FOOD OF INSECTS. 225 



Lepisma sacchanna (the common " wood " or " sugar fish ") in a pill-box 

 containing only a few grains of magnesia, found it, to his great surprise, 

 alive arid active in June, 1833, after this protracted confinement, without 

 food, of two years. 1 



In some cases the very want of food, however paradoxical the proposi- 

 tion, seems actually to be a mean of prolonging the life of insects. At 

 least one such instance has fallen under my own observation. The aphidi- 

 vorous flies, such as Sccsva Pyrastri, &c., live in the larva state ten or 

 twelve days, in the pupa state about a fortnight, and as perfect insects pos- 

 sibly as long, the whole term of their existence in summer not exceeding at 

 the very utmost six weeks. But one 2 , which I put under a glass on the 2d of 

 June, 181 1, when about half-grown, and, after supplying it with Aphides once 

 or twice, by accident forgot, I found, to my great astonishment, alive three 

 months after ; and it actually lived until the June following without a par- 

 ticle of food. It had, therefore, existed in the larva state more than eight 

 times as long as it would have lived in all its states, if it had regularly 

 undergone its metamorphoses, which is as extraordinary a prolongation of 

 life as if a man were to live 560 years. It is true that its existence was 

 not worth having even to the larva of a fly. For the last eight months it 

 remained without motion, attached by its posterior pair of tubercles to the 

 paper on which it was placed, manifesting no other symptoms of life than 

 by moving the fore part of the body when touched, and replacing itself on 

 its belly if turned upon its back. But this was quite enough to prove it 

 still alive. I can attribute this singular result to no other circumstance 

 than its having been deprived of a sufficient quantity of food to bring it 

 into the pupa state, though provided with enough for the attainment of 

 nearly its full growth as larva. Possibly the same remote cause might act 

 in this case, as operates to prolong the term of existence of annual plants 

 that have been prevented from perfecting their seed ; and it would almost 

 seem to favour the hypothesis of some physiologists, who contend that 

 every organised being has a certain portion of irritability originally im- 

 parted to it, and that its life will be long or short as this is slowly or 

 rapidly excited no great consolation this for the advocates for fast-living, 

 unless they are in good earnest in their affected preference of a " short life 

 and a meriy one ;" though it must be admitted that they would have the 

 best of the argument, were the alternative such a state of torpid insensi- 

 bility as that with which our larva purchased the prolongation of its 

 existence. 



After this general view of the food of insects, and of circumstances con- 

 nected with it, I proceed to give you an account of some peculiarities in 

 their modes of procuring it. 



1 Entom. Mag. i. 526. 



2 Not having ever met with another specimen, I am unable to say of what 

 precise species of aphidivorous fly it is the larva ; nor can I find a figure of it, 

 though, it approaches near to one given by De Geer (vi. t. 7. f. 1 3,). Its 

 shape is oblong-oval, length about four lines, and colour pale red speckled with 

 black. Each of the seven or eight segments which compose the body projects 

 on each side into three serrated flat aculei or teeth ; three or four similar but 

 smaller aculei arm the head ; and two, much larger than the rest, the anus, one 

 on each side of the usual bifid protuberance which bears the respiratory plates. 

 A bifid tubercular elevation is also placed in the middle of the back of each 

 segment. 



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