lepidopterous larvce and pupcB. 165 



expansion of the end of the peduncle into two knobs 

 arranged one after the other), and also after the develop- 

 ment of the parasite to show the way in which its 

 posterior end is at first so firmly attached to the egg- 

 shell, and subsequently to the old skins which have been 

 shed and shuffled down to the posterior end of the body. 

 I have not yet found any record of the black shield 

 which is part of the egg-shell covering the head of the 

 parasitic larva (except a doubtful reference to part of 

 one of DeGeer's figures). Its relation to the latter 

 should also be disclosed by sections. As the white larvae 

 increase in size they become green, from the blood of their 

 host, which fills the digestive tract, and shows through 

 the transparent skin and subcuticular tissues. On this 

 point DeGeer accurately states that the green stripe down 

 the back and green patches on the sides are due to internal 

 substances showing through the transparent skin. 



On August 12th seven had shown signs of develop- 

 ment, but to a very variable extent. By August 18th 

 two had been already crowded to death, and two more 

 were evidently dwarfed and unable to develop. When 

 once a larva begins to slacken in its rate of growth, as a 

 consequence of crowding, it is sure to die, probably 

 because of the pressure on its body exerted by the 

 increasing size of its neighbours. It is very probable 

 that at first the motive force which drives the blood 

 from the body of the host into the digestive tract of the 

 parasite is entirely supplied by the contracted body walls 

 of the former, which keep the blood under considerable 

 pressure. If this be the case, it is easy to understand 

 how growth must be at once arrested by external 

 pressure on the body of the parasite, which lessens or 

 may even equalise the pressures in its body and in that 

 of the host. Or, if a pumping apparatus is present in 

 the parasite, the same facts would tend to produce the 

 same results, for in the overcrowded larvae the apparatus 

 would have to pump from a reservoir under high pressure 

 into another also under considerable pressure, instead of 

 into one with much less pressure as in the uncrowded 

 larvae. That the cause of death in such cases is due to 

 simple physical laws is seen in the fact that the middle 

 larva, out of three side by side, is the one which always 

 perishes, being crowded from both sides ; while a larva 

 which developes later in the midst of a group of larger 



