79 



genera have been recorded from rats; a number of others are known 

 from mice and other rodents, and some of these will probably yet be 

 taken upon rats. 



These four forms are separated, as follows: 



1. Eyes large and distinct; beak very short; thorax plainly longer than broad 



Pediculus capitis. 



Eyes small, beak longer; thorax about as broad as long 2 



2. In male the pleura of abdominal segments 3 to 6 above and below have a promi- 



nent tooth-like projection; a tooth on the hind femur of female 



Hoplopleura acanthopus. 



In male the pleura of abdominal segments without such projections; no tooth on 

 hind femur of female 3 



3. Last joint of antennae much more slender than those before; an acute tooth at sides 



of segments 4 to 7; head much narrower in front; antennae two- thirds as long as 



head Polyplax miacantha. 



Last joint of antennae not much more slender than others; head quite broad in 

 front; antennae as long as head Polyplax spinulosus. 



Pediculus capitis De Geer. 



It is pale grayish in color, with {aint dark markings at the sides of 

 the thorax and abdomen; the last segment of the abdomen in female 

 is bilobed. The head is longer than broad and tapers in front. 

 Length, 2 millimeters. This is the head louse of man, and is said to 

 have been taken from rats, and is claimed to be able to transfer plague 

 from rats to man. Its occurrence on rats, however, appears to be 

 very uncommon. 



Hopopleura acanthopus Burmeister. 



In the male the pleura of the abdominal segments 3 to 6, which 

 reach up on the dorsum and over on the venter, have at their 

 inner ends a prominent projection, toothed in all except the third 

 on dorsum and the sixth on venter, which are spine-like. The head is 

 but little longer than broad, broad in front; and in the female there is 

 a recurved tooth on each hind femur. The last segment of the female 

 abdomen is bilobed behind. Length, 1.3 millimeters. It has been 

 taken from rats in Europe, but is more common on species of Microtus. 



Polyplax spinulosus Burmeister. 



The sides of the abdominal segments are acute, but the males do 

 not have the large tooth-like projection of Hoplopleura. The last seg- 

 ment of the female is truncate; the head is about as broad in front 

 as behind, and the legs are very short and stout; the antennae are as 

 long as head, and the last joint is but little smaller than the others. 

 Color, pale yellowish. Length, 1.4 millimeters. 



This is the common rat louse, and is probably as widely distrib- 

 uted as its host. Specimens have been ta"ken in both the eastern 

 and western parts of the United States, 



