260 



INVEETEBRATA 



CHAP. 



of reversion in Periplaneta, which agrees in every detail with this 

 process in Donacia. 



About the same time the endoderm cells have also completely 

 invested the yolk, and inside them the remaining yolk cells, which 

 from the beginning of the segmentation of the , egg have remained 

 in the yolk, form an almost continuous layer parallel with the 

 endodermal epithelium and lying on it. The yolk is progressively 

 liquefied by the action of these cells and then absorbed. When this 

 is complete the yolk cells degenerate, break, up, and disappear. 



The fexcretory organs of insects are known as Malpighian tubules ; 

 in the adult they are long thread-like tubules which open into the 

 proctodaeum at its inner end. In Donacia Hirschler describes them 

 as arising as three pairs of short sac-hke outgrowths of the procto- 

 daeum, in the stage when the embryo is still bent dorsally. When 



reversion takes place these sacs 

 •^('^ nim grow into long thread-like 



tubes. If Hirschler's account 

 is correct, the Malpighian tubes 

 are of ectodermal origin. It 

 will be remembered that, in 

 describing the development of 

 the Spider, we saw in that 

 animal the so-called Mal- 

 pighian tubes arise as out- 

 growths of the hinder 

 part of the mid-gut. It 

 would therefore seem to fol- 

 low that the structures 

 termed Malpighian tubes in 

 insects and spiders are in no 

 sense homologous with one 

 another. 



The account of the embryonic development of Donacia is now 

 completed. Provided with digestive, circulatory, respiratory, excretory, 

 nervous, and muscular systems, the embryo bursts the egg-shell and 

 emerges as the well-known "grub," or larva of the beetle, which 

 immediately begins to feed on the leaves of the plant on which it 

 finds itself It grows rapidly, moults several times, and then passes 

 into a quiescent or "pupal" stage, during which it undergoes an 

 extraordinary metamorphosis, and finally emerges from the pupal 

 skin as the perfect beetle or imago. Of the changes which convert 

 the grub into the beetle we have no exact account in the case of 

 Donacia. For the study of this process another type must be selected. 

 Before entering on this study, however, we shall give a very brief 

 survey of the most important points which have been made out in 

 the embryonic development of other Insecta, and we shall do this 

 in order to see how far they agree with the results which Hirschler 

 has attained in the case of Donacia. 



Fig. 206. — The "grub" oi Doryphora (Leptino- 

 tarsa) decemlineata in the third instar, from 

 the side. 



a.im, abdominal imaginal discs ; a?ti, antenna ; mrri, 

 first' maxilla; mx'^, second maxilla; stiy, stigmata; 

 th.im, thoracic imaginal discs ; w, rudiments of wings. 



