dips into 

 the water and rests in moist situations, 

 the gills probably supplement the spira- 

 cles. Further details on the respiration 

 ing respiratory tube. o f aqua ti c insects are given in Chapter 



Natural size. After 



HART. IV. 



Caterpillar of Para- 

 ponyx obscuralis, to show 

 tracheal gills. Length, 

 15 mm. After HART. 



FIG. 172. 



