June, 1912.] AlDRICH : BlOLOGY OF WESTERN EpPIYDRA. 95 



Idaho: Alarket Lake (overflow from irrigating ditch). 

 Nevada: Hazen (overflow from irrigating ditch). Winnemucca 

 Lake (alkaline). Walker Lake (alkaline). 

 California: Mono Lake (seepage near lake). 

 Washington: Soap Lake. Grand Coulee (alkaline). 



Habits. 



Comparing my observations of this species with those on the two 

 preceding, a striking difference appears in the density of the water 

 in which they live, as suhopaca occurs only in the less dense waters, 

 often in those with a specific gravity scarcely more than i. The 

 other two, as already stated, seem not to occur except in the dense 

 water or near it. 



In Box Elder Lake, a shallow expanse of slightly salty water two 

 miles north of Brigham, Utah, I found this species, the first Ephydra 

 of my trip. Using a hydrometer, I attempted to find out the density 

 preferred by the species, and here I learned my first lessen, empha- 

 sized all the way afterward — that in all these bodies of salt and 

 alkaline water, the density is subject to great fluctuations, and the 

 insects are able to adapt themselves to these. Along the side of the 

 railroad grade a stream of waste irrigating water ran into the lake; 

 it was possible to find all densities from r to 1.019, and the larvae of 

 the fly, though not numerous, seemed at home evervwhere, or at least 

 to a density of i.ooi. A few days later, near Garfield, Utah, subopaca 

 was found around waste water, sp. gr. i.oo; but it was seepage that 

 perhaps contained too little alkaline matter to be detected by my 

 instrument. I have recorded under distribution several cases like 

 this, and the interesting question is still unsettled whether some small 

 amount of salt or alkali is net a necessity in water that is to support 

 larvae of this species. The adults are often found at fresh water if 

 it is near a denser supply; in the road near the Mono Lake post- 

 oflfice this was especially in evidence, where there was a seepage of 

 very pure water with plenty of suhopaca adults standing on it (the 

 adults generally stand on the water, not on the shore). 



At Bodie, Cal., an old mining camp on the top of a mountain 

 range at an elevation of 8,400 feet, there were specimens of what I 

 took to be this species (certainly an Ephydra) standing on the surface 

 of reddish seepage water from a manure pile. 



