The Seamen Cried Out "It Was
Snowing Butterflies," And Such In Fact Was The Appearance.
More Species Than One Were Present, But The Main Part Belonged
To A Kind Very Similar To, But Not Identical With, The
Common English Colias Edusa.
Some moths and hymenoptera
accompanied the butterflies; and a fine beetle (Calosoma)
flew on board.
Other instances are known of this
beetle having been caught far out at sea; and this is the
more remarkable, as the greater number of the Carabidae
seldom or never take wing. The day had been fine and calm,
and the one previous to it equally so, with light and variable
airs. Hence we cannot suppose that the insects were blown
off the land, but we must conclude that they voluntarily took
flight. The great bands of the Colias seem at first to afford
an instance like those on record of the migrations of another
butterfly, Vanessa cardui; [5] but the presence of other insects
makes the case distinct, and even less intelligible. Before
sunset a strong breeze sprung up from the north, and this
must have caused tens of thousands of the butterflies and
other insects to have perished.
On another occasion, when seventeen miles off Cape Corrientes,
I had a net overboard to catch pelagic animals.
Upon drawing it up, to my surprise, I found a considerable
number of beetles in it, and although in the open sea, they
did not appear much injured by the salt water. I lost some
of the specimens, but those which I preserved belonged
to the genera Colymbetes, Hydroporus, Hydrobius (two species),
Notaphus, Cynucus, Adimonia, and Scarabaeus. At
first I thought that these insects had been blown from the
shore; but upon reflecting that out of the eight species four
were aquatic, and two others partly so in their habits, it
appeared to me most probable that they were floated into the
sea by a small stream which drains a lake near Cape Corrientes.
On any supposition it is an interesting circumstance
to find live insects swimming in the open ocean seventeen
miles from the nearest point of land. There are several
accounts of insects having been blown off the Patagonian
shore. Captain Cook observed it, as did more lately Captain
King of the Adventure. The cause probably is due to the
want of shelter, both of trees and hills, so that an insect on
the wing with an off-shore breeze, would be very apt to
be blown out to sea. The most remarkable instance I have
known of an insect being caught far from the land, was that
of a large grasshopper (Acrydium), which flew on board,
when the Beagle was to windward of the Cape de Verd
Islands, and when the nearest point of land, not directly
opposed to the trade-wind, was Cape Blanco on the coast of
Africa, 370 miles distant. [6]
On several occasions, when the Beagle has been within
the mouth of the Plata, the rigging has been coated with
the web of the Gossamer Spider. One day (November 1st,
1832) I paid particular attention to this subject. The weather
had been fine and clear, and in the morning the air was full
of patches of the flocculent web, as on an autumnal day in
England. The ship was sixty miles distant from the land, in
the direction of a steady though light breeze. Vast numbers
of a small spider, about one-tenth of an inch in length, and of
a dusky red colour, were attached to the webs. There must
have been, I should suppose, some thousands on the ship. The
little spider, when first coming in contact with the rigging,
was always seated on a single thread, and not on the flocculent
mass. This latter seems merely to be produced by the
entanglement of the single threads. The spiders were all of
one species, but of both sexes, together with young ones.
These latter were distinguished by their smaller size and
more dusky colour. I will not give the description of this
spider, but merely state that it does not appear to me to be
included in any of Latreille's genera. The little aeronaut as
soon as it arrived on board was very active, running about,
sometimes letting itself fall, and then reascending the same
thread; sometimes employing itself in making a small and
very irregular mesh in the corners between the ropes. It
could run with facility on the surface of the water. When
disturbed it lifted up its front legs, in the attitude of
attention. On its first arrival it appeared very thirsty, and
with exserted maxillae drank eagerly of drops of water, this
same circumstance has been observed by Strack: may it not be in
consequence of the little insect having passed through a dry
and rarefied atmosphere? Its stock of web seemed inexhaustible.
While watching some that were suspended by a
single thread, I several times observed that the slightest
breath of air bore them away out of sight, in a horizontal
line.
On another occasion (25th) under similar circumstances,
I repeatedly observed the same kind of small spider,
either when placed or having crawled on some little eminence,
elevate its abdomen, send forth a thread, and then
sail away horizontally, but with a rapidity which was quite
unaccountable. I thought I could perceive that the spider,
before performing the above preparatory steps, connected
its legs together with the most delicate threads, but I am not
sure whether this observation was correct.
One day, at St. Fe, I had a better opportunity of observing
some similar facts. A spider which was about three-tenths
of an inch in length, and which in its general appearance
resembled a Citigrade (therefore quite different from the
gossamer), while standing on the summit of a post, darted
forth four or five threads from its spinners. These, glittering
in the sunshine, might be compared to diverging rays of
light; they were not, however, straight, but in undulations
like films of silk blown by the wind.
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