382 Annals Entomological Socie.y of America [Vol. VI, 



ADJUSTMENT TO ENVIRONMENT SHOWN IN THE STRUCTURE OF 



THE GILLS. 



The gills of May-flies are especially susceptible to mod- 

 ification by the character of their surroundings. They are 

 usually large and prominent. In other aquatic insects gills 

 are less directly exposed. Those of stone-flies are generally 

 tucked behind the legs upon the ventral side of the thorax, 

 and those of damsel flies at the hind end of the body. Most 

 May-flies have seven pairs of gills, one borne at each postero- 

 lateral angle of the first seven tergites. They are usually 

 large, sometimes unweildy and always a conspicuous feature 

 of the body. Situated as they are, they extend along the whole 

 side of the abdomen and brush against everything with which 

 it comes in contact. 



The gills of Leptophlebia are the most generalized of any 

 which have been examined. They appear to lack modi- 

 fications both for respiration in any particular situation or 

 for protection. The seven pairs are identical in shape and 

 nearly so in size. Each one is entire at the base, but deeply 

 cleft into two long narrow divisions which lie in one plane. 

 Their surfaces are without markings or local thickenings. 

 One large trachea enters the gill and sends a branch to each 

 of its divisions. In these there is but a scanty supply of 

 tracheoles. The attachment to the abdomen is exposed 

 above and below so that the only protection for the gill is in 

 the ease with which it may be detached and regenerated. 



In Blasturus the first pair of gills are like those of Lep- 

 tophlebia, but the other six pairs are broadened so that a much 

 greater respiring surface is provided. At the base a trachea 

 enters and splits once, but each arm gives off a good number 

 of branches which supply the whole surface of each gill division 

 or lamella. The two lamellae do not lie in the same plane, 

 but the outer one is twisted over at the base and lies on top of 

 the inner. A 'do.uble gill made of two overlapping lamellee 

 is thus formed. A variation of this same kind of development 

 is shown in the gills of Choroterpes. These gills have neither 

 ribs, nor bands upon their margins. In consequence of this 

 they hang limply from the sides of the body, but the main 

 tracheae provide some leverage for the muscles, and the gills 



