LEPIDOPTERA. 199 



Rows of hairy tubercles extend down the body. The 

 head is small, and the mouth parts are for biting. 

 The amount of food consumed by this animal is in- 

 credible. It is, in fact, one of the greatest eaters and 

 fastest growers. By experimentation, Trouvelot found 

 that when the young silkworm hatches, it weighs ^V 

 of a grain ; when lo days old, it is lo times its original 

 weight ; when 30 days old, 620 times ; and when 56 

 days old, 4140 times its original weight. The food 

 taken by a single silkworm in 5 6 days equals in weight 

 86,000 times the primitive weight of the worm ; of this 

 about ^ of a pound becomes excrement, 207 grains 

 are assimilated, and over 5 ounces have evaporated. 



The three pairs of legs are short and weak, while 

 the prop-legs, especially the last pair, are stout and 

 horny, being useful locomotive organs. Before the 

 last skin is shed, the larva makes a silken cocoon 

 (Fig. 151, natural size) protected on the outside by 

 leaves. During the win- 

 ter months the leaves 

 often get broken and 

 partly worn off, as seen 

 in Fig. 151. The pupa 

 (Fig. 152) remains 

 motionless for about 

 nine months ; then 

 during May, in the ^'^" '^'" 



vicinity of Boston, it changes to a moth. If the pupje 



relations with occasional or rare food plants, which could not be 

 used as food successfully by the more mature larva, goes a long 

 way towards the explanation of changes both in habits and 

 structure which must have occurred in the past. 



