On the Geographical Distribution of Insects. 337 



almost a complete solitude, and these animals are found only 

 in one or other of their three early states, that of egg, larva, 

 or pupa." 



Period of the appearance of the petfect Insects. Every spe- 

 cies, under vi^hatsoever climate it may live, has one or more 

 fixed periods for appearing in the perfect form, according to 

 the rapidity with which its various transformations take place. 

 These periods may either be accelerated or retarded by the 

 eifect of temperature ; but they are not the less regular, when 

 considered in a general manner. 



In cold and temperate countries, they commence with the 

 return of heat and vegetation, and a remarkable coincidence 

 exists between the appearance of the insect and the plant 

 which is to afford it food. Hence we may conclude, a priori, 

 that wherever vegetation is developed with extreme rapidity, 

 and as it were by a sudden impulse, the same thing will take 

 place with insects, and vice versa ; and this, in reality, is found 

 to be the case. Thus, in the polar regions, where heat equal 

 to that of the tropics all at once succeeds a degree of cold 

 much more intense than that of om- severest winter, while the 

 ground is not yet free from snow it becomes covered with 

 plants in flower, and the air filled with insects, not very va- 

 ried in regard to species, but of which the individuals are in 

 myriads. In proportion as we recede from these desolate re- 

 gions, and reach more southern latitudes, vegetation and in- 

 sects are found to be developed less suddenly, but the two 

 always continuing cotemporaneous. In our temperate re- 

 gions, the months of April, May, and .June, are the seasons 

 when these animals are in greatest abundance. The number 

 diminishes during the heats of summer, and this decrease is 

 the more perceptible the further south we advance. This 

 fact is deserving of being remarked, for it accords with what 

 takes place on a larger scale in intertropical regions. In Sep- 

 tember and October, a kind of augmentation again occurs, 

 which coincides with the flowering of certain autumnal plants. 



* For more ample details regai-din}^ stations, the numerous works on the 

 clas8 of insects may be consuUed, in which they are in general accurately 

 indicated. A Memoir by M. G. Silbermauu, inserted in the Rivuc entmno- 

 logique, torn. i. deserves particularly to be mentioned. 



