102 Geographical Distribution of Insects, 



The comparison of the maxima and minima of tempera- 

 ture between Rome and New York, affords the following ex- 

 planation of this difference. 



Mean temperature of the Mean temperature of the 



warmest mouth, coldest month, 



New York, + 28 1 R. 3. 7 



Rome, +25.0. + 5.7- 



Thus, the difference between the maximum and minimum 

 of temperatiu-e is 31°. 8 at New York, wliile at Rome, it is 

 only 19". 3. An insect in the former of these countries is, 

 therefore, subjected to much greater alternations of heat and 

 cold than one placed in the other. But, on the one hand, it 

 passes the cold season in the state of larva or pupa, without 

 sustaining harm from it ; and, on the other, it enjoys S^.l 

 more heat dm-ing the summer. If it belongs to an equatorial 

 genus, therefore, it is placed, at New York, under conditions 

 much more nearly allied to those of its intertropical congeners, 

 than an insect at Rome could be. 



Mr Macleay explains, on analogous principles, why the 

 coleoptera, hemiptera, hymenoptera, &c., are so limited in 

 species in the polar regions, while the culicidae are generated 

 in millions during the summer, and are even more annoying 

 in these northern latitudes than under the tropics. In these 

 regions winter continues nearly nine months, and the thermo- 

 meter often descends to — 40° R. while it ascends dm-ing the 

 summer to -j- 30" and even to 33". But this short duration of 

 the heat harmonizes with the short lived existence of the culi- 

 cidae, which, moreover, pass their early states in the water, where 

 they are sheltered from the extreme cold, while the coleop- 

 tera, living for a longer period in the perfect state, require a 

 longer continued heat, and as they pass their first stages in 

 the earth or the interior of vegetables, they are not so well 

 sheltered from the cold. 



forms may be prevented extending northward into Europe by the interven- 

 tion of the Mediterranean. M. Lacordaire states, that equatorial forms arc 

 represented in Europe by scarcely more than two species. Danais chrysip- 

 ^Ms, which is found in Calabria, and Charaxes jasius, which extends its habitat, 

 as far as the middle of France. Pimdia, Akis, Scaurm, Bnichycems, &c. 

 which are found around the basin of the Mediterranean, do not belong- 

 to the forms in question, but to those of the temperate zone in the vicinity 

 of the tropics. 



