102 



NATURAL HISTORY OF ARTHROPODS. 



Fig. 138. — Oribates ovU 

 vorus^ enlarged. 



Pig. 139. — Hoplophora arc- 

 tata, enlarged, showing 

 two positions assumed. 



or three pedicellate stigmata. The forms are all ten-estrial, occurring under moss, 

 the bark of trees, and stones. The American Oribates concentrica and glabrata, 

 are blackish species, while 0. ovivorus is a reddish brown. The 

 latter species has been observed by Dr. Packard to eat the eggs 

 of the canker-worm. Hoplophora arctata 

 in its shape strongly reminds one of a 

 fresh-water mussel. The cephalothorax is 

 much smaller than the abdomen, and so 

 flexibly articulated that it folds over the 

 latter, as Dr. Riley expresses it, like "the 

 lid of a box, whenever the animal with- 

 draws its head and limbs, which it does 

 on the slightest disturbance." Some of 

 the species are said to be hatched with eight legs, but one is known to have only six 

 on emerging from the egg. 



The members of the family Gaseasid^ are parasitic upon other animals, attaching 

 themselves to the outside of the body. They have the mandibles chelate, the legs 

 equal and hairy, with two terminal claws and no ocelli. Frequently specimens of 

 carrion beetles (Silphidse) are found covered with minute bright-orange parasites. 

 These usually belong to the genus Gamasus, and are nearly allied to if not identical 

 with G. coleoptratorum of Europe. Species of Uropoda also have similar habits. 

 They attach themselves to the host by means of an anal filament of excrementitious 

 matter ; and Dr. Riley has described in addition, in Uropoda americana, a long and 

 flexible organ composed of the maxillfe, each of which terminates in a pincer, which 

 serves to attach the parasite to its host after the fracture of the somewhat brittle anal 

 cord, or after that means of connection is broken by moulting. Bats are frequently 

 infested with parasites of this family belonging to the genera Pteroptus and Dermanys- 

 sus. A species of the latter genus is also occasionally found on birds and poultry. 

 The Hydrachnid^, or water mites, have an unsegmented body, with two ocelli on 

 the anterior portion. The legs are haired, and terminate in 

 two claws, which in some genera are retractile into a socket 

 in the end of the last joint. The palpi are five-jointed, 

 and are either hooked or needle-shaped. These forms, as 

 both then- common and scientific names denote, live in the 

 water, both salt and fresh, but most of the species are found 

 in rivers and lakes. Many are parasitic on fresh-water beetles 

 and bugs, at least in their early stages. Some pass their 

 lives as parasites on the gills of fresh-water 

 mussels, and others may be found running over fresh-water plants. 

 The princii^al genera are Hydrachna, Limnochares, Atax, and Pont- 

 arachna, the latter being marine. Atax ypsilophorus, a black 

 species with a sulphur-yellow median line, forked in front, is com- 

 mon to Europe and America, living in the former country in the 

 gills of Anodonta cygnea^ in the latter in those of A. fluviatilis. 

 Atax humerosa is white, with dark-brown markings. It is found ■Fm.ux.—Ataa imme- 

 in Unio oylindricus. The eggs of the Hydrachnidae are laid in the "'"' ^"'^^se'i. 

 spring, in the stems of water plants. Dr. Packard has described a marine form, 

 Thalassarachna verrillii, from Eastport, Me. 



Fig. 140. — Claw of Hydraolinid 

 a. Betracted. &. Extended. 



