HABITS OF TWO PARASITES OF BLOW-FLIES. 207 



The aperture made in the pupavium is usually just large 

 euough to allow the insect to extricate its head and antennae, 

 thorax and legs, and pai't of the wings and the base of the abdo- 

 men, when by the pressure exerted against the distended 

 abdomen, the voiding of the meconium is assisted, and is either 

 accompanied or followed by a white discharge — the white particles 

 previously referred to, — which instantly dries and has a cement- 

 like appearance. This substance, upon analysis, has been found 

 to contain sodium hydroxide, but whether some of this is used 

 to soften the silk has not been established. The compressed 

 meconium, which is a deep black-brown rod, is completely 

 enclosed in a sac of integument, which resists boiling in caustic 

 potash, giving the impression that it is chitinized and probably 

 the larval mesenteron, which during metamorphosis has become 

 closed at the anterior end and compressed. 



In some instances the exit-hole is made too large, and the 

 adult appears with its abdomen still distended ; then by constantly 

 passing the tarsi of the hind -legs along it, it exerts sufficient 

 pressure to enable it to void the meconium : this method is not 

 always successful, and results in one or two days in the death of 

 the insect. Flight is impossible with the meconium unvoided, 

 although vain efforts to rise have been observed. In some 

 instances the transverse cut made with the mandibles is extended 

 so far around the puparium that, on emerging, the insect com- 

 pletely breaks off the top of the puparium. Emergence generally 

 takes place from the cephalic end of the puparium, but in several 

 instances it has been observed that this was accomplished from 

 the posterior end. 



The wings are always fully extended before the adult attempts 

 to break out of the puparium. Only in a very few cases has it 

 been observed that the wings Iiave become damaged and torn by 

 the jagged edges of the emergence hole; this condition is generally 

 accompanied by injuries to the soft integument between the 

 abdominal plates, resulting in the death of the insect in one to 

 two days. 



Examination of intact puparia some time after the emergence of 

 the parasites has shown that for some reason unascertained — not 

 due to temperature — a small number never succeed in escaping, 

 whilst a few have been found with the cocoon-lining too thick to 

 extricate themselves. 



Nothing of the pupal exuvium of those insects, which void the 

 meconium whilst escaping, can as a rule be seen ; it is generally 

 buried under it, whereas the others are usually found with it 

 attached to the apex of the distended abdomen. Males emerge 

 before the females. The greatest emergence of males is from 

 3 to 4 days earlier than the females. 



The Sexes. — Size : — The size of the adults varies considerably, 

 but is, of course, relative to the size of the host. The measure- 

 ment of several discloses a range of from 2| mm. to 6| mm. in 

 lenyth. 



