344 Oh (he Geographical Distribution of Insects. 



The remaining 40,000 species may be divided, according to 

 M. Lacordaire, as follows : — Hymenoptei-a, 12,000 ; Lepidop- 

 tera, 10,000; Diptera, 10,000; Hemiptera, 5000 ; Neuroptera, 

 1500 ; Orthoptera, 1000 ; Parasita, 500. 



On the supposition that we are acquainted with one-third 

 of the Coleoptera, one-half of the Lepidoptera, one-fifth of the 

 Hemiptera, one-sixth of the Hymenoptera, Neuroptera, and 

 Orthoptera, one-tenth of the Diptera, and one-twentieth of 

 the Parasita, we would obtain the following numbers as the 

 absolute amoimt of the species existing on the globe : — 



Coleoptera, 120,000 



Diptera, 100,000 



Hymenoptera, .... 72,000 



Hemiptera, 25,000 



Lepidoptera, .... 20,000 



Parasita, .... 10,000 



Neuroptera, 9,000 



Orthoptera, .... 6,000 



362,000 



With regartl to the number of individuals of each species, 

 or, what is the same thing, its degree of rarity, it will readily 

 be perceived, that there are no data whereon to found a cal- 

 culation likely to afford even an approximation to the fact. 

 It may be merely remarked, that, with respect to the great 

 majority, they are the more common as we approach certain 

 countries, which are, as it were, the centre of their habitation ; 

 and that, on leaving this point, they cease more or less ab- 

 ruptly at distances varying according to the direction we fol- 

 low, so that they may be represented as radiating from the 

 centre in question. This fact is of importance to be noted, 

 for it is partly on it that the possibility rests of establishing 

 entomological regions. 



Absolute and Relative number of Species, Genera, and Fami- 

 lies in different countries. — The absolute number of the species 

 of a country depends on a multitude of circumstances which 

 are for the most part the same as those enumerated above. It 

 is evident that its richness, in this respect, will be proportionate 

 to its extent, its temperature, the nature of its vegetation ; to 

 the number and nature of its stations ; to the absence or pre- 

 sence of barriers separating it from neighbouring countries. 



