country; similar species, with the dorsal shield broken up into numerous small ones, are found in northern Europe.

The genus Raillietia Trouessart is based on the Gamasus auris Leidy, taken from the ears of cattle. It is closely related to Parasitus, the male having the second pair

FIG. 156.- Caelenopsis americana. (Author's illustra

tion.)

FIG. 157.-Sejus americanus. (Author's illustration.)

of legs calcarate. Trouessart thought he had observed parthenogenesis in this species but he was probably in error, as various cases of supposed parthenogenesis have been shown to have been mistakes.

FIG. 158.-Para-
greenia alfkeni•
(Original.)

FIG. 159.-Laelaps echidninus. (Original.)

Heterozercon is based on a South American species (only the female known) in which there is a suckerlike disk each side of the anus; the dorsal shield is entire, the peritreme long, and leg I with claws. Discozercon, from Java, is a similar form,

Parasitids of various kinds occur on the seacoasts, and often have the caroncles modified or enlarged and adapted to the moist soil. Several new genera have been based on such forms as Cyrthydrolaelaps, which is a Cyrtolaelaps, except the peculiar foot. Most of these maritime forms have evidently been developed independently from the adjacent parasitid fauna, and exhibit convergence in structure. Very similar forms are known from the coasts of Europe and from certain Antarctic islands.

The genus Podocinum is remarkable on account of the very long front legs, which the mite uses as feelers. The species occur under sticks on moist ground; one of our species is very common. The species of Paragreenia (fig. 158) (Greenia Ouds. and Greeniella Bks., both preoccupied) are remarkable for inhabiting a cavity in the basal abdominal segment of certain female bees of the genus Koptorthosoma. With them have been found other mites, especially of the genus Trichotarsus. The relation of the mites to the bees is not fully known; the basal segment of these bees is strongly concave and its edges applied closely to the tip of the thorax, thus forming a cavity with an opening above. Within this cavity the Paragreenia occur, often nearly filling it. These mites have the front legs provided with short retrorse spines; several species have been described, all from the Old World. In the female of an American species of Odynerus a similar cavity is found where the apical margin of the basal segment rests on the front of the nonconstricted part of the segment; a simi

FIG. 164.-Parasitus predator:
Leg II of male. (Original.)

lar but undescribed genus of mites occurs in

this cavity.

Anystipalpus is in general similar to Laelaps but with the palpus formed on the manner of the Trombidiidæ, the last joint forming a "thumb."

Of Myrmozercon (including Myrmonyssus) a large number of species have been described, mostly from southern Europe. They are found attached to the bodies of various species of ants, and doubtless feed upon them. They have short, stout legs with very large ambulacra.

A number of species of Laelaps occur on small mammals and in their nests; thus L. multispinosus Banks occurs on the muskrat, L. pedalis Banks on the chipmunk, L. propheticus Banks on the groundhog, and L. echidninus Berlese on rats. They are supposed to suck the blood of their hosts, but may feed to some extent on the dermal scales. L. echidninus Berlese (fig. 159) is a very bristly form; it is found on rats in all parts of the world, but probably has little to do with the transmission of any human disease. Two other species have also been recorded from rats. Unlike the ticks, these mammal-inhabiting mites rarely if ever attack man or the domestic animals, so that they are of little economic importance. But as vectors of plague from rat to rat they are factors that must be considered in the eradication of that dread disease. According to recent researches, Laelaps echidninus also carries a disease

FIG. 165.-Hyletastes sp. and anal plate. (Author's illustration.)

peculiar to rats, produced by a sporozoan, Hepatozoon perniciosus. The sexual cycle occurs within the Laelaps, so that it is a necessary factor in this disease.

The genera Poecilochirus and Gamasoides are very near Parasitus, but have a membranous appendage to the movable finger of the mandibles. Gamasoides has stout spinelike bristles above on the femora.

The genus Parasitus 1 (figs. 160, 161, 162) includes a host of species all over the world; but few of ours have been described, and several are apparently identical with some of the European species. Berlese and others have split the genus into a great number of subgenera, most of which are difficult to recognize in both sexes. The species of Parasitus (figs. 163, 164) are mostly found running freely among fallen leaves in the woods, or in heaps of refuse or débris. Rarely are they attached to insects or occur in ant nests. Several inhabit sea beaches, often below the high-water mark. They are always predaceous, and are sometimes beneficial in destroying springtails and injurious mites. Most of the species have the same general habitus, and are difficult of separation. The European forms have been monographed by Berlese.

In Seiulus (fig. 167) are placed many small species, which are quite simple in structure, often with some long bristles; they usually occur in moss, but sometimes on the leaves of plants, rarely attached to insects. They are predaceous in habit and feed on thrips and red spiders.

Melichares is closely related to Hypoaspis, but the dorsal shield of the female is slender, hardly more than one-half as wide as the body.

To the genus Hypoaspis (figs. 168, 169) belong a great number of species, most of which were formerly placed in Laelaps. Their habits are extremely varied; a number are found in the nests of ants, some attached to the ants, others feeding on the débris or attacking the eggs of the ants; several species are attached to beetles or other insects; others occur in moss, and a few in decaying vegetable matter, or among dead leaves. Berlese has divided the genus into 6 subgenera, depending on the condition of the ventral plates in the female, or on whether the second pair of legs of the male is provided with spurs. These subgenera are Laelapsis, Eulaelaps, Cosmolaelaps, Hoplolaelaps, and Androlaelaps. One of our species has been taken in an ant nest, and another (H. placidus Banks) from wet sphagnum moss.

1 This genus is known to all acarologists as Gamasus, and it is very unfortunate that this name is antedated by the less-known one of Parasitus.

Under the name Antennophori (fig. 170) are placed a number of species and genera usually associated with ants. This group is recognizable on account of the generally circular shape and short legs, the first pair of which lacks the claws and caroncle, but is provided with long hairs at the tip. The dorsum is covered by an entire shield, and the male genital aperture is in the sternal plate. The habits of these mites are intensely interesting. The adult mites occur on ants or myriapods, one species, however, on an earth-boring beetle, Scarites. Those on ants (fig. 171) are usually upon the head or venter. The ants are accustomed to touch one another by the antennæ in asking for food; the ant having food thereupon regurgitates a drop, which is taken by the other ant. The mites come in here and secure a bit; by touching with their front legs the ant they are on, or another ant that comes near, the mites imitate this request for food, the ants often responding. Janet has claimed that the mites arrange

FIG. 168.-Hypoaspis mexicanus, mandible, and enlarged hair. (Author' sillustration.)

FIG. 169.-Hypoaspis macropilis and caroncle of tarsus IV. (Author's illustration.)

themselves systematically on the ants so that they balance and do not disturb the equilibrium of the ant. Thus when there is but one mite present it clings to the median surface of the head; when two are present, one is situated each side of the head; if three, one each side and one in the middle; if four mites, then two each side, usually one pair on venter; if six are present, then placed as with four, and a median one on head and venter. One can hardly believe that any such definite arrangement is regularly maintained. The species occurring on myriapods are supposed to feed on the secretion of the repugnatorial glands. They do not arrange themselves symmetrically on the host, and move about very rapidly if disturbed. They can travel as easily sideways or backwards as forwards, and to this fact, Trägårdh thinks, is due their circular shape. Only the adults occur on insects, the immature stages, at least with the myriapodophilous species, according to Trägårdh, occurring in fallen decaying leaves. The