PARASITES OF SIMULIUM LARVAE 47 
STRUCTURAL PECULIARITIES AND HABITS OF SIMULIUM LARVAE 
BEARING ON THE SUBJECT OF PARASTITISM 
Simulium larvae are particularly difficult insects to study in 
the living condition because it seems to be impossible to keep 
them alive for any adequate length of time in captivity. There 
seem to be two reasons for this. It may be due simply to the 
fact that the respiratory gills are very small and are unable to 
extract sufficient oxygen from stagnant water, for it will be 
remembered that the larvae live only in very: fast flowing water. 
When captive larvae are closely watched they are seen to pass 
faecal matter very frequently and this activity is accompanied 
by very evident signs of hunger, for the larvae do not remain sta- 
tionary as before but turn their heads rapidly in all directions 
and seem to be searching about on the bottom of the receptacle, 
in which they are placed, for food. I have even seen them turn 
and re-ingest their own faeces as soon as they had been expelled. 
If small Chironomid larvae are present in the water, as is fre- 
quently the case, the Simulium larvae will rapidly sieze upon, 
and attempt to devour them, though owing to the peculiar mod- 
ification of the mouth parts they never appear to succeed in 
these attempts. It seems from these facts that hunger is very 
detrimental to the larva, and the probability of this statement 
is heightened by the fact that larvae which have died in captivity 
very rarely have any food in the mesenteron. This region of 
the alimentary tract is then filled with some secretions, probably 
digestive, which coagulate in the warm fixing fluid. Although 
life may be prolonged for several days in healthy larvae it is 
difficult to keep parasitised individuals alive for more than two 
days at the most. For this reason I have been unable to carry 
through any successful experiments on reinfection by the parasites 
or in transferring them to other species of larvae. Such experi- 
ments, will, it seems, have to be performed in the field and could 
probably be more readily accomplished in the spring when the 
larvae are most abundant. 
It is interesting to note, in passing, that the majority of Simul- 
ium pupae fail to hatch in captivity. Many of those which I 
