Savory - Arachnida 1977

86 II. DE ARACHNIDIS
.Man is the only animal that constructs devices or machines that
enable him to travel. '\'heeled vehicles, ships and aeroplanes have all
helped him to carry himself and his impedimenta wherever he has
wished to go. And very often Arachnida have accompanied him,
uHawares.
of these excursions have been trivial and of no significance
because the stowaways have been unable to survive in their new
surroundings: familiar examples are the so-called "banana spiders",
which periodically arrive at British ports in bunches of bananas.
To these may be contrasted the fair number of spiders recognized as
members of the British fauna, which have reached this country in
recent years .. They include Ostearius melanopygius, Hasarius adansoni,
Achaearanea tepidariorum and Argiope bruennichi . . l\lembers of other orders
arc known, if more rarely, to have made similar journeys, successfully.
The surprising report in 1914 by Herland of a species of Eukoenenia
apparently acclimatized to life in the Natural History Museum of Paris
was followed in 1933 by the finding of Eukoenenia mirabilis on the lower
slopes of ~lount Osmond, Adelaide, and quite recently Marples has
collected others in Samoa. Accompanying them in Samoa there were
some Schizomida, while Cooke and Shadab (1973) have found immigrant
Cropygi in Gambia, and Wcygoldt (1972) has reported Amblypygi
that have been living in the Dodecanese Islands of Rhodos and Kos.
Finally there may be mentioned the support gi\'Cn by Arachnida to
Wegencr's theory ofCcutinental Drift. Legc-ndre (1968) has ·written a
L1scinating essay on Gondwanaland and its problems, in which he
refers to a number of significant examples.
Forster ( 1949) reported the close affinity between certain Kew
Zealand Cyphophthalmi and members of the same order found in
South Africa and South America.
The order Ricinulei is known from the Amazon basin, the caves of
~lexico and vVcst Africa.
Among theraphosid spiders, Zapfc (1961) pointed out the occurrence
of the sub-family ~liginae in Chile, Australia and .0J ew Zealand.
Legendre himself instances the family Archaeidac, discovered in Baltic
amber before it was found living, whose type genus Archaea is known
from South Africa, ~hdagascar, New Zealand and Australia.
As striking as any of these is the distribution of the four remarkable
genera of spiders that catch their prey by means of a ball of silk at the
end of a swinging thread. These "bolas spiders" belong to the genera
Dichrosticus in Queensland, .Mastophora and Agatostichus in America
and Cladomelea in South Africa.
10
Ecology: Migration and Dispersal
The dispersal of animals of any group throughout the country of its
occupation depends ultimately on t\VO factors: the means of tra\'Cl
adopted by or a\·ailablc to the individual, and the physical conditions
that it encounters during its travels. The first, controlling the distance
that it can go, determines the area that can be colonized; the second
determines its sun·ival at the end of its journey.
Few Arachnida are great travellers. There arc many species of
scorpions that scarcely trouble even to move away from one another
and have been somewhat loosely described as li\·ing in colonies. But
there is no social organization in these gatherings: scorpions are
naturally lazy creatures, and if food is available they will not tran·l far.
Hunger is often one of the chief stimuli to acti\·ity.
Among spiders one method of migration stands out, the well known
gossamer flight. A spider about to migrate shows what seems to be an
irresistible desire to climb; it scales the nearest vertical object, be it
gatepost, railing or tree, with a persistence that has all the outward
appearance of determination and purpose. When it reaches the summit
it turns round so that it is facing the direction of the wind, and,
its abdomen, secretes a droplet of silk from its spinnerets. The wind
draws this out into a long, buoyant streamer, and soon the spider
releases its hold and flies away.
There is no cause for surprise at the interest that this method of
dispersal has aroused for so long, for it is without parallel among other
animals. Dispersal by drifting is common enough and is shown by all
the multitude of marine things that compose the plankton of the seas,
as well as by those that similarly float through the air. This aerial
plankton has been found to include spiders, plentifully up to heights of
61 m and occasionally up to 3,050 m. But these planktonic creatures
are drifting, like seeds and spores, only because they are so small and
light; none of them has produced an extended current-catcher like the
spider's long thread. This thread is comparable to a parachute or glider,