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.
Fig. [247].—Atax alticola, x 16. (After Canestrini.)
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 Trombidiidae are generally of the “finger-and-thumb” type. The tarsi are two-clawed, without caruncle. This group may be divided into six sub-families.
Fig. [248].—Tetranychus gibbosus, x 50. (After Canestrini.)
(i.) The Limnocharinae or “Mud-mites” connect the Hydrachnidae 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.