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Show 186 RIO PLATA. Dec. 1833. fine and calm, and the one previous to it equally so, with light and variable airs. Hence we cannot suppose that the 1· nsec t s wer e blown off the land' but we must conclude th. at they voluntarily took flight. The great bands of the Cohas seem at first to afford an instance like those on record of the migrations of Vanessa cardui ;.* but the pr~sen_ce of_ o~her insects makes the case distinct, and not so easily mtellig1ble. Before sunset, a strong breeze sprung up from the north, and this must have been the cause of tens of thousands of the butterflies and other insects having perished. On another occasion, when seventeen miles off Cape Corrientes I had a net overboard to catch pelagic animals. Upon dra~ing it up, to my surptise I _found a considerable humber of beetles in it, and although m 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 scarabreus. 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 habit~, it appeared to me most probable that they were floated mto the sea, by a small stream which drains a lake near Cape Corrientes. On any supposition, it is an interesting circumstance to find insects, quite alive, 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 in 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 ever knew of an insect being caught far from the land, was that of a large grasshopper (Acrydium), which flew on board, when the * Lyell's Geology, vol. iii., p. 63. Dec. 1833. AERONAUT SPIDERS. 187 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.* On several occasions, when the vessel has been within the mouth of the Plata, the rigging has been coated with the web of the Gossamer Spider. One day (November lst, 1832) I paid particular attention to the phenomenon. 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 alw~ys 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 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 maxillre drank eagerly of the fluid ; this same circumstance "' The flies which frequently accompany a ship for some days on its passage from harbour to harbour, wandering from the vessel, are soon lost, and all disappear. |