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XXV. On the Effects of certain Agents on Insects, in a 

 Letter addressed to William Spence, Esq., F.R.S., &c, 

 by John Davy, Esq., M.D., F.Tt.S. 



[Read 5lh May, 1851.] 

 My dear Sir, 

 In a letter which I had the honour to address to you last year 

 relative to the effects of change of temperature on insect life, in 

 a very limited way, I expressed the hope of being able to continue 

 the experiments and extend them to other agencies on the same 

 class of beings. This I have in some measure accomplished, and 

 I propose now to make known to you the results, which be pleased 

 to use in any way you may think proper. 



The inquiry was entered on in the last week of November, 

 and owing to the extraordinary mildness of the winter months has 

 been continued almost uninterruptedly to the present time, viz. 

 the middle of March. During the whole of this period the 

 thermometer here has never for twenty-four hours been below the 

 freezing point ; it has oftener, by day, been above 40° of Faren- 

 heit than below that degree. This mildness of atmosphere has 

 been accompanied with unusual humidity, and an extraordinary 

 fall of rain : in November the amount of rain here was 13 - 26 

 inches, in December 5*46 inches, in January ] 9-54 inches, as mea- 

 sured by the pluviometer ; and scarcely a day has passed during 

 the whole of the period that insects of several kinds have not 

 been seen on the wing or abroad in the open air. 



For the sake of order, and to be better able to compare the 

 results, I shall first notice those experiments which I have made 

 on the effects of temperature on insects ; next, of gases ; and 

 lastly, of vapours. Should my account of the trials be found 

 tedious, I trust it will be borne with, as without some minuteness 

 of detail, accuracy on such a subject is hardly attainable. For 

 the names of the species, when they are assigned, you know that 

 I am indebted to an able Entomologist, Francis Walker, Esq., 

 who at your request was so obliging as to examine most of the 

 individuals, the subjects of the experiments, and to return them 

 labelled. 



1 . On the Effect of Changes of Temperature on certain Insects. 



I shall first notice the effect of reduced temperature, that is, of 

 a reduction of many degrees below the annual mean, and also 

 below the freezing point. 



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