The Gamosidce are somewhat similar, with rounded bodies and a hard sldn, but have only a single pair of stigmatm at the base of the second pair of legs. The blood system is well developed. They often swarm on the under side of rove-beetles, carrion-beetles and other insects and some are parasitic on bats and birds; 35 genera and perhaps 200 species are already known.
The Ixodidce are the ticks, large blood-suck ing species, with ovoid bodies and leathery skin capable of great extension. They are tempo rary parasites of vertebrates, chiefly of birds and mammals. As a rule the eggs are depos ited in the ground; the newly hatched larvw have three pairs of long clawed legs with which they attach themselves to a host, insert the beak and suck the fluids. When ready to slough they may drop off, seek concealment until the skin is changed, when the same maneuver is repeated. After passing the larval and nymphal states the imagoes live among herbage and shrubbery and upon opportunity again attach themselves, but usually in pairs, to some warm-blooded host, inserting the strong beak and drawing blood while copula tion takes place. The fertilized female becomes greatly distended, often to a spherical form, drops off and deposits her eggs, often to the number of 20,000 to 30,000. The large cattle ticic (Boophilus bovis) of the western ranges, now known to be the intermediate host of the parasite of Texas fever and a great scourge to cattlp and other animals, is an ex ample. Others are the wood-tick (Ixodes uni punctata), so common in New England, and certain European species, one a parasite of poultry and introduced into the United States, as the related dove-tick also has been.
The Hydrachnida, are the attractive and familiar water-mites, another large group of 40 genera and about 500 species. The adults, re markable for their sexual dimorphism and bril liant colors, suck the juices of small crustaceans while the young are parasitic on aquatic insects and mussels. The brilliant scarlet eggs of some species are frequently found attached to aquatic plants and stones in a mass of jelly. A related family containing mostly marine predaceous forms is the Halacaride. An allied family is the Trombidiidte, including the scarlet mites and the red spiders so well known to horticultur ists. A very common one is Tetrarhyncltus tele arius, which spins a web on the under side of leaves and is very destructive to plants during hot dry weather. Others cause great damage to orange and lemon groves. The young are parasitic on insects.
The disgusting itch-mites form a family (Sarcoptida.) of short, rounded forms which lack eyes, trachem and stigmatx altogether. They are microscopic and burrow in the skin of various animals, causing the diseases known as itch and mange, which are very difficult to eradicate but usually yield to persistent applica tions of sulphur washes. No less than 68
genera and 550 species parasitic on mammals, birds and insects have been described. (See PITH). Sheep-scab and mange in various domestic animals are caused by related mites of the.genera Psoroptes and Symbiotes, They all have the feet wonderfully provided with ad hesive organs in the form of hooks, bristles and sucking cups. Closely allied are the Tyrogly phida., comprising perhaps 50 species of minute forms with biting jaws. They are chiefly re markable for their great reproductive capacity and the enormous numbers in which they occur in slowly decomposing vegetable substances, etc. Here belong the cheese-mites (Tyroglyphus siro and T. longior) which are cultivated and sown in certain cheeses in order to give them an ap pearance of maturity and an acid flavor. Many similar species infest stored grain, dried fruits, etc., and some, like AleurobMs and Rhisogly phus, destroy living roots, bulbs and grains. Another one abounds in unrefined sugars.
Two strictly parasitic families in which the body has become elongated and worm-like and otherwise degenerate are Demodicida' and Erio phydicke. The former live in the hair follicles and skin glands of man and domesticated ani mals. Demodex folliculorum sometimes causes the °blackheads') which appear about the human nose; similar species infest the pig, dog, sheep, ox and other animals. That of the ox some times so perforates the skin that it has little value for leather. In these forms the legs are very small and degenerated and one pair may be lost. The second family is that ot the gall mites which have but two pairs of legs, the posterior being sometimes represented by bris tles. They form galls in the buds and leaves of plants whose juices they suck. Most of the species are confined to a particular species of plant, so that the 235 which have been described are probably but a small representation ot those actually in existence. Some are parasitic in the galls made by others, recalling the in quilines among the gall flies.
Trouessart, (Revue sciences naturelles de l'Ouest' (1892) •, Michael, (London 1883-87) ; (British Tyroglyphidx) (London 1901) ; Banks, actions American Entomological Society,' XXI; Walcott, (Hydrachnithe' (Studies Univ. of Nebraska, No. 34) ; Packard. (American Journal of Arts and Science' (1871) ; Banks, (Red Spiders of the United States' (Bulletin Department of Agriculture, Washington, 1900); Nuttall, (Insects as Carriers of Disease' (Johns Hopkins Hospital Reports 1900)and the monographs since 1898 in Scfniltze's (Berlin) ; Murray, Entomology, Aptera' (London).