100 Cieoi/raphical Distribution of Insects. 



of exotic plants, which we cultivate either in green-houses or 

 in the open air. 



4. AVhen, on the contrary, a plant has congeners into the 

 country to which it is transported, it is subject to the attacks 

 of the insects belonging to that country. This is the case 

 with all the oaks, willows, and poplars of North America na- 

 turahzed in Europe, and also with the pine tribe, most of 

 which suffer w'hen introduced from the attacks of hylurgi, &c., 

 wliich are often so fatal to our indigenous fir-tree. 



But although we are unacquainted with any insect which 

 accompanies a particular vegetable wherever it may be con- 

 ve\-ed, it often happens that, when a plant extends beyond its 

 natural boundary, it is accompanied by one or more of the in- 

 sects unsupported in its native locality. Thus, it has happen- 

 ed, that since pine plantations were multiplied in the vicinity 

 of Paris, the Lamia cedilis, (the timmermau of the Swedes 

 and Laplanders, which they regard with a kind of religious 

 veneration), an insect of Northern Europe, and previously 

 unknown in that part of France, has begun to make its ap- 

 pearance. The corn weevil {Calandni granaria\ in like 

 manner, seems to exist in whatsoever place the cei'eal gi*asses 

 have been transported to. 



Influence of Temperature. — Temperatm'e actson insects me- 

 diately and immediately ; in the first instance, mediately by 

 its influence on vegetation, which is destroyed by an excessive 

 cold, and fostered by heat. The creo-saprophagous species, 

 that is, such as live on decomposed animal substances, are not 

 exempted from this influence, for the rarity of the species in 

 intertropical regions, formei'ly alluded to, probably arises 

 from the rapid decomposition of carcasses occasioned by the 

 excessive heat, which makes them entirely disappear almost 

 in a few hours. Whence, it follows, that only such species as 

 are developed with extreme rapidity, the muscidae, for ex- 

 ample, can alone live in these climates. The coleoptera, 

 whose gro\Ai:h is much more tardy, would have no time to 

 operate upon a dead body, and it is in this order in particu- 

 lar, that the rarity of which we speak is most notable. 



The direct effects of temperature are not less impoi'tant, 

 although they do not act so powerfully on insects as on plants. 

 The latter, in fact, require a determinate deo'ree of heat for 



