GEOGRAPHICAL DISTRIBUTION OF INSECTS. 4.83 
render calculations on this subject very complicated, and 
throw a great degree of uncertainty over them*. This 
learned Entomologist would judiciously consider ento- 
mological climates under another view,—that which the 
genera of Arachnida and insects exclusively appropriated 
to determinate spots or regions would supply>. Linné’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 general, even if the countries 
are in the same parallel, composed of different species.” 
The natural limits of a country,—as mountainous ranges, 
rivers, vast deserts, &c.,—often also say to its insect po- 
pulation, “No further shall ye come;” interposing a bar- 
rier that it never passes*. Humboldt observes, with re- 
spect to the Scmudia 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®: and Mr. W.S. MacLeay 
makes a similar observation upon that of Gymnopleurus 
Ill.£. So that the real insect climates, or those in which 
certain groups or species appear, may be regarded as 
a Géograph. Génér. des Ins. 5. b Tbid. 
° Tbid..7—. @ Ibid. 8, 11. 
© Personal. Narrat. E.T. v. 88. He says also that each stream 
almost has its peculiar species (Jéid. 98), and that they sometimes 
emigrate to stations they had not infested before. Zoid 106--. 
© Hor. Entomolog, 519. 
Saline 
