66 



ENTOMOLOGY 



invaginated, along with the underlying cellular layer, to make glands of 

 various kinds. 



Chitin. The skin, or cuticula, 1 of an insect differs from that of a 

 worm, for example, in being thoroughly permeated with a peculiar sub- 

 stance known as chitin the basis of the arthropod skeleton. This is a 



8 



T> 



B 



FIG. 87. Genitalia of a moth, Samia cecropia. A, male; B, female; a, anus; c, c, 

 claspers; o, opening of common oviduct; p, penis; s, uncus (the doubly hooked organ); 

 v, vestibule, into which the vagina opens. The numbers refer to abdominal segments. 



substance of remarkable stability, for it is unaffected by almost all ordi- 

 nary acids and alkalies, though it is soluble in sodic or potassic hypo- 

 chlorite (respectively, Eau de Labarraque and Eau de Javelle) and 

 yields to boiling sulphuric acid. If kept for a year or so under water, 



PIG. 88. Terminal abdominal appendages of a dragon fly, Plathemis trimaculata. A, 

 male; B, female, i, inferior appendage; 5, s, superior appendages. The numbers refer to 

 abdominal segments. 



however, chitin undergoes a slow dissolution, possibly a putrefaction, 

 which accounts in a measure for the rapid disappearance of insect 

 skeletons in the soil (Miall and Denny). By boiling the skin of an 

 insect in potassic hydroxide it is possible to dissolve away the cuticular 

 framework, leaving fairly pure chitin, without destroying the organized 



1 The cuticula of an insect should be distinguished from the cuticle of a vertebrate, the 

 former being a hardened fluid, while the latter consists of cells themselves, in a dead and 

 flattened condition. 



