MOSQUITOES, SILKIVORMS, AND DRAGON-FLIES 13 



the netting can be lifted up and the tray emptied. The 

 larvae will grow rapidly, plainly increasing in size each 

 day. After eight or nine days they will cease feeding 

 and crawling around. Each one stands on the legs of 

 the middle of the body, and usually holds up the head 

 and forepart of the body, and sometimes the tail. It is 

 preparing to moult, i.e., to cast the skin. The moulting 

 should be observed in detail. After moulting each larva 

 will be noticeably larger and paler, and the long hairs of 

 the body will be replaced by short ones. The feeding 

 begins again, and larger and larger supplies of leaves will 

 be found necessary to keep the worms well fed. 



The larval stage of the silkworm lasts about forty-five 

 days, with a moulting once every nine days (or eight or 

 ten). It is easy to see why these successive moultings 

 are necessary. The true skin of an insect is always cov- 

 ered over outside by a cuticle in which a horny substance 

 called cliitin is deposited. This chitin makes the cuticle 

 nearly inelastic, so that the growing insect finds its body 

 confined within a non-stretchable case. There is then 

 but one way out of this dilemma, and that is the simple 

 way of breaking out ! And so in the life-history of all 

 insects the phenomenon of moulting takes place. In 

 nearly all cases the cuticle splits along the middle of the 

 back from the head to about half way to the end of the 

 tail, and through this rent the body issues with new cuticle. 

 The cast skin being very light and thin, and usually color- 

 less, soon disappears from view, and unless the insect be 

 seen precisely at each moulting time it is not easy to 

 ascertain how many moults occur in the life of any par- 

 ticular species. 



The silkworms breathe through very small openings, 

 called spiracles, on the sides of the body. One pair of 

 spiracles occurs on each of nine of the body-rings. What 

 rings are these .'' Each spiracle opens into an air-tube 



