NO. 1382. 



THE A CARINA OR MITES— BANKS. 



35 



Fig. ,55. — Eylais si 



jointSj rather subeqiial in length, although the fourth pair is com- 

 monly the longest. The coxa? are often broad and entirely united to 

 the venter and frequently to each 

 other. The coxt^? or epimera thus 

 form coxal or epimeral plates — 

 sonietimes four, sometimes three, 

 and in a few genera they are united 

 into one plate. The tarsi often ter- 

 minate broadly, and usually have 

 two claws. Tiiey are provided with 

 hairs and bristles. Sometimes there 

 are rows of hairs for assistance in 

 swimming; these are most numerous 

 on the posterior legs. On the venter 

 are the genital and anal openings; 

 sometimes the former is very far back, nearly or quite on the poste- 

 rior margin. There is usually a group of sucking disks each side of 

 the genital aperture, the number and position of which 

 are quite characteristic of each species. On the dor- 

 sal side of the rostrum (capitulum) are two spiracles 

 that lead to the trachea?; in some forms there are no 

 trachea^. 



The adult mite lives free in the water as a rule, but 

 in Afa.r they are parasitic in the gills of mollusks. 

 The 3'oung are fre(iuently parasitic on insects or mol- 

 lusks. Many of the species are red or bluish-green in color; often 

 they are prettily marked 

 with yellow or black, and 

 var}" considerably in mac- 

 ulation. Hydrachnids are 

 rather al)()ve the average 

 size of mites, some species 

 being full}^ <S millimeters 

 long and most of them 

 over 1 millimeter. 



There are frequently 

 ditf'erences in structure 

 between the sexes; in 

 Arrenurus and some other 

 genera the male has an 

 elongated abdomen tipped 

 with a median projection 

 called the petiolus. In 

 some forms the males have some of the joints of the legs especially modi- 

 fied. The female lays spherical eggs on water-plants, stones, or in the 



Fig. 56.— Eye-plate 



of eylais. 



— Akeenurus sp., male. 



