Agriculture Reference
In-Depth Information
Litter assimilation by saprophages is of low efficiency. It was estimated at
ca.
25 %
for a community of cryptostigmatid mites in an old field and only 5 % of the assimilat-
ed energy was allocated to production. Estimations of assimilation made by a number of
authors are in the range 24-52 % and the percentage allocated to production in the range
3-28 % (see review by Luxton, 1982).
Post-embryonic development may last for several months. The parasitic Mesostigmata
have only two immature stages, a hexapod larva and a quadriped nymph before moulting
to the adult form. Acari of the three other non-parasitic groups pass through six develop-
ment stages,
i.e.,
prelarva, hexapod larva, protonymph, deutonymph, tritonymph and adult.
Pupation occurs between the larval and nymphal stages and again between nymphal and
adult stage. As a result, inactive forms may be relatively common in acarine populations:
Cryptostigmata,
e.g.,
may spend nearly 30 % of the annual cycle in moulting or resting
stages (Luxton, 1982). Most Cryptostigmata have one generation per year. Nevertheless,
larger species, or those from boreal and arctic environments may take 2-3 years to com-
plete their life cycles (Luxton, 1972; Mitchell, 1977). Similar figures are reported for
Mesostigmata Uropodida (Athias-Binche, 1985).
Reproduction is generally bisexual but certain species may be parthenogenetic (
e.g.,
Oliver, 1971; Athias-Binche, 1985); as in the Collembola, males deposit spermatophores
and females fertilise themselves. Cryptostigmata females lay only 1 to 6 eggs on
average (maximum 16 or more) which hatch from 1 to 6 weeks later, depending on
temperature. In other groups,
e.g.,
Prostigmata, the number of eggs produced varies
between 10 to 100, depending on the family and species involved (Kethley, 1990).
Resistance of Acari to water and temperature stress appears to be greater than that
of the Collembola. Cryptostigmata can withstand desiccation up -6.0 MPa (pF 5) before
moving to wetter areas (Vannier, 1970). As a consequence and in contrast to most soil
invertebrates, their population densities may be highest during the dry and hot seasons.
This is the case,
e.g.,
in the savannas of Lamto (Côte d'Ivoire) where maximum
densities were observed at water potentials equivalent to -1.6 to -5.0 MPa (pF 4.2-4.7) and
soil temperatures of 33-42 °C (Athias, 1976). In tundra soils at Point Barrow (Alaska), an
inverse correlation is observed between the density of Acari and soil moisture in certain
habitats (McLean, 1975). They are regularly found in such extreme environments as littoral
sands of the Mediterranean basin and in the fine sand dunes of the Namib desert (Coineau
and Seely, 1983). Application of water to a dry soil of the Chihuahuan desert in New Mexico
did not significantly increase their density (Kamill
et al.,
1985).
Acari also exhibit remarkable resistance to low temperatures which allows them to
colonise polar regions and they are the macroscopic organisms with the most southerly
populations. Wise and Gressitt (1965) reported the prostigmatid mite
Nanorchestes
antarcticus
to be the most southerly terrestrial animal species.
Alaskozetes antarcticus
and
Cryptopygus antarcticus,
two species from Signy island (Antarctica) had supercooling
points of -25 to -30
°C
(Block
et al.,
1978). Resistance to low temperatures was greatest
in starved individuals and nymphs were more resistant than adults.
Ecological categories
In most ecological studies, Acari are simply divided into the four suborders Cryptostigmata,
Astigmata, Mesostigmata and Prostigmata which exhibit some homogeneity in their
