46 



REPORT 108, U. S. DEPARTMENT OF AGRICULTURE. 



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 posterior margin. There is usually a group of sucking 

 disks (fig. 76) each side of the genital aperture, the number and position of which 

 are quite characteristic of each species. On the dorsal side of the rostrum (capitulum) 

 are two spiracles that lead to the tracheae; in some forms there are no trachese. The 

 adult mite lives free in the water as a rule, but in Atax they are parasitic in the gills 



Fig. 72. 



-Limnochares: Venter, 

 illustration.) 



(Author's 



Fig. 73.— Limnochares: 

 Eyes and dorsal 

 plate. (Author's 

 illuslration.) 



of mollusks. Hydrachnids are the most beautifully colored of all acarians. Some 

 are a dehcate green, others blue with yellow marks, some species are very brilliant, 

 and others are prettily marked with yellow and black, but vary considerably in 

 maculation. Hydrachnids are rather above the average size of mites, some species 

 being fully 8 mm. long and most of them over 1 mm. 



There are frequently differences in structure between the sexes; in Arroiurus, and 

 some other genera, the male (fig. 77) is larger than the female and has an elongated 



Fig. 74.— EyIaU- sp. (Author's illustration.) 



FlG.75.— Eye plate of 

 Eylais. (Author's 

 illustration.) 



abdomen tipped with a median projection called the petiolus. In some forms the 

 males have some of the joints of the logs especially modilied. In some cases the male 

 IS much smaller than the female, and more flattened. The female lavs spherical 

 eggs on water plants, stones, or in the mantle folds of lamcllibranch molhisks. The 

 eggs on plants are often red in color, placed side by side in a mass and covered with 

 a mucous substance. During the course of development a membrane is separate. 1 

 from the embryo while yet inclosed in the eggshell; the ostq; in this condition is tlic 

 deutovum. After the eggshell is broken the <Mnbryo contiiuios to develop within this 



