30 AN INTRODUCTION TO ENTOMOLOGY 



larly stratified. This is due to the fact that the nuclei of different 

 cells are in different levels. 



The trichogens. Certain of the hypodermal cells become highly 

 specialized and produce hollow, hair-like organs, the setae, with which 

 they remain connected through pores in the cuticula. Such a hair- 

 forming cell is termed a trichogen (Fig. 41, tr); and the pore in the 

 cuticula is termed a trichopore. 



The cuticula. Outside of the hypodermis there is a firm layer, 

 which protects the body and serves as a support for the internal 

 organs; this is the cuttcula (Fig. 41, c). The cuticula is produced by 

 the hypodermis ; the method of its production is discussed in a later 

 chapter where the molting of insects is treated. The cuticula is not 

 affected by caustic potash; it is easy, therefore, to separate it from 

 the tissues of the body by boiling or soaking it in an aqueous solution 

 of this substance. 



Chitin. The well-known firmness of the larger part of the cuticula 

 of adult insects is due to the presence in it of a substance which is 

 termed chitin. This substance bears some resemblance in its physical 

 properties to horn; but is very different from horn in chemical com- 

 position. In thin sheets it is yellowish in color; thicker layers of it 

 are black. It is stained yellow by picric acid and pink by safranin. 

 Chitinized and non-chitinized cuticula. -When freshly formed, the 

 cuticula is flexible and elastic, and certain portions of it, as at the 

 nodes of the body and of the appendages, remain so. But the greater 

 part of the cuticula, especially of adult insects, usually becomes firm 

 and inelastic ; this is probably due to a chemical change resulting in 

 the production of chitin. What the nature of this change is or how it 

 is produced is not yet known, but it is evident that a change occurs; 

 we may speak, therefore, of chitinized cuticula and non-chitinized 

 cuticula. This difference is well-shown in sections of the cuticula 

 stained by picro-carmine, which colors the chitinized portions yellow 

 and the non-chitinized parts pink; it can be shown also by other 

 double stains, as eosin-methylene-blue. 



Chitinized cuticula is inelastic, while non-chitinized cuticula is 

 elastic. The elasticity of non-chitinized cuticula is well-shown by the 

 stretching of the body- wall after a molt and before the hardening of 

 the cuticula. It is also shown by the expanding of the abdomen of 

 females to accommodate the growing eggs, the stretching of the body- 

 wall taking place in the non-chitinized portions between the segments . 

 An extreme case of this is shown by the queens of Termites. 



