1186 
Journal of Agricultural Research 
Vol. XXVIII, No. 12 
There is, however, no evidence that cells or parts of cells of the epithelium are 
set free in the lumen, as described by Snodgrass (46) for the imago. The cyto¬ 
plasm of the cells of the mid-intestine is always deeply stained in sections (Pl. 5,. 
A, Mint) and has the appearance of being traversed by numerous parallel 
fibrillae which run at right angles to the surface of the epithelium. A similar 
appearance is found in the secreting cells of the silk gland in certain Lepidoptera 
(12) and in the epithelial cells of the mid-intestine of Ptychoptera (11). The 
nuclei are spherical, as a rule, and contain a large number of chromatin granules. 
In fixed material, at least, these granules, along with the achromatic material, 
are agglomerated into a dense spherical mass within the nuclear membrane, 
leaving a peripheral space vacant. This condition may, however, be an artifact. 
On their mesial surfaces the cells of the mid-intestinal epithelium show a well- 
developed striated border (PI. 5, B, StrBor) similar to that observed in Ptychop¬ 
tera (11) and other insects. Wedged in between the bases of the cells, next to 
the basal membrane (intima) triangular groups of minute cells are seen here and 
there. These are the imaginal cells of the mid-intestine (PI. 5, B, and C, ImCls ). 
The mid-intestine is lined throughout by a homogeneous layer of unknown 
chemical nature, apparently of gelatinous consistency, and having a thickness 
two or three times that of the epithelium, the so-called “peritrophic mem¬ 
brane’ ’ (PI. 5, C, Pmb). In sections it is more or less finely granular in struc¬ 
ture. Peripherally it is in intimate contact with the epithelium of the intestine; 
centrally it is sharply demarcated from the enclosed food mass. Not a little 
has been written concerning this somewhat problematical structure in various 
insects, but a full review of the literature will not be attempted here. Rengel 
(44) has described it in the larvae of the wasp (Vespa) and honeybee, Strindberg 
(47) in the ant embryo, and Snodgrass (46) and Petersen (41) in the adult 
honeybee. Rengel states that in larvae the peritrophic membrane has a lami¬ 
nated structure consisting of several concentrically arranged membranes, as is 
plainly the case in the adult, but in all of the writer’s preparations of this mem¬ 
brane he has never observed this structure except at the anterior end of the 
mid-intestine. Elsewhere the membrane is thick and homogeneous, but here 
the epithelial cells grow rapidly and progressively smaller, forming a ring around 
the oesophageal valve (PI. 5, C, x) and are separated from the latter by a narrow 
space. 4 These cells also differ from those of the remainder of the mid-intestine 
not only in being of smaller size but also in lacking, at least to a large extent, 
the striated appearance of the cytoplasm. From the surface of these cells thin 
streams of secretion can plainly be seen running caudad and joining with the 
principal mass of the peritrophic membrane within the mid-intestine. Streams 
of this material also appear leading inward from the three rows of epithelial 
cells next to the ring of smaller cells, as shown in Plate 5, C, and this appearance 
is characteristic of all of the sagittal sections examined, including preparations 
of both young and old larvae. It suggests that the cells of the anterior rim and 
those of the two or three adjacent rows are responsible for the secretion of the 
peritrophic membrane; an interpretation precisely in line with the view ad¬ 
vanced by Van Gehuchten (11) in the case of the dipterous larva Ptychoptera 
and Cuenot (8) in Orthoptera. Strindberg, however, has advanced against this 
view the fact that in the ant larva peritrophic membrane is continuous over the 
closed posterior end of the mid-intestine, which, he states, could not be the 
case if the membrane were secreted by an anterior rim of cells. This objection, 
however, would have less weight if, as seems probable, the peritrophic membrane 
is fluid or semi-fluid, since from its nature it would tend to form a uniform layer 
over the area of contact and therefore would flow together over the posterior 
end. It should be noted that the peritrophic membrane described above has 
nothing whatever to do with the cuticle secreted by the oesophagus, the free 
