Savory - Arachnida 1977

200 Ill. PROLES ARACHNES
the protonymph, the deutonymph and the tritonymph. In one of the
groups of ticks, commonly called the hard ticks, there is only one
nymphal stage whereas in the other group, the soft ticks, there may be
as many as eight. In the free-living mesostigmatid mites there are
usually two nymphal stages (Fig. 77).
In many astigmatid mites, the deutonymph is a resistant stage and
differs from the preceding and succeeding nymphs in morphology and
behaviour. It is called the hypopus and is formed when conditions are
unfavourable. A hypopus does not occur in the life cycles of the skin
parasites (Sarcoptidae and Psoroptidae) which pass through only one
larval and two nymphal stages before becoming adult.
[
Adult
+·
Nymph
J
Deutovum
t
Egg (ov~m)
------ Larva
Fw. 77. The life cycle of the Acari.
Although Acari show a wide variety of adaptations regarding their
feeding habits, the mouth parts may be relegated to a common plan
which shows similarities with those of other Arachnida. The mouth
parts (chelicerae and pedipalpi) are borne on the anterior gnathosoma
which is little more than a hollow tube leading to the mouth. The roof
of this tube is the tectum capituli, the lateral walls are the fused pedipalpal
coxae and the floor is composed of the subcapitulum and the
endites of the pedipalpal coxae which are prolongated to form the
hypostome. This is especially conspicuous in ticks (Fig. 78).
The chelicerae together with the pedipalpi are the food-acquiring
organs. The chelicerae are variously modified, being chelate in some
scavenging and predatory forms where they are used for prehension,
or stylet-like, or armed with movable cutting digits as in some parasitic
forms. Variation is seen in the pedipalpi also and these may be leglike
, or large and clawed , or reduced in size and used as sense organs. In
certain fur mites of the .'\stigmata there are two lobe-like expansions of
the pedipalpal coxae which form clasping organs.
Acari are fluid-feeders and either feed on liquid food or render their
meal fluid by secreting enzymes from the salivary glands. Once liquefied
the meal is imbibed by the action of the suctorial pharynx.
Owing to the large number of species of Acari which exist and because
of their great diversity, it is more convenient to deal with the character-
26. THE ORDER ACARI 201
Interne I mala
Corn1culus
FIG. 78. Gnathosoma of a tick, Ixodes ricinus.
}""""··
1stics of each sub-order in turn. In the classification used here the
order Acari is divided into seven sub-orders. The distinction between
these taxa and some details of the biology of the members are given
below.
N otosti g mata
Members of this sub-order are large mites (1,000 ,urn+), elongated,
unsclerotized and leathery in texture. Their legs are long and slender
and they resemble harvestmen superficially. They have four pairs of
small dorso-lateral spiracular openings, located on the hysterosoma at a
level behind the fourth coxa. Notostigmatid mites show a preference
for dark, semi-arid environments and may be found under stones, rocks
and organic debris in parts of North America, South America, Central
Asia and the Mediterranean. These mites are omnivorous, feeding on
small arthropods and pollen grains. There is only one family, the
Opilioacaridae, containing a few genera such as Opilioacarus.
Tetrastigmata
Mites in this sub-order are large (2,000-7,000 ,urn), heavily sclerotized
and non-segmented externally. They possess a pair of ventro-lateral
spiracular openings at the level of the third coxae. A second pair of
openings located behind the fourth coxae, and called the air sac pores,
may be homologous with the expulsory vesicles found in some free-living
Mesostigmata.