12 REPORT 108, U. S. DEPARTMENT OF AGRICULTURE. 



in these forms. Doubtless the secretions from each kind of gland is peculiar and for 

 some special purpose. In the Metastigmata and Oribatidae there are ver}' few, usually 

 none, of these salivary glands: in the TATOghiDliidse and Sarcoptidse there are usually 

 one or two pairs. 



In some mites there is a well-developed dorsal pulsating organ or heart, but in most 

 mites it is not present. In these latter there is an irregular motion of the body fluids, 

 kept up by the action of the muscles in other movements. ^lany mites have an elabo- 

 rate tracheal system for breathing. The tracheae open in various parts of the body; 

 in many common forms they open near the mandibles. In licks and parasitids they 

 open near the hind legs. In other forms they open in the acetabula or coxal cavities. 

 The opening of the tracheae are through stigmata or spiracles which A-ar>^ in different 

 forms. Commonly there are one or two main tracheal tninks in each side of the body, 

 each giving off many branches. In some forms there is a short tracheal tnmk which 

 at its tip gives rise to a great number of fine long tracheae, each of which is unbranched. 

 Many mites, however, have no internal respiratory system whatever, and in these 

 forms the skin is soft, and they absorb oxygen by osmosis through the general surface 

 of the body. 



The muscular system of mites varies greatly in the different genera. Strong mus- 

 cles are attached to the mandibles for extending and moving these organs. Still more 

 powerful ones are attached to the legs, and those for the 

 pharynx and organs of generation are often prominent. 

 There are also muscles connecting various parts of the 

 integument and in some cases one or two pairs attached 

 near the middle of the dorsum. Tliese latter often pro- 

 duce the depressions in the dorsal surface which are 

 commonly found in many of the soft-bodied mites. Thor 

 has studied the structure of the skin of various mites, 

 Fig. 6.— Embryo of Parasitm. and separates at least three layers, the outer, called 

 (Author's Ulustration.) epiostracium, an inner layer, ectostraciiun, and the in- 



nermost, the hypoderm. The body and appendages of mites are clothed more or less 

 thickly with hairs of various kinds. Usually some are stout and form spines; others 

 are feathered, others clavate or scalelike; some are very long and delicate; some are 

 movable, some arise from pores, and many have nerve connections. Haller has 

 attributed to them functions of touch, hearing, and smell, but with little, if any, 

 evidence. In a number of water mites hairs are sometimes developed in tufts on the 

 legs and aid the creature in swimming. In a few cases there are sexual differences 

 in the hairs. 



The life histories of mites are extremely variable, and will be described under each 

 family. There are typically four stages — egg, larva, nymph, and adult or prosopon. 

 Many mites, however, have developed a more complicated arrangement of forms that 

 almost obscures this fundamental simplicity. Most of these additional phases occur 

 during the nymphal stage, the gro^ving stage of the acarian. 



Nearly all mites deposit eggs, but there are a few forms known to be viviparous, or at 

 least ovo viviparous, and one, Pediculoides, brings forth adult males and females. 



The normal plan of development is as follows: The egg is usualU deposited by the 

 female. Often ^^ithin this egg, while the embr>-o (fig. G) is developing, an inner 

 membrane is formed which incloses the young mite; this stage is the "deutovum." 

 The outer shell may be cracked so as to sho"vv the membrane, or it may be wholly 

 discarded. The larva at birlh has but six legs. It feeds for a while, then pasvses into a 

 resting stage which in time discloses the eight-legged nymph. The added pair of 

 legs is the foiirth, at least usually. During the nymphal period the mite may molt 

 one or more times and change its appearance, but is always destitute of true genital 

 orifices. At the end of the nymphal stage it passes again into a quiescent condition, 

 and in due time molts into the adult mite (fig. 7). During these resting stages much 

 of the internal anatomy of the mite may undergo histolysis, each new stage being 



