Insects of unusual Occurrence. 39 



described to me as a small, green, caterpillar ; and its numbers 

 must have been immense, to have extended over a tract of 

 country more than a hundred miles long. But no wing- 

 ed insects were observed, which might have been supposed 

 to have laid their eggs on the leaves. I examined some very 

 old men, two of them near eighty years of age, but they could 

 not recollect any thing similar to the destruction of the foliage 

 of the birch which they then witnessed. 



Such examples as these might very well be cited as instan- 

 ces of equivocal generation, were it not that a little considera- 

 tion leads us to believe that there is nothing equivocal in na- 

 ture. Some have supposed that the ova of insects, like the 

 seeds of many vegetables, may, in certain circumstances, be 

 preserved during very long periods, until some accidental oc- 

 currence place them in a condition to produce a living crea- 

 ture. I do not think, however, that the analogy with seeds is 

 good ; and it appears to me, that it is not necessary to seek for 

 distant analogies to explain such phenomena, since we may 

 find a better analogy in the habits and transformations of in-, 

 sects themselves. It is not so much the sudden appearance 

 of insects unknown to the generation which suffers by their de- 

 predations, that is to be wondered at, but their as sudden and 

 total disappearance. It is this latter circumstance which I 

 think explains the former. It is well known, that many tribes 

 of insects exist in three states, that of the ovum, the pupa, 

 and the imago, or perfect insect. It is also known, that while 

 most insects l'emain but a short time in the first state, the 

 time during which they are in the second varies very much. 

 When ova are deposited, it is on or very near that substance 

 or element which is to afford food to the larva ; and, conse- 

 quently, the time for being in the ovum is limited by the du- 

 ration of the substance on which it is placed. The birch- 

 trees already referred to had grown up in the memory of 

 those persons whom I examined ; and, indeed, none of them 

 could exceed the age of forty years. But, during the whole 

 period of their growth, nothing had happened similar to the 

 devouring of their leaves. The ova must, therefore, have been 

 deposited on the leaves the same season in which the caterpil- 

 lars were produced. The disappearance happened when the 



