856 



FRESH-WATER BIOLOGY 



existence the larval appendages drop off and the animal takes on 

 the character of a pupa, which increases greatly in size, drawing 

 nourishment from its host, and beneath the skin of which new 

 appendages are gradually developed. From this quiescent pupa 

 emerges an active, eight-footed nymph (Fig. 1321) possessing legs 

 and palpi frequently quite similar to those of the adult, but with 

 smaller epimera and with a genital field lacking the structures which 



d e f 



FlG. 1321. Figures showing the ventral surface of the body of the nymphs of several genera ot water-mites. 

 (Appendages are similar to those of the adult and not shown here; compare with figures of adults on 

 succeeding pages.) a, Arrhenurus; b, Limnesia; c, Sperchon; d, Hygrobates; e, Piona; f, Lebertia. 

 (Modified from Piersig.) 



distinguish the adult. During this nymph stage the mite is not 

 usually parasitic except in the case of the mussel parasites. How- 

 ever, Unionicola crassipes has been found by Soar, in all stages, in 

 the fresh-water sponge, and the author has taken the differ nt de- 

 velopmental stages of a species of Piona in the gelatinous matrix 

 of a colonial protozoan. Another moult must occur before the 

 mite becomes adult, but this is passed through rapidly and in the 

 forms in which the nymph is free frequently occurs while the animal 

 is clinging to aquatic plants. This moult may or may not be ac- 



