72 
Experiments with Flies 
having had no time either to expand its wings or void the fluid waste products of 
the pupal period, the semi-fluid contents of its body were greater in volume and 
differently coloured from those of the other flies experimented on. It was no doubt 
owing to the fact that its body was still wet and that it had not undergone the 
normal purge, that it sank while in the control tube.) 
The first tube remained sterile, the second showed a strong growth of B. pyo- 
cyaneus in 12 hours. 
The slowness of growth in those of the controls which were infected 
compared with the rajnd development from crushed pupae points to 
some slight passage between the interior of the puparium and the 
surrounding fluid when the insects were allowed to remain in broth for 
such lengthy periods as 18 hours. The view I have expressed as to the 
cause of slow and late cultural development being due to different 
sources of infection is supported by the later experiments on flies and 
there seems to be no doubt but that the ‘pupal gut is heavily infected. 
With the adult flies the suggestion is that they are infected internally 
on emergence. The rapidity of the growth from the freshly emerged 
specimen, which had had no opportunity of voiding the contents of the 
gut, compared with those that had this opportunity, points clearly to 
the infection of the gut and affords additional support to the correctness 
of this conclusion in regard to the pupa. 
The very rapid development of the growth in the control experi¬ 
ments with “ fired ” pupae may have been due to the heat producing 
an outward current of air or vapour through the air passages, so 
that when the insect was plunged into cool broth immediately after 
“ firing ” an inward suction of the media occurred owing to the partial 
vacuum set up. It is also possible that the pupae used in Exp. 4 
were actually ruptured by the heating process, since three others 
treated at the same time flew off the platinum loop during the firing 
process, a fact which suggests that a considerable vapour pressure 
was developed within the pupa caseb 
Quite apart from the suggestiveness of this result and its bearing 
upon my special line of researcb, the fact that organisms ingested by 
the larva with its food may persist throughout the pupal stage and still 
be present in the adult insect is of importance. It is of significance in 
connection with typhoid and infantile diarrhoea, as the possibility of the 
larvae being reared in infected excreta may render the danger acute. 
’ Later experiments show that if the ends of the puparia are varnished, so as to 
block the stigmata, that after being allowed to soak in 10°/o solutions of lysol or formalin 
for periods up to 39 hours and then in control tubes of sterile broth for further periods 
up to 14 hours, the pupae will still give a positive result when cracked or pierced in the 
culture tube. The control tube remains sterile. 
