222 THE UNIVERSITY SCIENCE BULLETIN. 



creek. Since this Bool's Backwater was surveyed nearly every day 

 from earliest spring to near the end of the season, the field notes taken 

 in that connection afford a splendid outline for the development of this 

 species in nature. 



It would appear that this boatman prefers permanent and quiet but 

 fresh waters, perhaps best suited to its needs if some part of the body 

 of water has depth. Along the shores of Beebe lake, only a few hundred 

 yards distant from Bool's Backwater, a large form of this same species 

 was found. The two bodies of water are connected by Fall creek, and 

 the presence of two very distinct sizes seems remarkable. The larger 

 form was never taken in Bool's Backwater, but a very few of the 

 smaller form were taken in Beebe lake, along with the much more 

 abundant large variety. Aside from the decided differences in size, the 

 females are much alike. The males, however, present differences in the 

 arrangement of the palar pegs and in the strigil and abdominal segments. 

 From the material examined, it would seem that the pegs of the smaller 

 form show a greater tendency to crowd out of line, thus forming two lines 

 at one or both ends of the series. The smaller species has a larger, 

 coarser strigil than the other, while the right lateral margin of the 

 abdominal segments are more spinously produced in the larger form. 

 These points are figured on plate XXVII, which see. 



Life Cycle in Nature. 

 Overwintering nymphs were so abundant in Bool's Backwater and 

 other species so rare that it was determined to follow the development 

 of this unusual Corixid in this way. Collecting in early April brought 

 forth only fourth instar forms. April 18, while the waters were still 

 quite cold, the A. alternata present were mating and the P. buenoi were 

 still in their overwintering stage. There was no change in matters until 

 May 2, when a few in the fifth instar were taken. A count of one catch 

 showed 73 per cent still in the fourth instar. It was on this date I 

 noticed the May fly nymphs, which I have called the "scooter." This 

 nymph is mottled, with about the same shades as a Corixid nymph, and 

 is fully as inconspicuous. They occupy the same foraging grounds, and 

 it is the protective resemblance and agility in both that insure their 

 perpetuity in the world. On May 9 adults of A. alternata and the over- 

 wintering nymphs were notably "mited." A collection of the A. alternata 

 showed 85 per cent of them to carry the water mites. By May 19 most of 

 the nymphs were in the fifth instar. May 21, 97 per cent of the catch 

 were in fifth instar. In a count of 300 nymphs, only one per cent of them 

 were infested \vith mites, and in this case only one mite on each. Those 

 infested with mites were delayed in molting. June 4 was a fine, warm 

 day, following two or three before it, and the first adults of the P. buenoi 

 were taken. There were 95 per cent still in the fifth instar. The first 

 instar nymphs of A. alternata were taken on this date, which indicates 

 the ease with which the overwintering species could be followed up to 

 this time. Most of the adults of P. buenoi were new fledged, as indicated 

 by the softness of their integument. In fact, some were creamy yellow, 

 showing that transformation had just taken place. Another item of 



