PSE UDONE UROP TERA. 65 



generation. The greater part of this time is passed, however, beneath 

 the surface of water ; and after the insect emerges into the air and 

 assumes the adult form its existence is very brief. With many 

 species the individuals leave the water, undergo two transformations, 

 mate, lay their eggs, and die in the course of an evening or early 

 morning. And although the adults of many genera live several days, 

 the existence of these insects is very short compared with members 

 of other families. 



The females lay their eggs in water. Some short-lived species dis- 

 charge the contents of each ovary in a mass. Specimens are often 

 found in which there project from the caudal end of the body two 

 parallel subcylindrical masses of eggs, one protruding from each of 

 the openings of the oviducts. Egg-clusters of this kind "laid upon 

 the water rapidly disintergrate, so as to let the eggs sink broad-cast 

 upon the river-bed. The les's perishable species extrude their eggs 

 gradually, part at a time, and deposit them in one or the other of 

 the following manners : either the mother alights upon the water at 

 intervals to wash off the eggs that have issued from the mouths of 

 the oviducts during her flight; or else she creeps down into the 

 water enclosed within a film of air, with her wings collapsed so as to 

 overlie the abdomen in the form of an acute narrowly linear bundle, 

 and with her setae closed together to lay her eggs upon the under 

 side of stones, disposing them in rounded patches, in a single layer 

 evenly spread, and in mutual contiguity." (EATON.) 



With most May-flies the general form of the body in the immature 

 stages resembles somewhat that of the adult. The newly-hatched 

 nymphs respire through the integument at large. During the first 

 few days after their birth the young cast their skin several times, 

 the intervals between the moultings lengthening by degrees. Rudi- 

 ments of tracheal gills begin to appear when the insect 

 is eight or ten days old ; they bud forth from the hinder 

 lateral angles of some or all of the first seven abdominal 

 segments ; and, like the parts of the mouth, are modi- 

 fied considerably in detail before they acquire their 

 ultimate shapes. They may be either thread-like, 

 tufted, or plate-like in form. In Oligoneura and folia 

 there is a pair of tracheal gills attached to the base of 

 the maxillae. This is a striking exception to the general 

 rule that external organs of respiration do not appear 

 on the head of insects. Fig. 65 represents a nymph of a May- 



