GEOGRAPHICAL DISTRIBUTION OF INSECTS. 495 



elements that render calculations on this subject very 

 complicated, and throw a great degree of uncertainty over 

 them a . This learned Entomologist would judiciously 

 consider entomological climates under another view, 

 that which the genera ofArachnida and insects exclusively 

 appropriated to determinate spots or regions would sup- 

 ply 5 . Linne's dictum with regard to genera will here also 

 apply ; " Let the insects point out the climate, and not the 

 climate the insects." If you expect invariably to find the 

 same insects within the same parallels of latitude, you 

 will be sadly disappointed; for, as our author further 

 observes, " The totality or a very large number of 

 Arachnida and insects, the temperature and soil of whose 

 country are the same, but widely separated, is in gene- 

 ral, even if the countries are in the same parallel, com- 

 posed of different species ." The natural limits of a 

 country, as mountainous ranges, rivers, ' vast deserts, 

 &c., often also say to its insect population, " No 

 further shall ye come ;" interposing a barrier that it never 

 passes d . Humboldt observes, with respect to the Simulia 

 and Culices of South America, that their geographical 

 distribution does not appear to depend solely on the heat 

 of the climate, the excess of humidity, or the thickness 

 of forests ; but on local circumstances that are difficult 

 to characterize 6 : and Mr. W. S. MacLeay makes a 

 similar observation upon that of Gymnopleurus f . So 

 that the real insect climates, or those in which certain 



a Geograpk. Gener. des Ins. 5. b Ibid. 



c Ibid. 7 d Ibid. 8, 11. 



c Personal Narrat. E. T. v. 88. He says also that each stream 

 almost has its peculiar species (Ibid. 98), and that they sometimes 

 emigrate to stations they had not infested before. Ibid. 106 . 



f Hor. Entomolog. 519. 



