THE ACAEINA OR MITES. 17 



■ the best clue to the systematic position. About 650 species are known from this 

 country, where there are doubtless 2,000 or more, so that the student must not be 

 surprised to find forms that do not fit into tables. Several of the families are not 

 natural, and future study will greatly modify existing systems. 



I have not made many new genera in the American mites. I believe in keeping 

 genera as broad as possible. Division of a genus should be made only on grounds 

 of convenience or tho^e of zoological necessity. Because a group of species in a genus 

 differs from the other forms by some structural character there is not, I think, suffi- 

 cient reason for a new genus. But whenever a species, or group of species, differs 

 from the others by several disassociated points of structure, together with differences 

 iu life history or habits, then a new genus is ad\T.sable. However, in using an 

 accepted classification it is sometimes not easy to place a new form without either 

 making a new genus or modifying the characters of existing genera. The history of 

 acarology warrants the student in using much caution in the creation of new genera, 

 or higher groups. 



In regard to the nomenclature I have in general adopted that in common use in 

 Europe. A number of changes have been made from the names used in the first edition 

 of this work, several of which were then evident, but one dislikes to hasten in the use 

 of proposed changes until it is seen that it will be accepted by the majority of workers 

 LQ the group. A number of these changes were foreseen by Gervais in 1845, and had he 

 been more generally followed the nomenclature would not present so many synonyms. 

 Dr. Oudemans, an able Dutch acarologist, has adopted many other changes; but these 

 involve principles of nomenclatui'e not generally accepted by naturalists, and so are 

 not recommended at present, and lately Dr. Oudemans has apparently abandoned some 

 of them. 



To the ordinary observer of nature mites do not exist. He may walk abroad and 

 see birds and insects about him on every side; occasionally he may notice a tick or 

 a harvest mite; yet a little careful searching would reveal a world of these tiny crea- 

 tures at his feet. Among the fallen leaves of the forest, in the moss or lichens, under 

 stones and loose bark, in fungi, in the loose upper surface of the soil, in the galls of 

 plants, in the streams and ponds, and even in the depths of the sea there are mites 

 umumerable. Hidden is this world of mites to the general naturalist as completely 

 as though it were in another planet. 



Although Miiller early described many water mites, and Hermann (1804) and von 

 Heyden (1816) had gathered a few, this world of mites was practically unknown until 

 discovered by C. L. Koch about 1840. Since then a number of naturalists have par- 

 tially explored its shores, and sometimes penetrated into the interior. Now there are 

 more than a score of persons who seriously study acarology, as many more who devote 

 themselves to the water mites, and others study the ticks, since these have been 

 shown to be vectors of certain diseases. 



SYNOPSIS OF SUPERFAMILIES. 



|l. Abdomen annulate, prolonged behind; very minute forms; often with but 



four legs r Demodicoidea. 



Abdomen not annulate nor prolonged behind ; always with eight legs 2 



With a distinct spiracle upon a stigmal plate on each side of the body (usually 

 below) above the third or fourth coxae or a little behind; palpi free; skin 



often coriaceous or leather}^; tarsi often with a sucker 3 



No such distinct spiracle in a stigmal plate on this part of body 4 



Hypostome large, furnished below with many recurved teeth; venter .with 



furrows; skin leathery ; large forms, usually parasitic Ixodoidea. 



Hypostome small, without teeth; venter without furrows; body often with 

 coriaceous shields, posterior margin never crenulate; no eyes Parasitoidea. 



