644 TEXT-BOOK OF ENTOMOLOGY 



and others, we see that metamorphosis is, after all, only an exten- 

 sion of embryonic life,. the moults and great changes being similar to 

 those undergone by the embryo, and that metamorphosis and alter- 

 nation of generations are but terms in a single series. Moreover, 

 the metamorphoses of insects are of the same general nature as 

 those of certain worms, of the echinoderms, and the frog, the 

 different stages of larva, pupa, and imago being adaptational and 

 secondary. 



AVhile the changes in form from the larva to the pupa are appar- 

 ently sudden, the internal histogenetic steps which lead to them are 

 gradual. In the Lepidoptera a few days (usually from one to three) 

 before assuming the pupa stage, the caterpillar becomes restless and 

 ceases to take food. Its excrements are now hard, dry, and, accord- 

 ing to Gonin, are " stained carmine red by the secretions of the 

 urinary tubes." Under the microscope we find that they are almost 

 exclusively composed of fragments of the intestinal epithelium. 

 These red dejections were noticed by Reaumur, and afterwards by 

 Herold, and they are sure indications of the approach of the trans- 

 formations. It now wanders about, and, if it is a spinner, spins its 

 cocoon, and then lies quietly at rest while the changes are going on 

 within its body. Meanwhile, it lives on the stores of fat in the fat- 

 body, and this supply enables it to survive the pupal period. 



The amount of fat is sometimes very great. Newport removed from the larva 

 of Cossus ligniperda 42 grains of fat, being more than one-fourth of the whole 

 weight of the insect. He adds that the supply is soon nearly exhausted during 

 the rapid development of the reproductive organs, "since, when these have 

 become perfected, the quantity that remains is very inconsiderable." 



Although the larval skin of a lepidopterous insect is suddenly 

 cast off, the pupa quickly emerging from it, yet there are several 

 intermediate stages, all graduating into each other. If a caterpillar 

 of a Clisiocampa, which, as we have observed, is much shortened 

 and thickened a day or two before changing to a pupa, is hardened 

 in alcohol and the larval skin is stripped off, the semipupa (pro- 

 nymph, pro-pupa of different authors) is found to be in different 

 stages of development, and the changes of the mouth-parts are 

 interesting, though not yet sufficiently studied. 



Newport attributes the great enlargement and changes in the 

 shape of the thoracic segments of the larva of }'<i,ic.-<^i ml !<-n- at 

 this time, to the contraction or shortening of the nmscles of the 

 interior of those segments, "which are repeatedly slowly extended 

 and shortened, as if the insect were in the act of laborious respira- 

 tion." This, he adds, generally takes place at short intervals during 



