MIGRATIONS. 101 



animals are regulated by the temperature as to 

 the season at which they prepare to retire for 

 the winter, except as to insects, which, with few 

 exceptions, are of that description. My learned 

 coadjutor, Mr. Spence, in our Introduction to 

 Entomology, has some remarks on this subject, 

 which seem, at first sight, to prove that the disap- 

 pearance of insects, at least those of the Coleoptera 

 order or beetles, is not preceded by any remark- 

 able lowering of the temperature ; on the con- 

 trary, he observed a great number of various 

 genera congregating with this view when the 

 thermometer was fifty-eight degrees in the shade. 1 

 This was about the middle of October. But there 

 is one circumstance to which he has not adverted, 

 which may tend to reconcile this fact with the 

 received opinion. The nights, at this time of the 

 year are often cold when the days are hot, the 

 latter also are much shortened and the former 

 lengthened, so that the sum-total of heat received 

 from the sun is very much diminished, which 

 may be the exciting cause of their hybernating 

 at this time, when the diurnal temperature is so 

 considerable. 



With regard to the swift, these birds seem to 

 avoid heat, they lie by in the middle of the day, 

 and only appear in the morning and evening. 

 Their early migration from this country may 



1 IntrocL to Ent. il. 433. 



