188 



TEXT-BOOK OF ENTOMOLOGY 



responds to a single cell of the underlying hypodermis. Each field is bounded 

 by a distinct polygonal outline, and its surface is either covered by a large num- 

 ber of extremely minute projecting points, as on the dorsal arch of the segment, 



or is smooth, as upon the articular membrane and ven- 

 tral arch. Upon the sides of the dorsal arch and upon 

 the spiracular membrane each field has a projecting 

 spine or sometimes two or even three. (See also pp. 

 28, 30.) 



The cuticle of lepidopterous larvse has also been 

 described and figured by Minot. In the caterpillars 

 FIG. -2hi. p/ianiritM peg- of different groups investigated by him, the cuticle was 



AftiT Graber? 1 Mexil '' ~ found to be TOU S h with microscopic teeth or spinules, 



erect or flattened and scale-like, and either densely 



crowded or scattered, and affording excel- 

 lent generic and specific characters. In 



the slug- worms (Limacodids) we have 



observed that the cuticula is unusually 



rough, especially on the spiniferous 



tubercle of Empretia, Parasa, etc. (Fig. 



213, c). The skin of the body between 



the tubercles is seen to be finely sha- 



greened, due to the presence of fine teeth, 



which are more or less curved and bent, 



these teeth arising from a very finely granulated surface (d). The cuticle of 



neuropterous, trichopterous, and tenthredinid larae will probably afford similar 



cases. The integument of the larva of Datana is, on the black bands, rough 

 and nodulated, the irregular nodules being filled with a black 

 pigment, and forming a layer (p) external to the true cuticula 

 (Fig. 211). 





FIG. 211. Section of internment of 

 Datana minixtra: e, cuticnla; fii/ji, hypo- 

 dermis; p, outer pigmented nodulated layer. 





The integument of many insects contains fine 

 canals passing through the chitinous layers and open- 

 ing externally in minute pores. Certain of the pore- 

 canals communicate with hollow setae 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 tricliogen. 



Setae ("hairs" and bristles). - -The setae of insects 

 are, as in worms, processes of the cuticle originating 

 Fm. 212. irairs from certain of the hypodermal cells. They arise 



Datana: /, forma- ... 



two hair cdi ; c, cuti- either from a ring-like pit, or from a minute tubercle, 



cula ; ]>, pigmented . e 



layer; hy, hypoder- and are usually situated at the outlet of a pore-canal, 

 which connects with an underlying cell of the hypo- 

 dermis (Fig. 212). They are, then, bristle or hair-like processes 

 arising from the hypodermis. Where the hairs or setae are rubbed 

 off, their site is indicated by a minute ring like a follicle in the 



of 



