THE EXTERNAL ANATOMY OF INSECTS 31 



The formation of chitin is not restricted to the hypodermis, but is 

 a property of the invaginated portions of the ectoderm ; the fore- 

 intestine, the hind-intestine, and the tracheas are all lined with a 

 cuticular layer, which is continuous with the cuticula of the body-wall 

 and is chitinized. The most marked case of internal formation of 

 chitin is the development of large and powerful teeth in the proven- 

 tricu'us of many insects. 



The epidermis and the dermis. Two quite distinct parts of the 

 cuticula are recognized by recent writers; these are distinguished as 

 the epidermis and the dermis respectively. 



The epidermis is the external portion ; in it are located all of the 

 cuticular pigments ; and from it are formed all scales, hairs, and other 

 surface structures. It is designated by some writers as the primary 

 cuticula, (Fig. 41, e). 



The dermis is situated beneath the epidermis. It is formed in 

 layers, which give sections of the cuticula the well-known laminate 

 appearance. It is sometimes termed the secondary cuticula (Fig. 41, d) 



The basement membrane. The inner ends of the hypodermal cells 

 are bounded by a more or less distinct membrane; this is termed the 

 basement membrane (Fig. 41, bm). The basement membrane is most 

 easily seen in those places where the inner ends of the hypodermal cells 

 are much smaller than the outer ends; here it is a continuous sheet 

 connecting the tips of the hypodermal cells. 



b. THE EXTERNAL APOPHYSES OF THE CUTICULA 



The outer surface of the cuticula bears a wonderful variety of pro- 

 jections. These, however, can be grouped under two heads: first, 

 those that form an integral part of the cuticula; and second, those 

 that are connected with the cuticula by a joint. Those that form an 

 integral part of the cuticula are termed apophyses; those that are con- 

 nected by a joint are termed appendages of the cuticula. 



The cuticular nodules. The most frequently occurring out- 

 growths of the cuticula are small, more or less conical nodules. 

 These vary greatly in size, form, and distribution over the surface of 

 the body in different species of insects, and are frequently of 

 taxonomic value. 



The fixed hairs. On the wings of some insects, as the Trichoptera 



and certain of the Lepidoptera, there is in addition to the more 

 obvious setae and scales many very small, hair-like structures, which 



