872 



FRESH-WATER BIOLOGY 



59 (60) Posterior margin of ep. IV rounded; genital area midway between 

 epp. IV and the posterior end of the body, genital plates 

 elongated transversely Najadicola Piersig 1897. 



A large mite, i-S to 2.5 mm. long, 

 the gravid female often very large, 

 reaching a length of even 6 mm., living 

 in fresh-water mussels and laying eggs 

 in masses between the gills. Honey- 

 yellow in color, more or less distinctly 

 finely vermiculate with white lines. 

 One North American species, generally 

 distributed. 



Fig. 1348. Najadicola ingens (Koenike) . 

 Epimeral field and genital area, male. 

 X 23. (.After Koenike.) Inner side, pal- 

 pus, male. X 80. (After Wolcott.) 



60 (59) Posterior margin of ep. IV straight; genital area at the posterior 

 end of the body, genital plates not elongated transversely. 

 (Parasitic species) Unionicola Haldeman 1842. 



Varjdng from small to large in size, 

 or from 0.4 to 1. 9 mm. in length. Some 

 are active, free-swimming mites with 

 long legs, with swimming-hairs, and 

 leg I frequently with movable, dagger- 

 like spines. Others are mussel para- 

 sites, with shorter legs and no 

 swimming-hairs, leg IV in some cases 

 being characteristically modified in 

 the male sex. In both types strong 

 spines adjacent to the genital opening 

 serve together as an ovipositor. Cer- 

 tain free-swimming forms are regularly 

 pelagic and very transparent; the para- 

 sitic forms are dull-colored. Species 

 numerousand widely distributed, many 

 of them very abundant, especially the 

 parasitic forms. The latter are usually 

 mussel parasites though one species 

 has been recorded from a South 

 American imivalve. 



Fig. 1349. Unionicola crassipes (MiiUer), 

 a common and widely -distributed, free- 

 swimming species, common to North 

 America and Europe. Ventral surface, 

 female. X 22. Palpus.outer side, female. 

 X 63. (Modified from Piersig.) 



61 (54) Posterior margin of ep. IV with a prominent acute angle. 



62 



