POSTEMBRYONIC CHANGES OF FLIES 667 



seems to be turned to deep-seated and complicated internal develop- 

 mental processes. These inner changes involve an almost complete 

 destruction of many organs of the larva, and their renewal from 

 certain germs (the irnaginal buds) already present in the larva, as 

 will be seen in the highly modified Muscidae. Only a few larval 

 organs become directly transferred into the body of the pupa and 

 imago. Such are the rudiments of the genital system. The heart 

 also, and the central portion of the nervous system, suffer only 

 slight and unimportant, almost trivial, internal changes. On the 

 other hand, most of the other organs of the larva become com- 

 pletely destroyed : the hypodermis, most of the muscles, the entire 

 digestive canal with the salivary glands ^ while their cells, under the 

 influence of the blood corpuscles (leucocytes), which here act as 

 phagocytes, fall into pieces, which are taken up by them and become 

 digested. Simultaneously with this destructive, histolytic process, 

 the new formation of the organs by the imaginal buds, already 

 indicated in the embryo, is accomplished in such a way that the 

 continuity of the organs in most cases remains unimpaired. This 

 process of transformation can only be understood by considering 

 that of the embryonal germs of the organs, (1) only a part is des- 

 tined for the use of the larva in growth, and for the performance of 

 certain functions which exhaust themselves during larval life, so 

 that it is no more capable of farther transformation, and finally 

 becomes destroyed ; while (2) a second part of the embryonal germs 

 or rudiments persists first in an undeveloped condition, as imaginal 

 buds, in order to undertake during the pupa stage the regeneration 

 of the organs. 



Though Swammerdam knew that the rudiments of the wings were already 

 present under the skin of the larvse, we are indebted, for our present knowledge, 

 to the thorough and profound investigations of Weismann on the metamorphosis 

 of the Diptera, and also to the researches of Ganin and others who have worked 

 on the pupae of Muscidse, in which the development is most complicated and 

 modified. In the more generalized and primitive Diptera, such as Corethra, the 

 processes of formation of the pupa and imago are much simpler than in the 

 muscids and Pupipara. These processes are still simpler in the Lepidoptera and 

 Hymenoptera, and for this reason we have given a summary of what has been 

 done on these organs by Newport, Dewitz, and especially by Bugnion. 



Our knowledge of this subject is still very imperfect, only the more salient 

 points having been worked out, and, as Korschelt and Heider state, there is still 

 lacking certain proof as to how far the relations of the internal changes known 

 to exist in the Muscidse also apply to other orders of insects, though it must be 

 considered that in the pupa of Lepidoptera, Hymenoptera, perhaps also the 

 Coleoptera, and we would add in the Neuroptera as well as the male Coccidse, 

 very similar metamorphic processes take place. 



