Savory - Arachnida 1977

88 II. DE ARACHNIDIS
capable of transporting for hundreds of kilometres an animal which,
without it, would be wafted as many centimetres.
In the spring and autumn, seasons in which large numbers of spiders
are hatched, gossamer is more than usually prevalent and carries
chiefly young
; in the summer the aeronauts include many
half-grown and mature examples of the smaller species. The earliest
opinion was that one ~pecies, "the gossamer spider", was responsible
for all these threads, but this is not so. Emerron in 1918 found 69
American species in the Bristowe in 1930 listed 30 British ones
belonging to six families, and Bishop in 1945 identified 25 species in
six families from heights between 6·1 and 1,524 m. It is believed today
that nearly all the families contain species which undertake aerial
travel at some stage in theirlives, though it is clear that this cannot be
expected of the largest, which are too heavy to fly.
Even so, it cannot be maintained that the "ballooning habit" is
common to all, or even to the majority of spiders. Take Emerton's total
of69, Bristowe's total of30, and Bishop's total of25; add them together
and then, not to seem parsimonious, multiply the sum by ten. This
makes a generous allowance for more in different countries,
but the product is only 1,240, a very small fraction of the number of
species of spiders known to exist.
A different method of dispersal is shown by some of the smallest mites
that are parasitic on fruit t1 ees. These in the summer, either
by grasping the legs of passing insects, or standing erect and leaping
vertically if they feel a puff of air. The leap may then help them to be
blown some distance. The common British mite Belaustium nemorum is
often found holding the legs ofTipulidae; and several ofuropod
mites attach themselves by a thread of their own excrement to the bodies
of beetles.
A more peculiar method of travel is found among false scorpions,
and is known as phoresy. False scorpions are essentially lovers of darkness,
and yet on occasions they are to be seen clinging to the legs of
flies, harvestmen and other creatures, and thus being carried about at
no effort to themselves. They grasp the fly's leg in their chelicerae but
they are not parasites, for they neither injure nor feed on the fly.
1\\·enty-five or more species belonging to about 20 genera have been
found in these circumstances. The habit is not equally prevalent at all
times of the year, but during the short time that it appears it is reasonably
frequent. Thus Vachon ( 1932) recorded the finding of 78
mens on the legs of 57 harvestmen in one week at the end of August.
All belonged to the same species, and all but one were mature females.
The highest number found on the legs of any one harvestman was
eight.
l 0. ECOLOGY: MIGRATION AND DISPERSAL 89
The maturity of the travellers shows that the habit of phoresy has no
relation to the dispersal of a brood of youngsters. Vachon found that all
his captures were either recently fertilized or alternatively that they
had just left the brood-chamber in which they had reared and fed their
family. In either case they were in urgent need of food, and he concluded
that hunger had made them seize the legs of flies or harvestmen,
and that as a result they reached areas where food might be plentiful.
An alternative view, which has the support of the authority of
Muchmore ( 1971) is that the false scorpion grasps the fly's or harvestman's
leg in its search for f(>od, and that the bearer moves on before the
passenger lets go. The consequent dispersal is therefore only accidental.
A word should be added concerning plain, unspecializcd wanderings.
The impression that may be given by the last few paragraphs, or by
the casual statement that Arachnida can escape from their kin by
merely running avvay and seeking fresh fields, is one that can easily be
exaggerated. A reader may think of such an animal as one that has no
home, and which,
continuously, may find itself many miles
from its birthplace. This does not, in happen. Entomologists who
have painted coloured spots on ordinary house-flies, so that they can be
recognized if they are caught find that flights of a kilometre or
so are the exception.
Even the wolf spiders that one secs in a meadow arc likely to have
been born in it and are as likely to die in it: the individual, in fact,
lives normally in a circumscribed area over which it hunts and \\·here it
is too busily occupied in daily life to have the opportunity for foreign
travel. This is conspicuous when wolf spiders arc kept under observation
in the laboratory. In a large round pneumatic trough a solitary
spider has an area of about 2,600 cm 2 of sand and gravel, where it
energetically hunts and catches any insects that may be offered to it.
And somewhere in this field it has found a spot, under a stone or
between two stones, which, though to our eyes no different from other
spots, has attracted it and has become its "home". Here it usually rests
when not on the prowl, and here it returns after its forays, displaying
a homing instinct or mnemotaxis. This description of the act of homing
should be compared with the account of the function of the lyriform
organs, given in Chapter 3.
:\figration in such ways as these is so largely dependent on chance
that it is not inevitably followed by survival in the new surroundings.
The migrant must be adapted to its new circumstances, and the
methodical study of the relations between the organism and all the
features of its environment has grown into the established science of
Ecology. This may be said to present two sides,
as
autecology and synecology.