AQUATIC INSECTS OF FRESHWATER 



the end of the abdomen out of the water, jerking it under and quickly 

 crossing the hind legs over it, then descending to the nest and by opening 

 the legs the air bubble escapes into the nest. The hairs on the body keep 

 the surface from becoming wet, and in the nest the spider is as dry as on 

 land. The 40 to 100 cocoons, containing the round safFron-coIored eggs, 

 are laid in the nest during June, and in July the young are large enough 

 to spin a nest of their own. Another family of the eight-eyed group, the 

 Lycosidae, have semi-aquatic genera which almost exclusively live on the 

 banks of ponds and streams and prey upon aquatic insects, chasing them 

 on the surface of the water. Some species grow to nearly one inch in 

 length. Other smaller species are of similar habit. These often dive below 

 the surface when pursued by enemies from above the water or when in 

 pursuit of their prey. 



AcARiNA. This family consists of the lowest order of the Arachnidae, 

 which live in the earth and in both fresh and salt water. They comprise 

 the Mites and Ticks which differ from other Arachnids in their oval or 

 rounded forms, which are not articulated. The mouth parts are developed 

 for biting and sucking and they breathe by tracheal gills. Of this family 

 the Water-mites are known as Hydrachnidse, which have soft, oval un- 

 segmented bodies, and limbs adapted for swimming, terminating with 

 adhesive vesicles. They are parasitic on fishes and mussels, while some 

 of the smaller species live on the Hemiptera, Coleoptera and other insects. 

 The most generally distributed genera are Hydrachna, Atex, Limno- 

 chares and the marine Pontarachna. Atex has an oval solid body of bright 

 red color with curved, clawlike mandables, acute, pointed maxillaries, and 

 short, weak legs. The species inhabit some of the mussels, A. ypsilophorus 

 in Anadonta and A. humorosa in Unios. The eggs are laid in the spring 

 on the stems of water plants and when hatched seek hosts in which to pass 

 their subsequent existence. A bright red species, Bdella maritima, occurs 

 under stones between tidemarks. Three species of Trombidium are re- 

 presented in the United States, all of red color. 



Hydrachna or Water-mites, are common in ponds. They are 

 tenacious of life and steadily move about by the rapid movements of 

 their fringed legs. The larvae have six and the adult 

 eight legs, which increase in length, with the posterior 

 pair the longest. The body is slightly convex, the 

 mandables needle-shaped, the third joint of the max- 

 illae the longest, and all the species have two distinct 

 ^]^j ^l*' ^"""™"=' eyes. They are parasitic on both fishes and mussels, 



rlyaracnna geograpnica. J J z ' 



Enlarged. and are frequent external freshwater parasites, easily 



detected by their size. The largest and most widely distributed form is 

 H. geographica. Fig. 234, while H. globosa and H. triangularis are also 

 frequently occurring species. 



274 



