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 dbserved 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 fiying 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. 



