well shown in Table 85. At each station the mayfly larvae were 

 more abundant in late June than at any later time. In fact the 

 reduction between late June and early Au^^ust was on the average, 

 •nearly $0 per cent. In all probability, then, if samples had been 

 taken throughout the year of 1930, or if as many samples had been 

 taken before as after emergence, the Hexagenia count would have made 

 up well over 90 per cent of the total count of bottom organisms. 



The explanation for the greater abundance of Hexagenia 

 in 1930 than in 1929 is not known, bi)t there is no reason to doubt 

 that the difference in the two seasons was real rather than the 

 result of inadequate or improper sampling. It is highly improbable 

 that the abundance was the same in the two years, and that the times 

 of sampling in relation to emergence of the insects was responsible 

 for an apparent difference in abundance. If the samples of 1930 had 

 been taken earlier in the season than those of 1929 (that is, 

 principally before emergence in 1930, and principally after in 1929), 

 the observed difference would be expected. But in both years the 

 sampling period was the same (June l5-September l5), and successive 

 samplings were quite uniformly distributed through the season, A 

 second reason for believing that emergence was not involved is that 

 the season of 1930 was earlier than 1929 "with respect to temperature, 

 and emergence should have taken place earlier in the warmer season. 

 If emergence id take place unusually early in 1930, we should 

 expect lower average counts in that year than in 1929. The fact that 

 the opposite was found, strongly suggests that Hexagenia was actually 

 more abundant in 1930 than in 1929. 



It is evident from the large number of Hexagenia nymphs and 

 small number of tubificid worms that the bottom was not polluted, 



A review of the literature on North American lak« shows that 

 Western Lake Erie occupies a unique position by virtue of the over- 

 whelming abundance of Hexagenia in its bottom fauna. Lake Winnepeg 

 is the only other lake which has come to the attention of the writer 

 that even approaches Western Lake Erie in this respect. Neave (1932) 

 studied the two species of Hexagenia in this lake, and found them 

 abundant, although he did not state their abundance relative to other 

 forms. Samples taken before the annual emergence showed an average 

 of 68 per square meter in the southern part (average depth, 12 meters) 

 and 137 per square meter near the Narrows. These figures may be com- 

 pared with an average of 283 (in 1929), and 5lO (in 1930) in Western 

 Lake Erie, based on samples taken principally a fter emergence. It 



263 



