538 Cc. H. RICHARDSON 
cytoplasm contains many small vacuoles and numerous large 
and small canals. Intercellular canals are also present. 
It is highly probable that the salivary glands are the most 
important digestive glands in the body of the parasite. This is 
discussed at greater length below. 
The mesenteron constitutes by far the largest part of the di- 
gestive tract. Its length equals two-thirds that of the body 
and its greatest diameter two-thirds of the width of the body. 
Its cells are of the simple epithelial type, with ovoid, densely 
granular nuclei. The basement membrane is plainly visible, the 
intima very finely striated. The cytoplasm is filled with vacu- 
oles of various sizes, generally smaller and more scattered toward 
the basement membrane. At the juncture of the oesophagus 
with the mesenteron, the cells are somewhat enlarged and pro- 
duced into the lumen to form the peritrophogen. The cytoplasm 
of these cells is highly vacuolated, except near the periphery, 
where it is granular. The minute droplets of chitinous material 
secreted by the peritrophogen are carried backward by the peri- 
staltic movements of the mesenteron to form a rather thick, 
homogeneous peritrophic membrane (fig. 9) which completely 
surrounds the food mass. From its delicate structure one is 
led to believe that it is vestigial in Spalangia. 
The ileum consists of a thin strand of cells with a small lumen. 
Anteriorly where it comes in contact with the wall of the mesen- 
teron, it is wider and there is a layer of several cells between its 
lumen and the basement membrane of the mesenteron. The 
Malphigian tubules appear as small evaginations of the ileum 
just behind the mesenteron. The ileum is constricted just before 
it reaches the colon, a large bulbous structure, possessing a 
somewhat ramified lumen. Beyond this is the long narrow rec- 
tum lined with chitin. 
Although tracheal invaginations are formed during this stage, 
it is evident from their incomplete structure that they are not 
functional. The question naturally arises: How does the Spa- 
langia larva obtain its oxygen supply? Our knowledge of the 
physiological processes in insects is at the present time extremely 
meager and will hardly admit of an adequate explanation in this 
