Packard.] INSECTS OF THE POND AND STREAM. 147 
enlarging towards the head. So slightly developed, how¬ 
ever, is the tracheal system in Chironomus, and so thin are 
the walls of the body, that I am inclined to think that these 
nearly transparent larvae breathe in part through their skin. 
Now we have in the singular ghost-like larva of Corethra 
another plumed gnat, a being which has no spiracles nor tra¬ 
cheae, and which breathes, as Weissmann says, through the 
skin. The air thus absorbed is contained in four reservoirs, 
forming swimming bladders, and thus the density of the 
water is measured by this living hydrostatic apparatus. 
Two of these kidney-shaped bladders are lodged in the tho¬ 
rax and are larger than the two near the end, in order to 
support the heavier front end of the worm. These singular 
larvae may be found in winter in ponds by breaking through 
the ice, as well as the Belostoma and various water beetles, 
and can be kept alive in jars of water. 
The finger-like appendages we have described in the larva 
of the plumed gnat afford the simplest form of “tracheal 
gill.” Did the blood penetrate into them and accompany 
in closed vessels the air tube, it would be exactly comparable 
with the gills of fishes and larval amphibious reptiles; but 
it does not; it is not a true gill, and the term “false gill” 
or “tracheal gill” has been applied to this organ. 
From the young Chironomus, with its four tracheal finger- 
shaped gills attached to the extreme end of the body, we 
may pass to a singular larva of a European crane fly, 
called Cylindrotoma, which according to DeGeer breathes 
by means of numerous hollow flexible filaments scattered 
over the body, and which, as Westwood says, appear to be 
traversed by tracheae. A similar looking creature is the 
caterpillar of Paraponyx, which respires under water by 
means of a number of fasciculate filaments situated on the 
sides of the abdominal rings. 
In fact this caterpillar has been anticipated by the case 
worms, the young of the Caddis flies, in which the caterpil- 
19 
