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Show 136 EFFECT OF INSECTS ON [Ch. VIII. twenty rrenerations ''1.'. Mr. Curtis t observes, that as among caterpil~rs we find some that are con~tantly and unalterably attached to one or more particular species of plants, a~d .others that feed indiscriminately on most sorts of herbage, so It Is precisely with the aphides; some are particular: others. mo~e general feeders; and as they resemble other I~sects m this respect so they do also in being more abundant m some years than o;hers. In 1793 they were the chief, and in 1798 the sole cause of the failure of the hops. In 1794, a season almost unparalleled for drought, the hop was perfectly free from them, while peas and beans, especially the former, suffered very much from their depredations. The ravages of the caterpillars of some of our smaller moths afford a good illustration of the temporary increase of a species. The oak-trees of a considerable wood have been stripped of their leaves as bare as in winter, by the caterpillars of a small rrreen moth (Tortrix irridana,) which has been observed the ;ear following not to abound t· The Gamma moth (Plttsia gamma), although one of our common species, is not dreaded by us for its devastations, but legions of their eaterpillars have, at times, created alarm in France, as in 1735. Reaumur ob. serves, that the female moth lays about four hundred eggs; so that if twenty caterpillars were distributed in a garden, and all lived through the winter and bec£_tme moths in the succeeding May, the eggs laid by these, if all fertile, would produce eight hundred thousand~· A. modern writer, therefore, justly ob· serves, that did not Providence put causes in operation to keep them in due bounds, the caterpillars of this moth alone, leaving out of consideration the two thousand other British species~ would soon destroy more than half of our vegetation II· In the latter part of the last century an ant, most destructive "' Kirby and Spence, vol. i., p. 174. t Trans. Linn. Soc., vol. vi. ~ Lib. Ent. Know., Insect Trans., p. 203. See Haworth Lep. § Reaumur1 ii. 237. II Lib, Ent. Know., Insect Trans., p . . 212. Ch. VIII.] TilE NUMBER OF SPECIES. 137 to the sugar-cane (Formica saccharivora), appeared in such infinite hosts, in the island of Grenada, as to put a stop to the cultivation of that vegetable. Their numbers were incredible. The plantations and roads were filled with them ; many domes ... tic quadrupeds, together with rats, mice, and reptiles, and even birds, perished in consequence of this plague. It was not till 1780 that they were at length annihilated by torrents of rain, which accompanied a dreadful hurricane*· We may conclude by mentioning some instances of the devastations of locusts in various countries. Among other parts of Africa, Cyrenaica has been at different periods infested by myriads of these creatures, which have consumed nearly every green thing. The effect of the havoc committed by them may be estimated by the famine they occasioned. St. Augustin mentions a plague of this kind in Africa which destroyed no less than eight hundred thousand men in the kingdom of Masanissa alone, and many more upon the terri ... tories bordering upon the sea. It is also related, that in the year 591 an infinite army of locusts migrated from Africa into Italy, and, after grievously ravaging the country, were cast into the sea, when there arose a pestilence from their stench which carried off' nearly a million of men and beasts. In the Venetian territory also, in 1478, more than thirty thousand persons are said to have perished in a famine, occasioned by this scourge ; and other instances are recorded of their devastations in France, Spain, Italy, Germany, &c. In different parts of Russia also, Hungary, and Poland,-in Arabia and India, and other countries, their visitations have been periodically experienced. Although they have a preference for certain plants, yet, when these are consumed, they will attack almost all the remainder. In the accounts of the invasions of locusts, the statements which appear most marvellous relate to the prodigious mass of matter which encumbers the sea wherever they are blown into it, and the pestilence arising from its putrefaction. Their dead bodies are said to have been * Kirby and Spence, vol. i., p. 183. Castle, Phil. Trans., xxx., 346. ' |