30 ^ A^ I NT ROD UCTION TO ENTOMOLOG Y 



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

 cells are at 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 cuticiila. Such a 

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

 the cuticula is termed a Mchopore. 



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 cuticula (Fig. 41, c). The cuticula is produced by 

 the hypodermis ; the method of its production is discussed on p. 171, 

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

 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. — This word was introduced into entomology by Odier in 

 1823 for the colorless, flexible covering of the arthropods after the 

 integument had been boiled in caustic potash and the albuminous, 

 oily, coloring and mineral substances had been removed thereby. 

 By a not unusual turn in the use of words, chitin has come to mean, 

 as stated by Newport (1836-1839): "The peculiar substance that 

 constitutes the hard portion of the dermo-skeleton [in insects]." 

 From 1870 and onward the words chitinize and chitinization have 

 come to mean the hardening of the cuticula by the incorporation of 

 chitin; and they are used with that meaning throughout this work. 

 (For references to chitin, see p. loio). 



Rigid and flexible cuticida. — When freshly formed by the hypo- 

 dermis, 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 in adult insects, usually 

 becomes firm and inelastic ; this is due to a change in which the 

 hardening substance is developed within or upon the original soft 

 cuticula. What the exact nature of this change is or how it is pro- 

 duced is not known. This change is usually spoken of as chitinization ; 

 and the hard parts of the cuticula are then said to be chitinized, and 

 the soft parts, as at the nodes, non-chitinized. The hardened or 

 chitinized cuticula is rigid and inelastic while the soft or non-chitinized 

 cuticula is flexible and elastic. The elasticity of the soft cuticula is 

 well shown by the stretching of the body wall after a molt. It is 

 also strikingly shown by the expansion of the soft, intersegmental 

 cuticula to accommodate the growing eggs, as in the queens of 

 Termites. 



