The Geographical and Ecological Distribution of Arboreal Psocoptera

Annu. Rev. Entomol. 1985.30:175-196. Downloaded from arjournals.annualreviews.org
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192 THORNTON
quantity of microepiphytes increase with altitude, thus allowing the coexistence
of a greater number of species (although modification of lowlan,ds by man
is also an important factor). There have been no specialist studies of the vertical
distribution of psocid food on trees, but Turner (97) found significan~Ily greater
epiphyte densities above 1.5 m than nearer to the ground.
Several species of some eight families are known to be specialist exploiters
of dying and dead vegetation and in fact can be collected by first: breaking
branches and then returning later to collect a sample. Ectopsocus briggsi is a
typical example. Turner (96) noted that this small species, which feeds on the
spores and hyphae of decomposer fungi, had the greatest biomass of any psocid
in the late summer in one of the three years of his study, yet was otherwise rare.
He ascribed this to unusually moist conditions in late summer of that year,
following a summer drought, that favored the development of fungi on dead
plant material. Unpredictable climatic phenomena such as this as well as
random local disturbances to the canopy will have an important effect on the
distribution of such opportunistic species.
HUMIDITY Humidity clearly plays a key role in the life of psocids and in
aboreal species its effects may be related to food supply. Liposcelis species are
capable of absorbing water vapor from an atmosphere down to 58% relative
humidity (R.H.); below 58% R.H. water loss leads to death. Rudolph (66)
demonstrated this process in 22 psocid species of diverse habitats and representing
all major groups. The rates of both water loss and uptake were shown to
be markedly high for arthropods, and the uptake-to-loss ratio, a measure of the
efficiency of the water balance mechanism, was among the highest of all
arthropods studied. This ability appears to be characteristic of the order,
unrelated to specific environmental conditions. Differences in critical equilibrium
humidities (58-85%) and water loss and uptake rates were not correlated
with habitat, apart from the observation that domestic and nidicolous species
have lower critical values (58-76%) than arboreal species (76-85%). Intensive
and extensive studies of the microhabitats of species that differ greatly in
uptake/loss ratios are now needed.
The mechanism’s operation was shown to be highly dependent on food
supply, particularly in arboreal species in which food storage ability is evidently
poor and the physiological mechanism is upset unless the insects can feed at
short intervals. The indications are that above about 75% R.H., food supply,
rather than relative humidity itself, may be the limiting factor.
AIR POLLUTION AND PSOCID DISTRIBUTION Gilbert (28) has den~tonstrated
the indirect effects (through microepiphytes) of air pollution on the distribution
of corticolous psocids in the Newcastle area of the industrial north of England.
He showed a very high correlation between the diversity of the psocid fauna of