Packard.] IXSECTS OF THE POND AND STREAM. 147 



enlarging towards the head. So slightly developed, how- 

 ever, is the tracheal system in Chironomns, and so thin are 

 the walls of the body, that I am inclined to think that these 

 nearl}-- 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 "NYeissmann saj's, 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 

 larvie 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 bodj', 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 trachcic. 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 anticipjited by the case 

 worms, the young of the Caddis flies, in which the caterpil- 



19 



