ANATOMY AND MORPHOLOGY OF INSECTS. 9 



ing cells of the nymph or imago — at least, in the metabola and 

 hemi-metabola. 



Chitin is a nitrogenous substance more nearly related to 

 mucin than to any other substance found in vertebrates. The 

 large hypodermic cells of many larvae exhibit cup- shaped 

 cavities on their outer surface, which present a great simila- 

 rity to those of the well-known goblet cells of the mucous 

 membranes. Latreille regarded chitin as the result of the 

 degeneration of the external portion of the cell-substance 



Fig. I.— Sections of the skin of the Blow-fly larva. — i, a vertical section showing the 

 hypodermis h, with the super-imposed cuticular layers ; 2, a similar section, 

 showing the cuticular prisms ; 3, a sensory papilla ; 4, a sub-hypodermic cell. 

 «/, Cuticle ; e, nerve end organ ; t, terminal portion of the end organ ; n, nerve ; 

 s, sub-hypodermic tissue ; tr, trachea. 



[9, p. 882], a view in which I must concur. Chitin is very 

 insoluble in solutions of the caustic alkalies, a property which 

 enables the microscopist to make beautiful preparations of the 

 exo- and endo-skeletons of insects. 



The cuticle consists of two distinct parts, which correspond 

 with the epiostracum and endostracum of the Crustacea.* I 

 shall, therefore, use these terms to distinguish them. 



* Huxley, T. H., 'The Crayfish.' London, 1880, p. 192. 



