284 



Arthropoda. 



abundant in small pieces of water, where it builds a beU-Uke web, tbe cavity 

 of which, is filled with air, carried by the animal from the surface of the water 

 in its velvety covering of hairs. 



Order 3. Acarina (Mites). 



The Mites are small, frequently even microscopic Arachnids, in 

 which the cephalothorax and abdomen are generally fused 

 into a single mass. They possess from one to three pairs of eyes, or 

 none at all. The mouth-parts are usually short. The pedipalpi, 

 as well as the cheliceras, may be chelate, they are sometimes used 

 for biting, sometimes for stabbing. A heart has only been demon- 

 strated in some of the Acarines ; special respiratory organs are 

 frequently absent, but in many there is a tracheal system, 

 opening by a pair of stigmata. On hatching, the Mites possess only 

 three pairs of legs, the fourth pair develops later; in other respects 

 also, the larvae may differ more or less from the adult. Many 

 undergo a resting period before moulting. 



The genital aperture, as in 

 other Araohnida, is situated 

 anteriorly on the ventral sur- 

 face of the abdomen. In some 

 females {Tyroglyphus, probably 

 also iu the Sarcoptidse), there 

 is, besides, the ordinary genital 

 opening, a second one pos- 

 teriorly, just in front of the 

 anus, by which the spermatozoa 

 are received during copulation, 

 whilst the anterior serves as 

 the oviducal pore. 



1. rrom6ii£w(i», red, velvety, 



quadi'angular animals, some of 



The larvae live as parasites on 



The Water- 



Fig. 231. Diagram of the anatomy of a Tyro- 

 glyphus ; legs cut off. a anus, c cerebral ganglion, 

 d mesenteron, ij oheKoera, 1:2 pedipalp, Jlf Malpighian 

 tube, n ventral ganglionic mass, Ov ovary; $ aper- 

 ture of the oviduct, $ ' copulatory aperture. — Orig. 



which are among the largest of the Mites. 

 Phalangium, Spiders and Insects; the adults ai'e predaceous. 

 mites (Hydrachna and others), are roimdish animals, often of a red colour, 

 which swim about in the water by means of their haiiy legs; the hexapod 

 larvae are parasitic on aquatic Insects, whilst the adults are usually free-hving 

 (one species of this group is in the adult stage parasitic upon the gills of 

 the Fresh-water Mussel). The Beetle-mites (genus Gamasus) frequently occui- 

 on Beetles, Bimible-bees, etc. They are small animals, with an oval, flattened, 

 rather hard, brownish body ; they run about freely on the body of the host. An 

 allied, but thin-skinned form, the common Bird-mite {Dermanyssus avium) 

 occurs on Birds (Fowls, Canaries), and sucks their blood ; like some of the 

 Hemiptera, it is a temporary parasite preying upon the Birds by night. The 

 Ticks (Ixodes) are flat, with a fairly hard, but very extensible exoskeleton ; 

 they can move about, but attach themselves to Mammals, Birds, and Reptiles, 

 in order to suck their blood; the female increases enormously in size when 

 gorged. The species of the G-enus Tyroglyphus (Oheese-mites and Flour-mites), 

 live in old cheese, meal, and many other half-dried organic substances; they 

 are white, shining, almost microscopic animals. All these Mites, with the 

 exception of Tyroglyphus, possess tracheae. 



