Savory - Arachnida 1977

206 Ill. PROLES ARACHNES
26. THE'. ORDER ACARI 207
pods and arthropod eggs, and the colourful spider mites some of which
seriously damage orchard trees and food crops, for example Tetranychus
cinnabarinus, a pest of cotton. In addition to these free-living and plantparasitic
species there are many which parasitize animals. Among them
are the tram biculids such as Trombicula autumnal is (Fig. 83), the European
harvest mite, which may cause dermatitis in man. Species of
Trombicula are parasitic in the larval stage only, the nymphs and adults
being free-living predators. In the Oriental and Australasian regions
Trombicula species transmit Rickettsia tsutsugamushi which causes scrub
typhus.
A stigmata
With this cosmopolitan group of mites there is a departure from
a predacious existence. Instead many members are fungivores and
FIG. 82. Prostigmata. Scirus longirostris. After von Vitzthum.
predators, phytophages and parasites. Smaller members are only I 00 or
so fLm in length whilst the upper limit oflength is in the order of 1·5 cm.
One-hundred-and-thirteen families are usually recognized in this
sub-order.
.VI embers of the Prostigmata are cosmopolitan in their distribution,
and their habitats are almost unlimited. The group contains the aquatic
and semi-aquatic water mites which are small, round-bodied, brightly
coloured inhabitants of fresh and salt water. The larvae of water mites are
usually parasitic on other arthropods whereas the later stages are predacious.
The gall and rust mites such as Aculus cornutus found on peach and
Aceria fraxinivorus on ash are also members of the Prostigmata. Several
gall mites have been demonstrated to be the transmitters of plant virus
diseases. Other members of the group include the snout mites or Bdellidae
which are active red, brown or green mites preying on small arthro-
FIG. 83. Prostigmata. Trombicula autumnalis, larva. After von Vitzthum.