chap, ii.] DISPERSAL AND MIGRATION. 33 



and pupae of insects have their abode in solid timber, that they 

 might survive being floated immense distances. Great numbers 

 of tropical insects have been captured in the London docks, 

 where they have been brought in foreign timber ; and some have 

 emerged from furniture after remaining torpid for many years. 

 Most insects have the power of existing weeks or months with- 

 out food, and some are very tenacious of life. Many beetles 

 will survive immersion for hours in strong spirit ; and water a 

 few degrees below the boiling point will not always kill them. 

 We can therefore easily understand how, in the course of ages 

 insects may become dispersed by means which would be quite 

 inadequate in the case of the higher animals. The drift-wood and 

 tropical fruits that reach Ireland and the Orkneys ; the double 

 cocoa-nuts that cross the Indian ocean from the Seychelle Islands 

 to the coast of Sumatra; the winds that carry volcanic dust and 

 ashes for thousands of miles ; the hurricanes that travel in their 

 revolving course over wide oceans ; all indicate means by which 

 a few insects may, at rare intervals be carried to remote regions, 

 and become the progenitors of a group of allied forms. 



But the dispersal of insects requires to be looked at from 

 another point of view. They are, of all animals, perhaps the 

 most wonderfully adapted for special conditions ; and are so often 

 fitted to fill one place in nature and one only, that the barriers 

 against their permanent displacement are almost as numerous 

 and as effective as their means of dispersal. Hundreds of species 

 of lepidoptera, for example, can subsist in the larva state only on 

 one species of plant ; so that even if the perfect insects were 

 carried to a new country, the continuance of the race would de- 

 pend upon the same or a closely allied plant being abundant 

 there. Other insects require succulent vegetable food all the 

 year round, and are therefore confined to tropical regions ; 

 some can live only in deserts, others in forests ; some are de- 

 pendent on water-plants, some on mountain-vegetation. Many 

 are so intimately connected with other insects during some 

 part of their existence that they could not live without them ; 

 such are the parasitical hymenoptera and diptera, and those 

 mimicking species whose welfare depends upon their being 



