83 



There are, however, quite a number of species that are genuine 

 parasites of insects and other animals. Those occurring on rats 

 belong to two genera, Myonyssus and Laelapp. They can be sepa- 

 rated as follows: 



1. Anal plate small, much smaller than the ventral plate Lxlaps. 



Anal plate large, larger than the ventral plate Myonyssus. 



Myonyssus. — This genus is made by Tiraboschi for one species : 

 Myonyssus decumani Tiraboschi. 



Body oval; legs short and stout, all tarsi with a large caroncle 

 with two short claws; coxae II have a .large tooth on the anterior 

 border, none of coxae with spines; sternal plate much broader than 

 long, with three bristles each side; ventro-genital plate much longer 

 than broad, broadest behind, bordered with bristles; anal plate very 

 large, nearly one and a half times as broad as long; three large spines 

 each side on venter. Length, 0.95 millimeter. 



Found in Italy on Mus decumans. 



Lselaps. — ^This genus embraces a large number of species, several 

 of which occur on small animals, such as the muskrat, ground hog, 

 and chipmunk. Three have been recorded from rats, one of these 

 from California. The dorsal plate is covered with hairs or bristles, 

 and there are usually stout bristles on the margins of the plates on 

 the venter. There is also a bristle, or a spine, at the tip of the anal 

 plate. The legs are short and stout, with a distinct caroncle, and 

 two claws. 



1. Dorsum with numerous fine hairs; no stout spines on coxae L. stabularis. 



Dorsum with fewer, but stouter spine-like bristles ; each coxa has a stout spine 2 



2. Body but little longer than broad; ventral plate longer than broad L. agilis. 



Body much longer than broad; ventral plate about as broad as long. . L. echidnimis. 



Lxlaps echidninus Berlese. 



Dorsum of body almost wholly covered with a shield, with rows 

 (six in front, eight behind) of stout, curved bristles, a longer pair 

 near front margin, and some around lateral and posterior margins. 

 Legs short and stout, tarsi about twice as long as preceding joint; 

 each coxa bears a stout spine near middle. Palpi very short; sternum 

 with three stout bristles or spines each side; ventral plate with four 

 stout bristles each side-; anal plate with a stout apical bristle, and a 

 small one each side. Length, 1 millimeter. 



Occurs commonly on rats in warm countries, and known from 

 California. It may possibly aid in the transmission of disease. 

 Lselaps agilis Koch. 



Similar in many respects to L. echidninus but differ in the shorter 

 and proportionately broader body, barely longer than broad, and in 

 the weaker and shorter spines on dorsum and on the ventral plates; 

 there are also some small short spines on the general surface of the 

 venter. Length, 0.7 millimeter. 



