1 64 ARACHNOIDEA. 



CASE 



XI. Genus Zercon {Koch). 



Koch describes and figures a genus under the name of Zercon. 

 It has parallel sides, and a transverse 

 line across the middle. We give a copy 

 of his figure. 



Genus Sejus {Koch). 



Without any separation on the back 

 between the thorax and abdomen, and 

 without any soft portion exposed behind. 



This is a genus of Koch's which we adopt 



Zercon dimidiatus (copied from 



About ^So'f^apts point. wlth satisfactlon for a well-known portion 

 of the Gamasidse, which is usually referred 

 by naturalists to the genus Dermanyssus. Deducting Koch's two 

 genera, the Gamasidae above noticed are limited to those Gama*. 

 sid^ in which the body is enclosed in whole or in part in a chito- 

 nous skin. They are all parasitic on insects. There are other 

 species which have the skin soft and not chitonous, for -which the 

 genus Dermanyssus was established, and its original type was a 

 species (D. avium) that infests fowls. But there are many species 

 of soft-skinned Gamasidse whose aflinity is greater with those that 

 are parasitic on insects than with those that feed on warm-blooded 

 animals. We find it a great convenience to remove these from the 

 latter, and to bring them alongside Gamasus ; but in a separate 

 group confined to the soft-bodied species that are either parasitic 

 on insects or found free. The other characters by which they may 

 be distinguished from Dermanyssus are, that the posterior two 

 pair of legs are placed further back than in that genus, the body 

 less elongate, and not so swollen behind. The facies of the two 

 are sufficiently distinct : Dermanyssus reminds us of Ixodes, with 

 which its habits correspond ; Sejus reminds us of Gamasus, with 

 which its habits agree. Dermanyssus will then be confined to 

 the soft-bodied Gamasidae parasitic on warm-blooded animals. 



