106 PROCEEDINGS OF THE ACADEMY OF [March, 



respiration. Other organs must take up a respiratory function after 

 the removal of the caudal "gills" from the body, but this idea is 

 scarcely conceivable since it would be almost impossible for the 

 animal to adjust any such structures in so short a time. Since 

 these other organs seem especially adapted to a respiratory function, 

 their existence makes the possible respiratory function of the 

 caudal gills seem quite improbable. 



II. The rectal folds or gills. The structure of these, associated 

 with the observed intake and outgo of water into the rectum through 

 the anus, seems to warrant their inclusion in the consideration of 

 possible respiratory functions. The only other function which could 

 be assigned to the intake and outgo of water into and from the 

 rectum is that of a locomotor one. This, however, is rather improb- 

 able for several reasons. These larvae, when moving in the water, 

 do so by a worm-like wriggling of the body and do not "shoot" 

 through the water as do the Anisopterous larvae where the .rectum 

 does have a locomotor function. The water when expelled from 

 and taken into the rectum does not create currents strong enough 

 to propel the larva through the water. They are produced when 

 the larva is at rest and in this case do not move it. 



The function of respiration, then, can be assigned to the rectum, 

 until another possible means for this function can be found. The 

 possible respiratory function of the skin will have to be considered 

 before one can draw a final conclusion, however. 



III. The skin. The skin, or hypodermis and chitin, of the 

 larvae, varies in thickness and hardness according to the length 

 of time from the preceding moulting period. Just after exuviation 

 the chitin is soft and quite transparent. Soon it becomes hard, 

 thickens and becomes quite dark in color. Ramifying throughout 

 the surface of the larva are innumerable, fine, thread-like tracheae 

 which join larger branches and finally these latter unite with the 

 main tracheal trunks. The thread-like tracheae being found over 

 the entire surface of the larva's body, afford a very large area exposed 

 directly to the water by which the larva is surrounded. Thus the 

 structure of the skin seems to warrant its consideration as a means 

 of respiration. 



IV. Spiracles. Spiracles on the thorax and abdomen are sup- 

 posed to function when the larvae crawl up out of the water into the 

 air. This idea, perhaps true, need not be considered here because 

 we shall only deal with respiration as carried on when the larvae 

 are actually submerged. However, when the larvae come to the 



