Effects of certain Agents on Insects. 209 



The tenacity of life in insects, as exemplified in their suspended 

 animation, whether from want of free air, — as when immersed in 

 water, or buried in the earth, or surrounded by azote or carbonic 

 acid, may serve a useful purpose in the economy of nature. They 

 are the favourite food of many birds and fishes ; and in their torpid 

 state, with just sufficient life to preserve them from decomposition, 

 they are most easily obtained, and that at a time when such a 

 supply is peculiarly wanted ; and even their quitting their state of 

 torpor in temperate weather in winter, when they themselves ap- 

 pear to exist without food, may conduce to the same end, — the 

 affording of food to other animals higher in the scale of organiza- 

 tion, and especially such as are roused from a like state of torpor 

 by elevation of temperature. 



I am, my dear Sir, 



Yours very truly, 

 Lesketh How, Ambleside, J. DaVY. 



March 19th, 1851. 



To William Spence, Esq. 

 F.R.S., &c. 



P. S. — In the note with which you have favoured me of the 

 15th of April, you have called my attention to the effects of the 

 vapour arising from the bruised leaves of the common laurel on 

 insects, and also to that of prussic acid. Before describing the 

 experiments I have made in consequence, I shall, with your per- 

 mission, quote your words : — " There is," you remark, " one 

 vapour, on which, though now employed to kill insects, we want 

 more exact facts — that of the prussic acid let loose from the 

 bruised leaves of the common laurel. Entomologists find that 

 insects introduced into a wide mouthed phial, having a few of 

 these bruised leaves at the bottom, die almost instantly ; but 

 there is this inconvenience in employing them, that the watery 

 vapour, which they also give out, condenses on the side of the 

 phial, and injures the wings of the flies, &c, put into it." You 

 add : " It would be desirable to know how small a quantity of 

 the vapour of prussic acid suffices to kill an insect of medium 

 size ; whether some insects (as beetles) are less affected than 

 others (as flies, &c), and whether the size of the insect has any 

 thing to do with the result." 



Preparatory to making any trials with the prussic or hydro- 

 cyanic acid, I thought it right to institute one or two with the 

 bruised leaves of the laurel. 



Two leaves just gathered, torn into small pieces, and bruised in 



VOL. I. N. S. PART VII. — DEC. 1851. P 



