GEOGRAPHICAL DISTRIBUTION OF INSECTS. 495 
elements that render calculations on this subject very 
complicated, and throw a great degree of uncertainty over 
them*. This learned Entomologist would judiciously 
consider entomological climates under another view, — 
that which the genera of Arachnida and insects exclusively 
appropriated to determinate spots or regions would sup- 
ply b . 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 c ." 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* '. So 
that the real insect climates, or those in which certain 
3 Geograph. Gene?: des Ins. 5. b Ibid. 
c Ibid. 7—. d Ibid. 8, 11. 
e 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. 10G — . 
1 Hor. Entomolog. 519. 
