NORTH AMERICAN SPECIES OF BRANCHINECTA. 255 



eye, and by the greater number of eggs in the ovisac ; by the 

 absence of the basal tubercle on the second attennae of the male, 

 by the difference in the tips of the second attennae, and by the 

 longer and more slender caudal appendages. 



It can be distinguished from B. paludosa by the absence of the 

 teeth on the inner margin of the basal division, and by the 

 inturned distal end of the second division of the second antenna. 



Of the 82 females of B. coloradensis only two had both caudal 

 appendages perfect, and only 6 of the 164 single caudal appen- 

 dages were perfect. Forty males of B. coloradensis had only 1 1 

 imperfect appendages. No imperfect appendages were found on 

 B. Lindahli. 



A careful examination of the larvae of B. coloradensis showed 

 only perfect caudal appendages. From this it seems practically 

 certain that the appendages of the adults were originally perfect. 

 The tips of the appendages were first to disappear, and the appen- 

 dages were then gradually shortened until only a small portion, 

 or nothing at all was left (PI. XII., 25 to 33). In all but a com- 

 paratively few, this change seemed to be a normal process. 

 There was no sign of outward injury, but all the appearance of 

 a gradual removal or absorption of the living substance. This 

 appeared to be the same process by which the large swimming 

 second antennae of the larvae are reduced to the much smaller 

 and more simple second antennas of the adult female. 



A number of the appendages of both males and females showed 

 parts of the appendage blackened by disease (PI. XII., 29, 34 to 

 38), but this could not be mistaken for the other change. This 

 blackening appeared in only a comparatively few of the appen- 

 dages, and the number of males and females thus affected was 

 about equal. 



The caudal appendages of B. coloradensis first make their ap- 

 pearance as little knobs at the end of the common unsegmented 

 body mass (PI. X., I, 2). The point of the knob is pushed out 

 to form a stout, short, curved projection which will ultimately 

 develop into the end or one of the end bristles of the mature 

 appendage. By the time twelve body segments have developed, 

 another small point is formed outside of the first bristle (PI. X., 

 3). When all the abdominal segments can be seen, two more 



