THOMPSON: ANATOMY OF MOSQUITO. 
189 
producing a spongy appearance, the striated border is dissolving, 
and occasionally cells protrude into the lumen of the canal. Be¬ 
tween the bases of the older cells the regenerative nuclei occur in 
increasing numbers. With the larvae of Tenebrio molitor (van 
Rees, ’88) the regenerative nuclei are found even in the first instar. 
I do not know how early they appear with the mosquito wriggler. 
In the last larval instar and the newly formed pupa they are very 
numerous. As these nuclei increase in number, they diminish in 
size, which suggests that they are derived by repeated divisions 
from those present in the larval stage. 
The changes advance swiftly after pupation. In a pupa one hour 
old (pi. 14, fig. 28) the epithelial cells are separating from one an¬ 
other, are elongate and protrude into the lumen, the striated border 
has been everywhere lost, and the cytoplasm is dissolving. By the 
third hour of pupal life, the regenerative nuclei, which are now 
about the size of the epithelial cells of the gut of the imago — less 
than one half as large as the regenerative nuclei of the larva — have 
forced the older cells from the basement membrane and form a defi¬ 
nite pavement epithelium with recognizable cytoplasm. The older 
cells lie in fused masses in the lumen of the gut, their nuclei reduced 
to dark granulate spheres. Here they are rapidly absorbed (pi. 16, 
figs. 40, 43, ca). This metamorphosis does not proceed with equal 
celerity in all regions of the midgut. The caeca, the cardia, and the 
iliac end of the stomach lag behind the major part of the stomach. 
The caeca decrease in size as their epithelium histolyzes, and by the 
fifteenth hour they have vanished. The epithelium of the cardia 
degenerates first at the posterior end of the chamber. The new 
epithelium, unlike that of the caeca and the stomach, consists of 
columnar cells from the beginning and appears to be formed as an 
advance forward beneath the older cells (pi. 16, fig. 38, ca ), instead 
of simultaneously over all parts of the wall. 
At the beginning of pupal life the hind gut regions, ileum, colon, 
and rectum are the same as in the mature larva, except that they 
are shortened and that a ring of small dark cells lies between the 
rectum and the anus (pi. 17, fig. 54, r an). This ring develops 
during pupation and the first hour of pupal life, but whether its 
cells are derived from the cells of the posterior end of the rectum 
or from cells of the liypodennis in the region of the anus could not 
be determined. The rectum and ileum are involved in degenerative 
O 
