8 7 2 



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, 1.5 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) . 

 Epiraeral 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. 



Varying from small to large in size, 

 or from 0.4 to i .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 

 numerous and 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 univalve. 



FIG. 1349. Unionicola crassifes (Muller), 

 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 



