TRANSFORMATIONS OF BUFFALO GNATS. 19 



thickenings of the cuticle known as the spiracles. The spiracles do 

 not seem to be functional as a means of respiration. 



Respiration apparently takes place by what are known as the 

 rectal gills (PL I, fig. 2, rg), which are extensions of the rectal wall 

 and are formed of the same epithelial layer. These extensions are 

 primarily 3-branched, and are either simple or bear numerous lobes. 

 Minute tracheola? penetrate these gills, joining the main trunks at the 

 base of the rectum. The gills are also filled with blood, are retractile 

 into the rectum, but are usually extended in running water. Accord- 

 ing to Headlee, they function both as blood gills and as tracheal gills, 

 as the presence of blood in them is undoubted, and since the trachea^ 

 in them are very minute. The tracheal system can be seen very dis- 

 tinctly in living larvae. Several larva?, still quite active, were placed 

 in a shallow dish of water and examined under the binocular by the 

 aid of strong sunlight against a black background. As the larva? 

 slowly died the trachea?, owing to the air collected in them, showed 

 a bright silvery appearance, so that even the very small ramifications 

 and branches could be clearly distinguished. The larva? seem to be 

 in a comatose condition an hour or so before death, and the silvery 

 appearance of the trachea? remains for about three hours after death, 

 so that there is a considerable period in which the trachea? may be 

 studied before the air is driven out. In larva? preserved in alcohol or 

 mounted in balsam the air, of course, is driven out of the trachea?, 

 which becomes almost transparent, so that it is very difficult to trace 

 them in this condition. The trachea? become greatly ramified toward 

 the caudal end of the body and seem to penetrate the wall of the 

 alimentary tract and to extend into the rectal gills. 



Larva? have not been found to survive longer than forty-eight 

 hours in still water, and on being transferred from flowing water will 

 turn back constantly in an effort to clean the rectal gills with the 

 mouth parts. Larva? placed in still water and only barely covered 

 will survive longer than those covered to a depth of 3 inches or more. 

 That the lack of oxygen is responsible for the death of the larva? 

 transferred from flowing to still water is further evidenced by the 

 fact that the larva? will survive longer in a tightly corked bottle at 

 a temperature of 32° F. than at a temperature of 75° F., because the 

 amount of oxygen that can be held in solution by the water varies 

 inversely as the temperature. Larva? seem to thrive equally well in 

 running water, whether the temperature is 32° or 75° F. Low tem- 

 perature seems to retard but not to injure development, and a rising 

 temperature up to 75° F. hastens development. 



The structure of the rectal gills and the number of lobes afford 

 excellent systematic and specific characters, as may be seen from 

 the accompanying figures (PL IV, figs. 5-9). The variation in indi- 

 viduals seems to be bounded by well-defined limitations. 



