THE Biotocy oF EPHYDRA SUBOPACA LOEW 591 
and drought raises it. The adaptability to such wide range in salinity 
enables the larvae to survive in frequently changed conditions; and the 
frequent variation of salinity in the season favors the larvae in one way 
or another during different instars, and, as a whole, gives them plenty 
of chance to avoid fatalities. 
Migration experiment.— The physiological significance of the larva’s 
preference for salt water has been shown in the last paragraph. Such 
preference can be shown by its behavior equally well. On this an experi- 
ment was performed: In a metal plate having two series of compartments, 
each series was prepared as miniature aquaria by filling with fresh and 
salt water in alternate order. In each of the aquaria containing fresh 
water, six larvae were placed A narrow piece of cheesecloth was placed 
as a bridge between each two adjacent compartments, with each end 
reaching as far as the center of the bottom of each aquarium. The gentle 
slope along the cheesecloth immersed in the water afforded the larva a 
path for climbing over the bridge. Just one day after they had been 
placed in the aquaria fourteen of the twenty-eight larvae migrated to the 
salt water. Later the fresh water unavoidably became salty through 
the capillary action in the cheesecloth, but the rising salinity in the 
salt water was checked to a certain extent by a little fresh water being 
dropped in from time to time. Finally two larvae migrated back, while 
the other twelve remained. These observations are perhaps too few 
to warrant drawing conclusions, but it may be assumed that osmotic 
pressure acts externally and internally, and that it is a constant stimu- 
lus to the larva when it is subjected to a hypertonic or hypotonic 
solution. The larva is sensitive to such difference from the two media 
through contact with the external medium by its body wall as well as by 
its alimentary canal. Moreover, even within such range (1 to 9 per cent) 
as previously mentioned, in which the larva is able to live, there is still a 
further preference for percentage of salt as an optimal range for each 
individual larva, or at least for each instar, as shown by the larva’s 
migrating back to the water with a salinity of 4 per cent from that of 
8 per cent. 
Factors influencing habits 
Absence of air.— The larva is well equipped with particularly developed 
structures that enable it to stand some of the unfavorable conditions 
