160 ANATOMY AND PHYSIOLOGY 



after it has reached its perfect state ; nor in pupae, 

 except among the Diptera; but they are found in 

 the larvae of all the orders, except the Orthoptera, 

 Pymenoptera, and Hemiptera. They are either in 

 the form of approximating fasciculi of hairs, or broad 

 thin plates, variously lobed and divided on the edges. 

 The former are most common, and may be witnessed 

 in the larvae and pupae* of common gnats ; the latter 

 in the larvae of whirlwigs (Gyrinus), Ephemerae, 

 Phryganidae, &c. In Dragon-flies, the small leaf-like 

 trachea! plates are placed in the caecum, where 

 water is alternately admitted and ejected, serving the 

 double purpose of supplying the air-pipes and aiding 

 locomotion. 



These appendages are usually kept in a state of 

 continual and intense agitation, probably for the pur- 

 pose of always bringing around them a new supply 

 of fresh air. When placed in expanded plates along 

 the margins, their continual undulating motion must 

 assist materially in maintaining the equilibrium of the 

 body and facilitating movement in the water. 



When the air has found admission into the body 

 of the insect by one or other of the various avenues 

 above described, it is received by the 



Trachea or Air-tubes. These are ramose tubes, 

 opening into a spiracle, or originating from a branchia, 

 and spreading through all parts of the body. As no 

 insect can exist without air, and as these are the only 

 channels by which it can be distributed, they are in 

 no case deficient or imperfectly developed. The 

 manner of their distribution is so varied as to pre- 



