CRUSTACEANS. 



entangled together, are carried into the air by the upward current causecl by 

 the rarefaction of the stratum near the heated ground, during the middle of 

 the day ; and that at night, the earth being cooled, the air descends, bringing 

 with it the accumulated webs, which, lying thick upon the herbage, are called 

 "gossamer." 



Mr. Darwin's observations relative 'to these gossamer spiders are very in- 

 teresting. His ship was sixty miles from land, in the direction of a steady 

 though light breeze, and vast numbers of small spiders covered the rigging 

 with their webs. 



" The little spider, when first coming in contact with the rigging, was always 

 seated on a single thread. The little aeronaut, as soon as it arrived on board, 

 was very active, 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. While watching some that were suspended 

 by a single thread, the slightest breath of air bore them out of sight. I re- 

 peatedly 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, with a rapidity which was quite unaccount- 

 able. I thought that I could perceive that the spider, before performing the 

 above preparatory steps, connected its legs together with most delicate threads." 



M. Virey has recorded some very curious observations t? Bulletin des 

 Sciences Nat.," torn. xix. p. 130), which seem to prove that small spiders in 

 an atmosphere perfectly tranquil, and without the aid of any web, have the 

 power of darting through the air ; and believes that, by means of a rapid 

 vibration of their feet, they walk the air. 



" After reading M. Virey's account," says Mr. Darwin, " it appears to me far 

 from improbable that, in the case above recorded, the little aeronaut actually 

 did, as was suspected, unite its feet together with some fine lines, thus forming 

 iirtificiai wings. I regret that I did not determine this point with accuracy, 

 for it would be a curious fact that a spider should thus be able to take flight 

 by the aid of temporary wings." " Voyage of the Beagle? 



CHAPTER XIII. 



FIFTH CLASS OF ARTICULATED ANIMALS. 

 CRUSTACEANS. 



THE Insects and Arachnidans described in the preceding 

 chapters are air-breathing animals : even in such species as 

 inhabit fresh water, respiration is strictly aerial. No insects or 

 spiders could live in the sea ; and, consequently, the waters of the 

 ocean would be utterly untenanted by corresponding forms of 

 life, had not a class of beings belonging to the articulated division 

 of the animal world been so constructed as to be capable of re- 



