after it has reached its perfect state; nor in pupæ, except among the Diptera; but they are found in the larvæ 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 larvæ and pupæ of common gnats; the latter in the larvæ of whirlwigs (Gyrinus), Ephemeræ, Phryganidæ, &c. In Dragon-flies, the small leaf-like tracheal plates are placed in the cæcum, 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 purpose 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
Tracheæ 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-
