CHAPTER XX 

 THE MITES 



General Account. — The mites and ticks, which constitute the 

 Order Acarina of the Class Arachnida, are only shghtly known by 

 the majority of people. Popular knowledge of them is usually 

 limited to a few species of ticks, chicken mites, and perhaps two 

 or three other species of mites. Yet the group includes a large 

 number of species, varying in size from some ticks which are half 

 an inch or more in length to mites barely visible to the naked eye. 

 The variety of body form is great and some species when magnified 

 appear ridiculously grotesque. The majority of the species are 

 more or less round or oval, with head, thorax and abdomen all in 

 one piece, but many have the cephalothorax (head and thorax 

 fused together) distinctly marked off from the abdomen, while 

 a few are quite wormlike in form. Many mites are free-living 

 and prey upon decaying matter, vegetation, stored foods and 

 the like; some are predaceous and feed upon smaller animals; 

 some are aquatic, even marine; and many are parasitic on other 

 animals during all or part of their life cycle, and some of these 

 serve as intermediate hosts for, and for dissemination of, danger- 

 ous disease germs. 



Like other Arachnida (spiders, scorpions, etc.) the mites and 

 ticks usually have two pairs of mouthparts and four pairs of 

 legs, though the last pair of legs is not acquired until after the 

 first moult. The first pair of mouthparts or chelicerse are some- 

 times needle-like, sometimes shaped like a grapnel hook, and 

 very often pincer-like, the pincers often being at the tip of a 

 long exsertile needle-like structure. The second pair of mouth- 

 parts, or pedipalps, are simple segmented palpi. In many kinds 

 of Acarina the anterior end of the ventral side of the body is 

 produced as a sort of chin or lower lip, the hypostome, which 

 may be needle-like or barbed and rasplike (Fig. 152). 



The digestive tract is in most cases well developed. Waves 

 of muscular contraction make a very efficient sucking organ of 



331 



