The integument of many insects contains fine canals passing through the chitinous layers and opening externally in minute pores. Certain of the pore-canals communicate with hollow setæ which sit directly over the pores; other pores form the external openings of dermal glands, but in many cases they are empty or only filled with air, and do not have any hairs connected with them. Each of these pores communicates with a hair-forming hypodermal cell, called by Graber a _trichogen_. =Setæ= (“hairs” and bristles).—The setæ of insects are, as in worms, processes of the cuticle originating from certain of the hypodermal cells. They arise either from a ring-like pit, or from a minute tubercle, and are usually situated at the outlet of a pore-canal, which connects with an underlying cell of the hypodermis (Fig. 212). They are, then, bristle or hair-like processes arising from the hypodermis. Where the hairs or setæ are rubbed off, their site is indicated by a minute ring like a follicle in the chitinous integument. The cuticular hair, says Leydig, is in its first condition the secretion of the cellular element of the skin, and a thread-like continuation of the cell-body may rise up through the pore-canal into the centre of the hair, remaining there permanently. While the setæ are usually simple, they are often branched, plumose, or spinulose, as in larval Hemerobiidæ, Anthrenus, and Dermestes, the larvæ of certain coccinellid beetles, notably Epilachna, and of Cassida, the larvæ of arctians, etc., and in bees (Anthophila, Megachile, Osmia, Colletes, Apis, etc.). The use of these spinulose, plumose, and twisted hairs in the bees is clearly shown by J. B. Smith, who states that as these insects walk over flowers, the pollen grains adhere to the vestiture, “and this also accounts for the fact, probably noticed by every observant fruit-grower, that bees frequently bury themselves completely in the blossoms, or roll over every part of them. Such insects are after pollen, not honey, and by so rolling about, the pollen grains are brought into contact with and adhere to the surface of the insect.” The syrphid flies also pollenize flowers, the pollenizing of chrysanthemums being effected, as Smith states, by _Eristalis tenax_, and he adds that the body vestiture of the syrphids “is often composed of spurred and branched hairs.” (For reference to gathering hairs, see p. 45.) [Illustration: FIG. 213.—Cuticular spinules of larva of Adoneta: _a_, _b_, _c_, _d_, different forms; _e_, _e′_, caltrops. ]