SOIL ORGANISMS - Soil Ecology - page 265

have fewer stadia than others. Development through the reproductive instars generally
takes 40 to 400 days and moulting is continuous throughout life (Massoud, 1971).
Reproduction may be parthenogenetic or bisexual. In the latter case, copulation does
not take place; males deposit spermatophores and females fertilise themselves by rubbing
their genital aperture with these spermatophores. A number of different behaviours may
facilitate this process, e.g., primitive or specialised courtships and the production of
pheromones (Schaller, 1953; Betsch-Pinot, 1977; Blanquaert and Mertens, 1977) although
Petersen (1980) considers this system to be inefficient. Parthenogenesis is more frequent
in species living in the soil interstices than in litter dwellers. This situation may have
been selected to obviate the problems faced by females in locating spermatophores in the
restricted environment of the soil pore space. Each female may lay 100 to 600 eggs
during her adult life time, which rarely exceeds one year.
Resistance to desiccation and low temperatures has been developed in some species.
Isotoma saltans is a common inhabitant of the surface of alpine glaciers and Anurophorus
subpolaris has been reported to survive temperatures of -50 °C in Antarctica (Wise, 1965;
Block, 1983). The physiological state of the individuals concerned partly determines
their ability to tolerate low temperatures (Vernon and Vannier, 1990).
A few cases of anhydrobiosis have been reported in Collembola (Poinsot, 1968).
Desiccation tolerance is generally limited to a metric potential of -5.0 MPa (pF 4.7)
(Vannier, 1970), although it varies greatly among species (Verhoef and van Selm, 1983).
The amount of water lost before activity ceased varied from 16.2 to 33.8 % of body
fresh weight in six liner-dwelling species from woodlands in the Netherlands (Vegter and
Huyer-Brugman, 1983). Vannier (1978) defined three types of transpiration regime
depending on the efficiency of body water control. Hydrophilic species exert no control,
mesophilic a limited control and xerophilic a strong control. Again this pattern separates
soil-dwelling from the liner-dwelling species which have a much greater drought
resistance (Figure III.31). Tolerance may change during life as juveniles are usually
much less desiccation tolerant than older animals (Betsch and Vannier, 1977).
In Allacma fusca, the difference was due to the absence of tracheae in the early stadia.
In some species, populations persist because of the survival of eggs whose drought
tolerance is greater than that of adults (Wallace, 1968).