Geographical Distribution of Insects. 101 



a certain period, and can resist only a certain degree of cold. 

 In absence of the former, the seeds fail to ripen ; if the latter 

 be exceeded, the plant is destroyed. In these respects, insects 

 are not confined within such narrow limits. 



It is true that they also require a certain temperature at a 

 pai'ticular period of their lives, that, namely, of their transfor- 

 mation into perfect insects ; but this period may be prolonged 

 almost to an indefinite extent, without compromising the life of 

 the animal. It has been proved that, by placing a chrysalis 

 in an icehouse, its exclusion may be retarded for one or two 

 years, and that during this stage of its life, as well as while a 

 larva, many insects may be frozen without causing death. 

 \ow, let us suppose, that, by some sudden change in the con- 

 .stitution of our planet, the temperature of winter were to con- 

 tinue for an entire year. On the return of warmth, almost 

 the whole vegetable kingdom would be destroyed in our cli- 

 mates, while insects would preserve the greater part of their 

 species. What maintains their races, is, then, the established 

 order according to which nearly all, as in our countries, pass 

 the winter either in the q^^, larva, or nymph state. Indivi- 

 duals which sometimes pass this season in the perfect condi- 

 tion, invariably take refuge in suitable retreats, and are thus 

 preserved from injury, like plants protected by a covering of 

 snow. 



The extremes of heat and cold, as has been remarked by 

 Mr Macleay, are much more essential in determining a locality 

 flian the mean annual temperature of the year. In this, we 

 can discern the reason of certain remai'kable phenomena, such 

 as the following ; that the intertropical forms of insects are 

 ftrolonged much farther north in the New than in the Old 

 Continent, which is the reverse of what botanists have ob- 

 sei'vcd in regard to plants. Thus, we find in the vicinity of 

 New York, 40° 46' Lat. N. Fhameiis carnifex, Butela sexpunc- 

 lafa, Gymnotis nitida, and a considerable number of other 

 species belonging to genera, essentially equatorial ; while the 

 insects of Oporto or of Rome, situate under the same paral- 

 lel, have a, fades infinitely less, resembling that of the species 

 of Asia or of equatorial Africa.* 



* This difference, liowever, it should be remarked, may partly arise f r om 

 the countries being continuous, or nearly so, in one case, while African 



