178 THE UNIVERSITY SCIENCE BULLETIN. 
upon the fact that they were never seen to prey upon the little Crustacea 
present nor were these forms depleted in numbers appreciably. 
The writer has had them under close observation many times and 
has frequently seen them catch and devour Ostracods and other small 
Crustacea. The process has been observed under binoculars and differs 
little from that of any other back-swimmer. The prey is grasped by 
the fore limbs and the victim rolled about until the stylets locate a vul- 
nerable point. Several small creatures have been captured and destroyed 
one after another by some Pleas under observation. Their relation, then, 
to the aquatic complex of life is that of a consumer of small crustacean 
creatures that abound in the same shelter of the submerged vegetation. 
They do spend much time clinging closely to the plant filaments, and 
when they leave it they go clipping off in a manner indicative of a definite 
goal. Their locomotion is even and rapid, but the journeys not extended, 
for they are content, as a rule, to dodge from the shelter of one stem to 
another. Their hind limbs are not heavily fringed, as is the case with the 
other genera, and their gait through the water an even scoot, rather than 
the jerky motion of the others, which use only their hind limbs in strong 
propelling strokes.* 
The writer has read that the hind wings do not exist and that the 
front ones are united into one. 
According to this, their distribution would be restricted to perma- 
nent pools to which they had been carried, as adults, attached to 
transported mats of aquatic vegetation, or perhaps entangled in mud 
upon the feet of birds or beasts, or as eggs upon or within their sup- 
porting plant growth. 
However, the writer wonders if a few may not be found which have 
functional flying wings, for of two specimens placed in a glass tumbler 
in which the water was one-half inch from the top, one disappeared over 
night. A study of many specimens shows that the fore wings are not 
united along the median line. They meet and interlock by a device 
figured on plate XXV. The hind wings are always represented by. 
distinct pads as shown in the figure on plate XXV. Sometimes these 
pads are of fair size, and until a very full series has been examined, the 
writer does not believe it can be said that they may not occasionally 
possess truly functional wings. A genus of Corixids was once described 
as having hind wings aborted, but among them the writer has found well-. 
developed flying wings, a point confirmed by Dr. Abbott, who created 
the genus. 
The egg stage has not heretofore been observed. The writer brought 
in a number of Plea from the field station on July 17, and placed them ~ 
in an aquarium with Elodea and Chara. Their eggs were inserted in the 
tissues of the plants as shown on plate XXV. A description of the egg | 
follows: 
* Wefelsheid records a diurnal migration from 2 meters from shore in day time to 
near the bank in evening. 
