ZOOLOGY. 353 



even \vith species of limited numbers, the same power of prolicity 

 could enable a single pair to reach the present numbers under any ex- 

 isting climate of the earth. 



Were it so, we should expect to find a very correct ratio subsisting 

 between the present numbers of any undisturbed species and its powers 

 of reproduction. But, since attention has been called to the subject, 

 and, with many of the lower animals, the ova counted, not only is 

 there no reason for supposing that such relation is present, but, in 

 many instances, the very opposite is true a fact of the truth of which 

 I have lately been the more and more convinced, from counting 

 the ova of many insects, and comparing the result with their well- 

 known habits and conditions of life. There are examples in which 

 there appears no escape from conclusions of this character. And, al- 

 though [ might detail many taken from the ranks of lower animals, 

 yet, from its well marked character and recent occurrence, I select 

 that furnished in the seventeen year locust as the subject of this 

 paper. 



The present year, (1851,) may be noted as containing an episode of 

 insect life of more than ordinary interest and value, for in it has oc- 

 curred the grand appearance of the locust. The regularity and prompt- 

 ness with which this insect appears at the end of an interval of seven- 

 teen years is well known in science. Justly does it excite our aston- 

 ishment that the conditions of its economy should be so unique. 

 During the last of May, I had the good fortune to witness their grand 

 appearance in the interior of Pennsylvania. They came forth in their 

 usual and almost incredible numbers, and a fine opportunity was given 

 me to learn something about their conditions of life. The insect ap- 

 pears in its perfect or imago condition simply for the preservation of 

 its species ; its period of life in this state is, therefore, quite brief. 

 Both male and female go about their functions immediately on escap- 

 ing the earth, after which they die. Their existence is, therefore, 

 almost entirely subterranean, and, considering the depth to which 

 they descend, almost as isolated from the agencies of civilization as 

 those of the tenants of the ocean. It appeared evident, from what I 

 saw of their movements, that, unless swept away by violent currents, 

 they remain generally in the locality of their birth, so that the comers 

 of this year may properly be said to be the lineal descendants of those 

 which there appeared fifty or more years since. This is important as 

 to our determining whether or not they really increase in number. I 

 made strict inquiries of several men who had witnessed this their fourth 

 appearance through the same tract of country, and their replies always 

 were that they did not think their numbers to vary materially either 

 way. Being men of sense and farmers, I thought them able to judge 

 of this matter, since they regarded the ravages of this insect with no 

 common eye. We will now look a little to its powers of reproduction. 

 The female has about 500 eggs, which, from certain relations of the 

 other sex, which I have made out microscopically, are, probably, all or 

 nearly all fecundated. We have, then, for every two individuals which 

 have appeared this year, a deposit of" 500 embryos, for the generation 

 to appear 17 years hence. Now, from what has just been stated about the 



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