1G0 
ANATOMY AND PHYSIOLOGY 
after it has reached its perfect state ; nor in pup®, 
except among the Diptera ; hut they are found in 
the larvae of all the orders* except the Orthoptera, 
Hymenoptera* 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 pup® of common gnats ; the latter 
in the larv® of whirl wigs ( Ch/rinus), Ephemer®, 
Pliryganid®* &c. In Dragon-flies* the small leaf-like 
tracheal plates are placed in the c®cum* where 
water is alternately admitted and ejected* serving the 
double purpose of supplying the air-pipes and aiding 
locomotion. 
These appendages arc 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 
Tracheae 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- 
