Fig. 135.—Nothrus ovivorus
Packard.
The family Orobatidæ includes a number of small terrestrial mites, which occur on the moss on trees and stones. Some species are known to feed on the eggs of insects, and the one shown in the accompanying figure, Nothrus ovivorus has been observed, by Dr. Packard to eat the eggs of the Canker Worm.
The members of the family Gamasidæ are parasitic upon animals, but chiefly upon insects. The Hydrachnidæ are parasitic also upon the aquatic insects, and also affect fish or mussels or occur on fresh-water plants.
One of the most important families of mites is the Trombidiidiæ which includes a large number of species, some of which occur in immense numbers. Most of them are vegetable feeders, but some species feed on the eggs of insects.
Fig. 136.—Trombidium locustarum: a, female with her batch of eggs; b, newly hatched larva—natural size indicated by the dot within the circle; c, egg; d e, vacated egg-shells.
The genus Trombidium includes a number of the Red Mites which feed on insects in all their stages. The Locust Mite, Trombidium locustarum Riley, is one of the most interesting as well as one of the most important of our locust enemies, and will serve to illustrate the habits of the group. It differs so much in infancy and maturity that it has been referred to different genera and is known under different names. The mature form lives on the ground and feeds on all sorts of animal or decomposing vegetable matter, and wherever the ground is filled with locust eggs these afford an abundance of food and the mites flourish and multiply rapidly. In the spring the female lays 300 or 400 minute spherical orange-red eggs in the ground ([Fig. 136]a). From these eggs, as shown enlarged at c, d, and e (the two latter being the vacated egg shells) emerge the six-legged larva shown at b. These are mere specks and crawl actively about, fastening themselves to the locusts mostly at the base of the wings or along the upper veins. They subsist on the juices of their host. They firmly attach themselves by the mouth and increase rapidly in size, the legs not growing and becoming mere rudiments. In this form they are shown at [Fig. 137]a. When fully developed they let go their hold, drop to the ground, and crawl under the shelter afforded by holes in the earth or under sticks. Here, in the course of two or three weeks, they transform within the larval skin to the pupal stage shown at b, and eventually break through the old larval skin and escape in the form shown at c and d. This mature form passes the winter in the ground and is active whenever the temperature is a few degrees above the freezing point. A larger species T. giganteum Riley, also attacks locusts, while a third species attacks the common House-fly. This was formerly known in the larva state only and was referred to the genus Astoma, to which also the larval form of Trombidium was referred. I have described the adult together with the larva and pupa as Trombidium muscarum. An allied mite, Hydrachna belostomæ, attacks the large aquatic water bug, Belostoma, and has a mode of development precisely similar to that of Trombidium.
