ANA TO MY 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 lilow-fly laiva. — i, a vertical section showing the 

 hypodermis /;, with the super-imposed ciilicular layers ; 2, a similar section, 

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

 cu. Cuticle ; c, nerve end organ ; /, terminal portion of the end organ ; «, nerve; 

 s, sub-hypodermic tissue ; ir, 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, 18S0, p. 192. 



