i6 



INSECTS 



It may be well to say just a few words as to this 

 outer skin which forms so important a feature in the 

 insect structure. It is soft yet resistant, and may 

 become so resistant that it is almost impenetrable to 

 corrosives or oils and with difficulty to be punctured 

 with needle or knife. It all depends upon the amount 

 of chitin deposited in the tissue and chitin is a secre- 

 tion from the lower layers of the cuticle, resembling 

 horn in texture and somewhat in qualities. No matter 



FIG. 2. Moulting of a grasshopper: a, nymph ready to change ; b, the skin 

 split along the back and the adult emerging; c, continues the process, and at d, 

 the insect is drying out. 



how soft the body wall it contains some chitin and as 

 this is not elastic, there is a limit beyond which it cannot 

 be stretched. When this limit is reached a new skin 

 forms beneath the old one which, thereupon becoming 

 lifeless, splits and permits the larger next stage to escape. 

 This sort of change is found also in the crustaceans, 

 and in some of the reptiles. 



The third, or pupal stage, may be a period of rest 

 or of continued activity, depending upon whether the 

 species has a "complete" or an "incomplete" meta- 

 morphosis. Returning again to our young grasshopper, 



