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Show 188 RIO PLA'fA, Dec. 1833. 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 in a lateral course, 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 is 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 rays of light; they were not, however, straight, but in undulations like a film of silk blown by the wind. They were more than a yard in length, and diverged in an ascending direction from the orifices. The spider then suddenly let go its hold, and was quickly borne out of sight. The day was hot and apparently quite 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 will almost always be evident. And this probably would be sufficient to carry with it so light an object as the little spider on its thread. 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 Dec. 1833. AERONAUT SPIDERS. 189 land, attached in vast numbers to the lines, proves that they ~re the manufact~r~rs of the mesh, and that the habit of sailmg through the air, IS probably as characteristic of some tr'b as tha.t o; diving i~ ?f the Argyroneta. We may then reJe:~ Latreille s supposition, that the gossamer owes its origin to the webs of the young of several genera, as Epeira or Thomisa: although, as we have seen that the young of other spiders do possess the power of performing aerial voyao-es. * During our different passages south of the Plata, I often to~ed aste.rn a net made of bunting, and thus caught many cuno~s ammals. Th~ structu:e of the Beroe (a kind of jelly fish) IS most extraordmary, with its rows of vibratory c'li • 1 re, and complicated though irregular system of circulation. Of Crustace~, th~re were many strange and undescribed genera. One, whiCh m .some respect~ is allied to the N otopods (or those ~rabs whiCh have their posterior legs placed almost o.n their backs, for the purpose of adhering to the under SI.de of !edges), is very remarkable from the structure of its hmd. pair of legs. . The penultimate joint, instead of being termmated by a Simple claw, ends in three bristle-like appendag~s of dissimilar lengths,-the longest equalling that o~ the entire leg. These claws are very thin, and are serrated With teeth of an excessive fineness, which are directed towards the base. The curved extremities are flattened, and * I was not at the time aware of M. Virey's very curious observations (Bulleti? des ~ciences Natur., tom. xix., p. 130) which seem to prove tha~ small spiders, m an atmosphere perfectly tranquil, and without the aid of any web! have the power of darting through the ai1·. M. Virey, believes that by means of a rapid vibration of their feet, they walk tlw air. Although in his case, the conclusion seems almost inevitable, yet in the one I have described, we must suppose that the several threads which were sent forth, served as sails for the atmospheric currents to act on. After read~ng M. Virey's account, it appears to me far from improbable, that the httle ~ero~aut actually did unite, as was suspected, its feet together by s~me fine hnes ; thus forming artificial wings. I regret I did not dete~ mme this point with accuracy ; for it would be a curious fact, that a sp1der should thus be able to take flight by the aid of temporary wings. |