SOME DIFFICULTIES IN THE LIFE OF AQUATIC INSECTS. 361 
The larva and pupa of the Dipterous fly, Ptychoptera paludosa, exhibit 
some interesting adaptations of the tracheal system to unusual con- 
ditions. The larva is found in muddy ditches, where it buries itself 
in the black ooze to a depth of an inch or two. Here, of course, it can 
procure no oxygen, either gaseous or dissolved. When it requires a 
fresh supply, it must reach the surface with part of its body, and to 
enable it to do so with the least possible exertion, the tail end of the 
body is made telescopic, like that of another and still more familiar 
Dipterous larva, Eristalis. The last segments are drawn very fine, and 
are capable of a very great amount of retraction or expansion. No vis- 
ible opening for the admission of air has been discovered, nor do the 
hairs form a floating basin, as in the Stratiomys larva. The larva may 
be often seen lying just beneath the surface, which is broken by the tip 
of the tail. Whether air can be admitted here by some very minute 
orifice, or whether it is renewed by the exchange of gases through a 
thin membrane, I can not as yet venture to say. In shallow water the 
larva may be occasionally found lying on or in the mud, and stretching 
out its long tail to the surface. In deeper water it often floats at the 
surface. 
Two tracheal trunks run along the whole length of the body, includ- 
ing the slender tail, where they are extremely convoluted and un- 
branched. Toward the middle of the body the trachez become greatly 
enlarged in the center of each segment, the intervening portions, from 
which many branches are given off, being comparatively narrow. Each 
tube therefore resembles a row of bladders connected by small necks. 
A cross section shows that the tubes are not cylindrical, but flattened, 
and that, while the lower surface is stiffened by the usual parallel 
thickenings, the upper surface is thrown into two deep longitudinal 
furrows, so that itis really inflated, becoming circular in section, and 
readily collapses again when the air is expelled. It seems likely that 
the buoyancy of the larva can thus be regulated, and a larger or smaller 
quantity of air taken in as desired. 
The pupa has a pair of respiratory tubes, which are carried, not on 
the tail, but on the thorax, close behind the head. One of these tubes 
is very long, the other very short. The long tube is twice as long as 
the body and tapers very gradually to its free tip. Here we find a 
curious radiate structure, rather like the teeth of a moss-capsule, which 
seems adapted for opening and closing. There is however no orifice 
which the most careful scrutiny has succeeded in discovering. A deli- 
cate membrane extends between the teeth, and prevents any passage 
inward or outward of airin mass. The tube incloses a large trachea, 
the continuation of one of the main tracheal trunks. This is stiffened 
by @ spiral coil, but at intervals we find the coil deficient, while the 
wall of the tube swells out into a thin bladder. However the tube is 
turned, a number of these bladders come to the surface. As the pupa 
lies on the surface of the mud, the filament floats on the top of the 
