Savory - Arachnida 1977

204 Ill. PROLES ARACHNES
26. THE ORDER ACARI 205
Hypostome
Chelicera within
sheath
Hypostome
Capttulum
Porose area
behtnd
Carapace or
scutum
4th walktnQ leg
FIG. 80. Metastigmata. Ixodes ricinus; dorsal aspect. After Snow.
evertsi using only two hosts and Boophilus annulatus parasitizing only one.
In the former case the larva and nymph feed on the same animal and
in the latter case all three stages utilize the same host. Another modification
regarding feeding is that in many species the adult males do not
feed and in those which do they imbibe only small quantities of food.
This situation compares dramatically with the females which may suck
over 50 times their own weight of blood and other fluids, taking several
weeks to complete their single meal. Whereas the hard ticks are found
mainly on mammals, the Argasidae are most commonly, although
certainly not exclusively, located on birds. They feed intermittently,
often on the same animal and keep in the nesting area of the host. Most
species are parasitic in all stages, e.g. Argas persicus, while some feed only
as larvae and nymphs (Otobius megnini) and others in all but the larval
stage (Ornithodoros savignyi).
FIG. 81. Metastigmata. Ixodes ricinus; ventral aspect. After Snow.
Many ticks are important pathogen transmitters disseminating such
important diseases as red-water fever, forms of encephalitis, relapsing
fever and tularaemia.
Prostigmata
It is generally recognized that this group is a heterogeneous assemblage
of mites which share certain common features. Amongst these characters
are that the spiracular openings, when present, are paired and are
located either between the chelicerae or on the dorsal surface of the
propodosoma. Prostigmatid mites are usually weakly sclerotized and
have chelicerae which vary from being strongly chelate to reduced,
and pedipalps which may be simple, fang-like or clawed. (Fig. 82).
The group contains terrestrial, aquatic and marine forms and includes