472 



ARACHNIDA ACARINA 



CHAP. 



The remaining families of the Prostigmata (Halacaridae, 

 Hydrachnidae, and Trombidiidae) all have raptorial palps, and 

 clawed or piercing chelicerae. 



Fam. 2. Halacaridae. This is a small group of marine 

 Mites. In their usually prominent capitulum they resemble the 

 Bdellidae. In some respects they recall the Oribatidae, having 

 hard integuments, and their legs being articulated near the 

 margin of the body. They do not swim, but crawl upon weeds 

 and zoophytes, or burrow in the mud. 



Fam. 3. Hydrachnidae. The Hydrachnidae are the Fresh- 

 water Mites. Their legs are provided with long close- set hairs, 



and thus adapted for swimming. 

 They are predaceous, and in their 

 young stages are often parasitic on 

 water insects. A familiar example 

 is Atax bonzi, which lives within 

 the shell of the fresh-water mussel. 



Fam. 4. Trombidiidae. - - The 

 predaceous palps of the Trombi- 

 diidae are generally of the " finger- 

 and- thumb" type. The tarsi are 

 two-clawed, without caruncle. This 

 group may be divided into six sub- 

 families. 



(i.) The LIMNOCHARINAE or " Mud-mites " connect the Hydrach- 

 nidae with the typical Trombidiidae. They are usually velvety 

 and of a red colour. They do not swim, 

 but creep. The larva of Limnocharis 

 aquaticus is parasitic on Gerris lacustris. 

 (ii.) The CAECULINAE bear a strong 

 general resemblance to the Harvestmen 

 or Phalangidae. Caeculus is so similar 

 to the Phalangid genus Trogulus that 

 it was considered by Dufour to belong 

 to the same order. 



(iii.) The TETRANYCHINAE or " Spin- 

 ning-mites" are phytophagous, and do 

 much injury to plants, sucking the sap 

 from the leaves and giving them a blistered appearance. 

 Tetranychus telarius is the " Eed-spider " of popular nomenclature. 



FIG. 247. Atax alticola, x 16. 

 (After Canestrini.) 



Flo. 248. Tetranychus gibbo- 

 sus, x 50. (After Canes- 

 trini. ) 



