PARASITES OF SIMULIUM LARVAE 49 
ing it up from the stones and débris of the stream, instead of, 
as at later stages, by simply ingesting what is strained out by 
their cephalic fans from the rapidly flowing current. 
Sections of the alimentary tract of Simulium larvae are of 
peculiar interest in connection with the peritrophic membrane, 
which, owing doubtless to the siliceous nature of the food (Dia- 
toms), is exceptionally thick and well developed. As I am led 
to believe that the presence of this thick membrane is of impor- 
tance in connection with oarasitism, and since it also shows some 
unique modifications in development, I will describe its forma- 
tion and structure in some detail. 
b. The peritrophic membrane 
Text figure 1 shows diagrammatically at A a median longitud- 
inal section through the proventriculus and anterior portion of 
the mesenteron of a larva. From this it will be seen that at 
the junction of the stomenteron and the mesenteron the former 
has been pressed into the lumen of the latter to such an extent 
that its wall has become everted for a considerable distance, so 
that it folds back upon itself, thus bringing the internal surface 
of the reflected portion into contact with the first twenty to 
thirty cells of the mesenteric epithelium. It is from this band 
of mesenteric cells, which is termed the cardia, that the peri- 
trophic membrane (p.m.) is produced. The cells are much mod- 
ified in size and shape and are sharply marked off from the more 
posterior mesenteric epithelium. So great is the modification that 
there has long been a discussion as to whether the cardia is of 
stomodeal (ectodermal) or mesenteric (entodermal) origin. Miuall 
and Hammond (’00) have shown, from a study of the embryol- 
ogy of Chironomus that in this fly the cells of the cardia are 
undoubtedly of entodermal origin, and there is no reason to 
doubt that this is also the case in the allied family Simultidae. 
The junction, then, between the stomenteron and the mesenteron 
is situated immediately anterior to the cardia, and the reflected 
portion which faces the cardia represents the extremity of the 
oesophagus. I shall refer to this portion of the oesophagus in 
JOURNAL OF MORPHOLOGY, VOL. 24, No. 1 
