WATER MITES 



41 



inhabiting fresh water. A few of the halacarid or essentially 

 salt-water forms have invaded the fresh water, while a few 

 of the hydracarid 1 or fresh-water forms have migrated to the 

 salt and brackish waters. Our study of the water mite will 

 be confined to the hydracarid forms Thyas venusta 

 (Limnocharidc-e) and Limnesia fulgida (Hygrobatida:), but 

 before considering these in detail it is desirable to know, 

 apart from the striking difference in the larval forms, the 

 conspicuous characters to be encountered in the more 

 commonly met with adult forms in these families. 



Hygrobatidce. 



Limnocharidce. 



Water mites sometimes attaining 

 a lengtJi of about 5 mm. Colour 

 red. Body, as a rule, not arched 

 (except in sub-fam. Hydrarach- 

 ninas). Skin soft, generally with 

 a papillated cuticle, frequently 

 exhibiting one or more chitinous 

 plates, rarely entirely hard. 



Unpaired median Eye frequently 

 present in addition to the normal 

 eyes, the latter usually enclosed 

 in a capsule. 



Capitulum, as a rule, ending in a 

 rostrum. 



Epimcra not covering much of the 

 ventral surface, almost without 

 exception in four groups. 



Genital Area never removed from 

 the epimeral region, as a rule, 

 without externally noticeable sex 

 dimorphism. 



Water mites generally of small size. 



Colour varied. Body, as a rule, 



highly arched. Skin ranging 



from soft to a rigid exo-skeleton. 



Chitinous plates almost entirely 



absent. 

 Unpaired median Eye never 



present, normal eyes not 



enclosed in a capsule. 

 Capitulum generally without a 



rostrum. 

 Epimera frequently of considerable 



size ; plates not seldom adjoining 



one another and partly fused 



with one another. 

 Genital Area often standing well 



back from epimeral area. Sex 



dimorphism frequently sharply 



defined. 



Thyas venusta C. L. Koch. 



It will be noticed at the outset that the body colour is 

 red, the legs and palpi being a little paler. This holds for 

 the family almost without exception. The body is flattened ; 

 in considering its outline, it is to be observed that the 

 anterior margin is slightly arched, and that the extremities 

 of this marein are continued to form well-defined shoulders. 



1 Proc, Roy. Phys. Soe., xviii. 92. 



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