then suddenly let go its hold of the post, and was quickly borne out of
sight. The day was hot and apparently calm; yet under such
circumstances, the atmosphere can never be so tranquil as not to affect
a vane so delicate as the thread of a spider's web. If during a warm
day we look either at the shadow of any object cast on a bank, or over
a level plain at a distant landmark, the effect of an ascending current
of heated air is almost always evident: such upward currents, it has
been remarked, are also shown by the ascent of soap-bubbles, which will
not rise in an in-doors room. Hence I think there is not much
difficulty in understanding the ascent of the fine lines projected from
a spider's spinners, and afterwards of the spider itself; the
divergence of the lines has been attempted to be explained, I believe
by Mr. Murray, by their similar electrical condition. The circumstance
of spiders of the same species, but of different sexes and ages, being
found on several occasions at the distance of many leagues from the
land, attached in vast numbers to the lines, renders it probable that
the habit of sailing through the air is as characteristic of this
tribe, as that of diving is of the Argyroneta. We may then reject
Latreille's supposition, that the gossamer owes its origin
indifferently to the young of several genera of spiders: although, as
we have seen, the young of other spiders do possess the power of
performing aerial voyages. [7]
During our different passages south of the Plata, I often towed astern
a net made of bunting, and thus caught many curious animals. Of
Crustacea there were many strange and undescribed genera. One, which
in some respects is allied to the Notopods (or those crabs which have
their posterior legs placed almost on their backs, for the purpose of
adhering to the under side of rocks), is very remarkable from the
structure of its hind pair of legs. The penultimate joint, instead of
terminating in a simple claw, ends in three bristle-like appendages of
dissimilar lengths--the longest equalling that of the entire leg. These
claws are very thin, and are serrated with the finest teeth, directed
backwards: their curved extremities are flattened, and on this part
five most minute cups are placed which seem to act in the same manner
as the suckers on the arms of the cuttle-fish. As the animal lives in
the open sea, and probably wants a place of rest, I suppose this
beautiful and most anomalous structure is adapted to take hold of
floating marine animals.
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