48 REPORT 108, U. S. DEPARTMEXT OF AGRICULTURE. 



700 species have been described. The species are often widely distributed and foun<] 

 amid very differing surroundings; but the aquatic environment is, as a whole, more 

 uniform than an aerial one, so that a species may have a distribution that would hv 

 very unusual for a terrestrial acarian. 



Various classifications have been made, one of which arranges them in 14 families. 

 However, it may be better in this paper to use a less complex s>^stem. Dr. Koenike. 

 who has studied these mites very extensively, considers that they represent a sub- 

 order, Hydracarina, which he divides into five families, as follows: 



1. Eyes in the middle of front 2 



Eyes on the sides ;> 



2. Eyes situate upon an elongate chitinous plate Limxocharid.i: 



Eyes connected by a transverse chitinous bridge Eylaid.e 



3. Eyes placed on a small chitinous plate (or eye-capsule I ; color red 4 



Eyes not on a chitinous plate Hygrobatid.i: . 



4. Bod}'' more or less depressed Hydryphantid^ . 



Body arched, rounded Hydrachxid^ . 



The Limnocharidse and Eylaidse have each but one genus, while the great bulk of 

 species belongs to the family Hydrobatidae. Other writers also have kept these 

 water mites well separated from all other acarians, but most authors now recognize 

 their close relationsliip with the Trombidiidae and some merge part of the hydrachnids 

 with the Trombidiidae. Nordenskiold believes the family to have tvvo separate 

 origins, and therefore not natural. However, their aquatic habit is a convenient, i; 

 wrong, incentive for treating them as a unit, and distinct from other families. ^More- 

 over, they have usually been studied as a unit, and by persons who have taken little, 

 if any, interest in other acarians. 



In the following table are found several common European genera which are not 

 yet recognized from this country, and doubtless other European genera will also Lx- 

 found to occur in North America. Dr. Koenike publi-shed a valuable paper on some 

 Canadian forms, and lately Dr. R. H. Walcott and Miss Ruth Marshall have issued 

 several excellent articles on our native species. 



1. LiA-ing in the sea Pontarachna, Xautarach)t<i. 



LiA-ing in fresh water 2 



2. Eyes close together near the median line :^ 



Eyes widely separated on the sides -1 



3. Hind legs far from front legs, and without swimming hairs; tips of tarsi 



obliquely truncate; body somewhat divided into cephalothorax and abdo- 

 men; in fact the whole creature is trombidiform . (Limnocharinae) Limiiocharcs. 

 Hind legs not far from front legs, with swimming hairs; tarsi pointed; no 



indication of di^-ision of body (Eylainae) Eylais. 



4. Penultimate joint of palpus prolonged beyond base of last or with a tooth or 



spinelike projection near, or at tip; eye capsule present. . (Hydrachnina>) 5 



Penultimate joint of palpus not prolonged beyond base of last, nor ending in 



a spine ; no eye capsule (Hygrobatinae) 10 



5. Mandibles one-jointed, needlelike; abdomen globose Hydrachna. 



Mandibles of two joints, with claw at end; abdomen depressed 6 



6. Lateral eyes far apart Diplodontus. 



Lateral eyes close together 7 



7. Without swimming hairs to legs 8 



With swimming hairs 9 



8. A\'ith a median eye on front part Thyas. 



Without median eye Panisus. 



9. With a median eye on front part 29 



Without median eye Tanaognathus. 



10. Fifth joint of palpus forming a claw opposiible to the apical part of the fourth 



joint; males frequently have the abdomen extended behind 11 



i-'ifth joint of palpus not forming a claw opposable to the apical part of fourth 



joint 12 



11. Capitulum or rostrum elongate, of two joints; palpi very small Krcndowskia. 



Capitulum short, entire Arrenurus. 



