52 GEOGRAPHICAL DISTRIBUTION. 



upon some special vegetable product for their existence, whether it 

 be, as it may happen, the leaf, the flower, or the juice of the plant 

 in question. Again, while in some cases the adult insect may be 

 entirely independent of such a circumscribed food-supply, the larva 

 may still be governed in its diet by a particular kind, without 

 which, consequently, the prolonged reproduction of the species 

 would be impossible. Such instances of limitation are exhibited 

 by numerous forms of caterpillars. Hence, it is not difficult to com- 

 prehend why, in regions which are affected by similar conditions of 

 climate, and which collectively show a general correspondence in 

 the character of the vegetation, certain species of insects should be 

 found at one locality and not at another, even where no physical 

 barrier separating the two should be interposed. In fact, the bar- 

 rier interposed by conditions of vegetable growth is fully as effective 

 in restraining a broad specific distribution as are the barriers re- 

 sulting from the physical conditions of the earth's surface, most 

 of which they are able to overcome, either voluntarily or involun- 

 tarily. The mature insect, from its lightness, is frequently carried 

 away in aerial currents from its native or favourite haunts to regions 

 widely remote, in a manner precisely similar to what obtains in the 

 case of birds. Hawk-moths have been caught on board ship at a 

 distance of two hundred and fifty miles from shore, and a large 

 Indian beetle (Chrysochroa ocellata) was captured some years ago, 

 in the Bay of Bengal, at a distance of two hundred and seventy 

 miles from the nearest land. During Captain King's expedition to 

 the Straits of Magellan dragon-flies flew on board his vessel when 

 still fifty miles out at sea (south of the Rio de la ( Plata) ; and Admi- 

 ral Smyth reports that, in the Mediterranean, myriads of flies were 

 brought to his ship by a southerly wind from a region fully one 

 hundred miles distant. A beetle is recorded by Darwin as having 

 been caught aboard the " Beagle" when the vessel was upwards of 

 forty miles distant from the nearest shore ; from what actual dis- 

 tance the insect may have come could, necessarily, not be deter- 

 mined. A locust was observed by the same naturalist three hundred 

 and seventy miles from land; and in 1844 swarms of these insects, 

 "several miles in extent, and as thick as the flakes in a heavy 

 snow-storm, visited Madeira. These must have come with perfect 

 safety more than three hundred miles, and, as they continued flying 

 over the island for a long time, they could evidently have travelled 



