FORMATION OF THE PRONYMPH FROM THE LARVA. 397 



exhibit an extra stage not separated as distinct in other Insects 

 is substantially justified. In the Lepidoptera the phenomena 

 of histolysis are partly carried out in the larva and partly in 

 the nymph, whilst in the Diptera they are completed in a 

 much shorter time, and a new stage of development becomes 

 manifest. In the Lepidoptera the imaginal discs are united 

 before the larval skin is shed, whilst in the Diptera their union 

 is delayed until the process of histolysis is almost complete. 



The Histolysis of the larval tissues proceeds from before 

 backwards, and from without inwards. It commences in the 

 anterior segments during the resting stage of the larva, and is 

 not complete in the posterior segments until after the forma- 

 tion of the nymph. The cells of the hypodermis and the 

 muscles are first attacked. The separation of the hypodermis 

 from the larval cuticle is the first change which occurs after the 

 pupa state is assumed, and, like the histolysis of all the tissues, 

 proceeds from before backwards. It is easy to remove the 

 anterior segments of the pupa-case on the first day of the 

 pupa, but it is not possible to remove the posterior seg- 

 ments of the case before the end of the second, without 

 injury to the pronymph, as the larval muscles and hypoderm 

 remain attached to the cuticle longer than those of the anterior 

 segments. 



a. Histolysis of the Larval Muscles. 



Kowalevski was the first who actually demonstrated the 

 manner in which the histolysis of the larval tissues is effected, 

 but he was led to undertake the investigation by the writings 

 of Metschnikoff [143] on intracellular digestion. All the 

 observations made_^ before_Metschnikoff' s great discovery of 

 the part played by the white blood corpuscles, phagocytes, may 

 be passed over in silence, as they have been completely super- 

 seded and shown to be erroneous by the investigations of 

 Kowalevski, first published in 18S4. All that was actually 

 known before that date may be summed up in the following 

 words. A number of large granule cells, called ' Kornchen 

 kugeln ' by Weismann, and ' corps rosea ' by Viallanes, make 



20 — 2 



