THE VOYAGE OF THE BEAGLE 175 



mosphere 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 evi- 

 dent: 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 dif- 

 ferent 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 



7 Mr. Blackwall, in his Researches in Zoology, has many excellent observa- 

 tions on the habits of spiders. 



