204 MR. A. M. ALTSUN ON THE LIFE-IJISTORY AND 



which is in contact with the fly pupariiun — may be silk or pupal 

 integument, and in those cases examined the tracheal system 

 was visible either outside the silk or between the silk and pupal 

 integument ; no cast skins however were detected, the search 

 was not very thorough. Sometimes the cocoon with the fly- 

 papal integument sliows the outline of the fly-nymph's legs etc., 

 indicating kite hatching of the egg and after histogenesis had 

 proceeded some time. 



The spinning of this cocoon raises an interesting point as to 

 the means whei'eby the larva, which appears to fill the puparium, 

 is able to move about within it, to accomplish the task of covering 

 every part of the inside, and also of absorbing the liquid remains 

 of the fly-nymph, taking into consideration that its intestine is 

 now filled with a semi-solid mass of undigested fly-pupa. It has 

 been observed that the spinning of the cocoon reiUices the size of 

 the larva to some extent, but observation has not definitely 

 established the means adopted ; some larvse have been found 

 contracted into a small compass as though able to revolve within 

 the puparium and thus change their direction, whilst othei'S 

 appear to be working in the manner common amongst silk- 

 spinning Lepidopterous larvas. The latter method is the most 

 probable, and is supported by the appearance of the larva. 



The lining strengthens the puparium considera.bly, and is a 

 sure indication of the presence of the parasite in its last stages. 

 It is weakest at the anterior end of the host puparium, where the 

 chitinized mouth-parts of the blow-fly larva form .an obstruction. 



Tlje length of larval life in the diflerent instars appears to vary 

 considerably, but no details were obtained ; in one instance, 

 however, a, fall-grown larva was found in a puparium 18 days 

 after the egg was deposited. Tliis is the shortest period recorded. 

 On the other hand, some intact puparia, which were from a lot 

 parasitized on 6th August, were opened on 26th November, 

 and 13 were found to contain full-grown healthy larvte, 112 days 

 after the eggs were deposited. The final instar lax-va is the stage 

 where the great variations in the life-cycle occurs. The time 

 taken to pass from this stage to that of the pupa varies from a 

 few days to many months. This point has been observed by 

 Graham-Smith (1), who also observed in 1915 that from material 

 parasitized in 1914, " the individuals emerging in the spring 

 were much larger than those which emei'ged in the autumn." 

 This and the observations made during the course of this work 

 have caused the writer to form the opinion that among the 

 factors responsible — beyond the question of food-supply — is that 

 of the size of the host puparium. The integument of the 

 puparium of an undersized blow-fly larva is much thinner than 

 that of the full-grown larva, consequently the parasite within the 

 former is more susceptible to the variations of the temperatuie, 

 which alone would retard development. 



In size the full-grown larvte vary considerably, but this is 

 naturally dependent upon the size of the host. 



