THE BroLtocy or EpHYDRA SUBOPACA LOEW 589 
of these two factors, but the stagnant and shallow conditions, neverthe- 
less, remain ever favorable to their life. The existing conditions at the 
overflowed areas, likewise, give a good evidence of the larva’s preference 
‘in such a habitat. The water in these areas, like that in the pools, 
is always still and never more than 10 centimeters deep, and here are 
found the larvae in great abundance. Over the surface, thick brown scums 
for sheltering and feeding make another favorable addition for the 
inhabitants. 
The running water in a narrow creek near this location has not been 
permanently inhabited by the Ephydra larvae. Once or twice, after 
several days drought, when the water: was reduced to a low mark and 
became fairly stagnant at its shore, adults began to gather around and a 
few young appeared, crawling at the muddy bottom. But not long 
afterward, when a rain brought the water up to its original depth, flies 
were no longer able to alight on the surface and all the larvae disappeared 
in the rapid currents. In the pools, the ditches, and the overflowed 
areas at the salt works in Syracuse, similar conditions exist. At different 
points where the water runs in a low stream, some larvae were found 
drifting along without being able to make a stop or to direct themselves 
to shift to a favorable recess. 
Preference for salt water 
General range of percentages of salt in water— Some marine animals 
placed in fresh water have their blood and body fluid disturbed through 
osmotic pressure; consequently death may ensue before osmotic equi- 
librium is established. On the other hand, if the percentage of salt in 
the water is greater than that in the animal’s blood and body fluid, the 
same result also may follow. So neither hypertonic nor hypotonic solution 
is suitable for the larvae to live in, but between them there is a general 
range of percentages of salt that serves as an optimal medium. To this 
physiological factor is largely due the distinction in adaptation between 
the salt- and fresh-water animals. The ful'y grown larva of Ephydra 
subopaca, however, does not seem to be materially affected by either salt 
or fresh water. It is partly due to the circumstance that it has stopped 
feeding when pupation is imminent, and partly due to the condition of the 
body wall, which is gradually hardening to a puparium and has become 
impermeable. Thus the larva has been enabled, to a certain extent. to 
