JL833.] FLOCKS OF BUTTERFLIES. 151 



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 ; * 

 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), Notaplms, Cynucus, Adimonia, and Sca- 

 rabseus. 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 Patagouian 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 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.t 



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 par- 



* Lycll's Principles of Geology, vol. iii. p. 63. 



j- 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. 



