28 



PSYCHE. 



[February 1897. 



Weak or fiinctioiiless organs in any ani- 

 mal are peculiarly susccptable to dis- 

 ease. Healthy organs ma)' be exposed 

 to the same influences without danger. 

 In the same way the imaginal discs, in 

 which there is an exceedingly active 

 metabolism, and all the larval orgatis 

 -which 7-emain fiinctional during the 

 metamorphosis ox during a greater part 

 of it are immune from the attacks of the 

 phagocytes. The heart in the miiscids 

 continues to beat, as Kilnckel d'Hercu- 

 lais has observed, during the entire per- 

 iod of the metamorphosis with the ex- 

 ception of a day or two in the latter 

 half of it. The nervous system must 

 continue functional during the entire 

 time. The three pairs of tlioracic 

 muscles which pass intact from the lar- 

 va to the imago are probably employed 

 in respiration during the metamorphosis. 

 The reproductive glands are, like the 

 imaginal discs, rapidly growing organs. 

 It would be interesting to know exactly 

 to what extent the blood corpuscles in 

 the other orders of insects besides the 

 biachyceran dipters become phagocytes 

 during the metamorphosis, and why 

 histolysis is so much more extensive 

 here than elsewhere. Tiie answer 

 would undoubtedly be in accoid with 

 what I have just said above. The bra- 

 chycera are the most highly specialized 

 insects ; the structural differences be- 

 tween the larva and imago are the most 

 profound ; their metamorphosis is the 

 the most complete ; but two or three 

 of the functional, larval organs remain 

 fimctional during the metamorphosis; 

 amoeboid blood corpuscles in the form 



of phagocytes consume the rest, sparing, 

 however, also the imaginal discs from 

 which the imaginal organs are built up. 

 Among the other holometabolic insects, 

 on the other hand, many or most of the 

 larval organs remain functional during 

 the metamorphosis, hence there is but 

 little histolysis. But the larval intes- 

 tine would always necessarily become 

 luifunctional, and, as we have seen, 

 Kowalevsky is of the opinion that the 

 larval midgut in all holometabolic insects 

 contains imaginal discs and undergoes 

 degeneration during the metamorphosis. 

 It is interesting to note that each of 

 the three thoracic and eight abdominal 

 segments which make up the larval 

 bodv in the brachyceran dipters has 

 two pairs of imaginal discs, a dorsal 

 and a ventral pair. Thus there is a 

 double rovv of discs extending the length 

 of the body on the dorsal side and an- 

 other on the ventral side of the larva. 

 This is seen especially well in Melo- 

 phagus where the thoracic discs are near 

 the surface and not in the center of the 

 larva as thev are in Musca ; the thoracic 

 discs are h.ere in straight rows with the 

 abdominal ones. The question now 

 presents itself, — are the thoracic and 

 abdominal discs homodynamous organs. 

 The different discs furnish very differ- 

 ent imaginal organs : legs, wings, pupal 

 spiracles, balancers, and hj'podermis 

 grow from the thoracic, and hypoder- 

 mis, rectal glands, and perhaps external 

 sexual organs from the abdominal discs ; 

 but yet they are all similar to each other 

 in several verv important respects. 

 In the first place the method of ori- 



