PARASITES OF SIMULIUM LARVAE 33 
ing larva which was examined in a cell slide under the low power 
of a microscope. Sections also show the different positions the 
cardia and proventriculus may assume with respect to each 
other (text figure 1, Cand D). The result of these movements 
is that the stout bristles on the basal half of the oesophageal 
fold are brought forward into contact with the newly formed, 
plastic secretion of the cardia. Owing to the direction in which 
these bristles are placed they are able to draw a quantity of 
this material backward when the proventriculus returns to its 
normal position. This material soon hardens and in a subse- 
quent forward movement of the proventriculus the bristles are 
withdrawn from the membrane only to become re-entangled with 
it at a point nearer to its origin, so that any subsequent retrogres- 
sive movement will draw more of the secretion backward till 
the circular membrane thus formed overhangs the orifice of the 
proventriculus and entirely surrounds the food, which is being 
passed from here into the lumen of the mesenteron. 
I have not examined in detail the musculature which is in- 
volved in these cardiac movements, but believe that the whole 
mesenteron is contracted by a series of external longitudinal 
muscle fibres which extend along its entire length (pl. 6, fig. 9). 
This would also account for the wrinkling up of the peritrophic 
membrane which is often seen in sections of the midgut. 
In Simulium larvae the peritrophic membrane is, as previously 
stated, exceptionally thick and well developed. This is almost 
certainly due to the fact that these larvae live to a great extent 
on diatoms and other siliceous matter which, but for the protec- 
tion afforded by such a membrane, would be very liable to damage 
the walls of the mesenteron. In these larvae the membrane 
also remains, for the greater part, intact in the proctenteron, 
still closely investing the food till it is voided from the anus. 
That large quantities are continually being formed is evidenced 
by the fact that the larvae pass faecal matter very frequently 
and that the faeces are always enveloped in a plentiful supply 
of the membrane. The membrane is doubtless impervious to 
anything but liquids as was emphatically stated by van Gehuch- 
ten (90) who concluded that the digestive fluids and the products 
