deutovum. The larva (figs. 80, 81) on hatching is six-legged, and in some cases is parasitic in the respiratory system of mollusks or attached to aquatic insects. Neumann has claimed that in one species of Limnesia the form hatching from the egg is eight-legged and therefore a nymph, as in Spinturnix. Kramer has arranged the larvæ in three series, which he considers is the basis of classification. As a whole, the larvæ differ greatly from the adults and many were described as different creatures. Some were the basis of the genus Achlysia. α The larva usually has very small mouth parts. It fastens to an insect by means of hooks at the tip of the short, stout palpi, inserts its jaws, and proceeds to feed. Gradually the body becomes swollen, the legs shrink, and the creature looks like an elliptical egg. The pupa is formed within this saclike body, and from it in time issues the adult mite. Frequently mature insects such as mosquitoes, whose early stages are passed in the water, have many immature hydrachnid mites clinging to them. These mites have made a serious mistake, and out of the water must soon die. It is probable FIG. 76.-Genital suckers of Hydrachnids: a, Hydrochoreutes, Piona; b, Hydrobates, Mideopsis, Lebertia; c, Limnesia; d, Atax; e, Arrenurus; f, Nesaea. (Original.) FIG. 77.-Arrenurus sp.: Male. (Author's illustration.) ance. that they were attached to the insect pupa, and at its transformation moved to the adult without suspecting the suicidal result. These mites in the water must often have to change when their host larva molts, so that they realize when a change is imminent, and relax from the old skin at the right time to attach to the emerging insect. It has been noted by Müller that when Libellula was heavily infested with the immature stages of certain red water mites there were about as many specimens on the wings of one side as those of the other side, so that the insect had a prettily and evenly maculated appear On most insects, however, there is no plan of attachment, but specimens often occur at the ventral base of the abdomen. Nearly all Hydrachnidæ live in fresh water, a few forms occur in brackish water, and several are known from the littoral zone of the sea. They are sometimes parasitic, as already noticed, and feed on any small animals they can catch, such as small Crustacea, infusoria, and minute insect larvæ. The water mites are found throughout the globe, but appear to be most numerous in temperate regions. Many species inhabit rapid streams and very cold water. Some species are rather sociable and occur in colonies among small patches of water weeds. A great many species have been described from tropical countries, most of which belong to European genera. Over 60 genera and 88854-15- 4 700 species have been described. The species are often widely distributed and found 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 be 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 system. Dr. Koenike, who has studied these mites very extensively, considers that they represent a suborder, Hydracarina, which he divides into five families, as follows: 1. Eyes in the middle of front...... Eyes on the sides................. 2. Eyes situate upon an elongate chitinous plate.. Eyes connected by a transverse chitinous bridge.. 2 3 LIMNOCHARIDE. 3. Eyes placed on a small chitinous plate (or eye-capsule); color red.... Eyes not on a chitinous plate.... 4. Body more or less depressed.. Body arched, rounded.. HYGROBATIDÆ. HYDRYPHANTIDE. HYDRACHNIDE. The Limnocharidæ and Eylaidæ have each but one genus, while the great bulk of species belongs to the family Hydrobatidæ. Other writers also have kept these water mites well separated from all other acarians, but most authors now recognize their close relationship with the Trombidiida and some merge part of the hydrachnids with the Trombidiida. Nordenskiold believes the family to have two separate origins, and therefore not natural. However, their aquatic habit is a convenient, if wrong, incentive for treating them as a unit, and distinct from other families. Moreover, 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 be found to occur in North America. Dr. Koenike published 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. Living in the sea. Living in fresh water. 2. Eyes close together near the median line.. Eyes widely separated on the sides.... Pontarachna, Nautarachna. 234 3. Hind legs far from front legs, and without swimming hairs; tips of tarsi obliquely truncate; body somewhat divided into cephalothorax and abdomen; in fact the whole creature is trombidiform. (Limnocharina) Limnochares. Hind legs not far from front legs, with swimming hairs; tarsi pointed; no indication of division of body. (Eylainae) Eylais. 5 4. Penultimate joint of palpus prolonged beyond base of last or with a tooth or spinelike projection near, or at tip; eye capsule present..(Hydrachninæ) Penultimate joint of palpus not prolonged beyond base of last, nor ending in a spine; no eye capsule... (Hygrobatina) 10 .Hydrachna. 5. Mandibles one-jointed, needlelike; abdomen globose Mandibles of two joints, with claw at end; abdomen depressed. 6. Lateral eyes far apart...... Lateral eyes close together.... 7. Without swimming hairs to legs. With swimming hairs..... 8. With a median eye on front part. Without median eye... 6 9. With a median eye on front part. 29 Without median eye. Tanaognathus. Fifth joint of palpus not forming a claw opposable to the apical part of fourth joint..... 11. Capitulum or rostrum elongate, of two joints; palpi very small. Capitulum short, entire........ 10. Fifth joint of palpus forming a claw opposable to the apical part of the fourth joint; males frequently have the abdomen extended behind.. 11 12 Krendowskia. .Arrenurus. 12. Epimera of both sexes united into one plate..... Epimera (at least of female) not united into one plate. 13. Fourth joint of palpus with a projection below. Fourth joint of palpus without projection.. 14. Dorsum without a bowed furrow. Dorsum with a bowed furrow. 15. Hind tarsi without claws.. Hind tarsi with claws.. 16. Bowed furrow open in front, ends extending over on venter. Bowed furrow not open in front.... 17. Second joint of palpus with a projection below; genital aperture at tip of body... 13 18 Xystonotus. 14 15 16 .Oxus. Lebertia. Axonopsis. 17 .Aturus. Second joint of palpus without projection; genital aperture much before tip of body... Mideopsis. 18. Fourth tarsus without claws; a long bristle at tip. Limnesia. Fourth tarsus with claw as with others.. 19 19. Genital opening with lip each side nearly covering the disks; palpi with spine below on joint 2.. Sperchon. Disks near genital opening fully exposed.. 20 20. Epimera of female in two groups... Koenikea. Epimera of female in three groups; genital opening usually with disks each Legs and palpi rather short, male with middle joint of hind legs enlarged. Acercus. 25. Legs I and II with spirally ringed bristles.. 27. Without swimming hairs on legs; fourth joint of palpus without three spurs below; genital area large.. Feltria. With a few swimming hairs; fourth joint of palpus with three spurs, or papillæ, below; genital area smaller. Atax. 28. Hind epimera with a pointed projection below; palpi larger than leg I. Najadicola. Hind epimera without a projection; palpi smaller, and ending in two or three small claws; the fourth has two papillæ below. 29. Median eye in the soft skin. Median eye on a plate.... Piona. Eupatra. Hydryphantes. The immature forms are known in many genera and these may be tabulated as follows: 1. Coxæ united into a large plate each side leaving only a narrow groove between them... 7 Coxæ not united into a large plate leaving only a narrow median line; beak large and prominent; palpi of three or four joints usually extended; no dorsal shields. 2 2. Coxæ I and II partly united, palpi curved under beak. Coxæ I and II separate, palpi extended.. .Eylais. 3 3. Beak nearly as long and as broad as body; tarsi subtruncate at tips...Hydrachna. Beak only about one-half length of body; tarsi tapering to fine point.. 6. Legs with six joints beyond coxæ. Legs with apparently but five joints beyond coxæ. 7. Hind legs fully three times as far from second as second from first; hind legs with fifth joint from tip several times as long as broad; body usually elongate.. Limnesia. Hind legs but little farther from second than second from the first pair.... Limnochares. 8 8. On venter the groove from coxæ II reaches the median groove near tip of body... .Frontipoda. 9 This groove (when present) reaches median groove near or before middle...... 9. The grooves from coxæ II and III are both rudimentary and neither reach near the median groove... Groove from coxæ II reaches median groove.. Hygrobates, Atractides. 10 11 12 .Arrenurus. 10. Groove from coxæ III also plainly reaches median groove. Groove from coxæ III does not reach median groove.. 11. Beak with palpi prominent from above; legs arise from margin. Beak with palpi hidden under front of body; legs arise well under body. Midea, Mideopsis. 12. Median groove with a transverse line or interruptions near tip. 13. Groove from coxæ III reaches one-half way to median groove. Median groove with two interruptions, or at least one much before tip. 15. In hind legs fifth joint from tip is very distinct. In hind legs fifth joint from tip is minute.... 13 16 14 15 Wettina. Laminipes, Hygrochoreutes. 16. Groove from coxæ III reaches one-half way to median groove. Groove from coxæ III rudimentary... 17. Legs arise from well under body.. Legs arise near margin.. 18. Body nearly circular.. Body elongate. 19. A short median process at tip of body. No such process. Tiphys. 17 19 .Brachypoda. 18 Teutonia. Curvipes. .Lebertia. Axonopsis. + FIG. 78.-Arrenurus sp.: Palpus. (Author's illustration.) FIG. 79.-Arrenurus sp.: Fe- FIG. 80.-Larva of a hydrachnid. (Original.) The genus Limnochares (figs. 72, 73) is practically an aquatic trombidian; the body is of the same shape, and red in color. It can not swim, but creeps over the mud and plants under water. The young are parasitic on water skaters (Gerrida). Our species is very like the European. We have three species of Eylais (figs. 74, 75). They are distinguished by having four simple eyes situate close together on a plate near the median line. They are most frequently found in ponds, and a larva of this genus has been found on mosquitoes. The genus Thyas (figs. 84, 85, 86), of which we have a few species, has no swimming hairs on the legs, and beside the lateral eyes there is a median eye in front. Abou: 20 species of Arrenurus (figs. 77, 78, 79) have been described from North America: and doubtless many more occur. They are often green in color and have a chitinous dorsum, which, in the male, has a circular furrow open behind. The males have the body extended behind, forming a median petiolus. They live most commonly in lakes and ponds. All come from Northern States and none from the far West. Krendowskia and Axonopsis are each represented by one species. Albia is similar to Aconopsis and the one European species also occurs in the Northern United States. In Aturus the epimeral plate covers nearly the entire ventral surface; one species is known from a small river in Canada. Of Xystonotus and Mideopsis but one species is known in each. The latter is also a common European water mite. The species of Sperchon (figs. 90, 91) are often found in cold and rapid mountain streams. Three species are known from Canada. They have the palpi enlarged at FIG. 83.-Tyrrellia circularis. (Author's illustration.) FIG. 84.-Thyas sp. (Original.) base, and the second joint bears a strong spur below. The sucking disks each side of the genital opening are nearly concealed by lateral flaps. The legs have no special swimming hairs, and coxæ III and IV are widely separated from I and II. FIG. 85.-Mandible of Thyas sp. FIG. 86.-Palpus of Thyas Limnesia (fig. 82) is a large genus. Three species have been recorded from Canada, two of them European. They have a soft body; the hind legs are well provided with swimming hairs, and the fourth tarsus lacks the claws, but is provided with a bristle |