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Show 134 EQUILIBRIUM OF SPECIES [Ch. VIII. of spect·e s a monO0 ' plants ' 'a nd thus rcOo 'nlating indit·ectly th. e relative numbers of many of the higher orders of terrestnal animals. . The peculiarity of their agency consists in thetr power of suddenly multiplying their numbers, to a degree. wht.ch could only be accomplished in a considerable lapse of ttme.m an! of the larger animals, and then as instantaneo.usly rclapst~g, wttl~out the intervention of any violent disturbmg cause, mto theu former insignificance. . . If for the sake of employing, on drfferent but rare occasiOns, a power of many hundred horses, we wer: unde~· the necessity of feeding all these animals at great cost m the mterval~ when their services were not required, we should greatly admtre the invention of a machine, such as the steam-engine, which was capable, at any moment, of exerting the ~ame d~gree of ~tren.gth without any consumption of food durmg perwds of mactlon. The same kind of admiration is strongly excited when we contemplate the powers of insect life, in the creation of \~hich nature has b,een so prodigal. A scanty number of mmutc individuals, only to be detected by careful research, are ready in a few days, weeks, or months, to give birth to myriads which may repress any degree of monopoly in another species, or remove nuisances, such as dead carcasses, which might taint the air. But no sooner has the destroying commission been executed, than the gigantic power becomes dormant-each of the mighty host soon reaches the term of its transient existence, and the season arrives when the whole species passes naturally into the egg, and thence into the larva and pupa state. In this defenceless condition it may be destroyed either by the elements, or by the augmentation of some of its numerous foes which may prey upon it in these stages of its transformation; or it often happens that, in the following year, the season proves unfavourable to the hatching of the eggs or the development of the pupre. Thus the swarming myriads depart which may have covered the vegetation like the aphides, or darkened the air like locusts. Ch. VIII.] PRESERVED BY INSECTS. 1:35 In almost every season there arc some species which in this manner put forth their strength, and then, like Milton's spjrits which thronged the spacious hall, " reduce to smallest forlils their shapes immense"- ----- So thick tho aery crowd Swarm'd and were straiten'd; till, the signal given, Behold a wonder! they but now who seem'd In bigness to surpass earth's giant sons, Now less than smallest dwarfs. A few examples will illustmte the mode in which this force operates. It is well known that among the countless species of the insect creation, some feed on animal, others on vegetable matter, and, upon considering a catalogue of eight thousand British insects and arachnidre, Mr. Kirby found that these two divisions were near] y a counterpoise to each other, the carnivorous being somewhat preponderant. There arc also distinct species, some appointed to consume living, others dead or putrid animal and vegetable substances. One female, of Musca car· naria, will give birth to twenty thousand young; and the larvro of many flesh-flies devour so much food in twenty-four hours, and grow so quickly, as to increase theit· weight two hundredfold ! In five days after being hatched they arrive at their full growth and size, so that there was ground, says Kirby, for the assertion of Linn reus, that three flies of M. vomitoria could devour a dead horse as quickly as a lion * ; and another Swedish naturalist remarks, that so great are the powers of propaO'ation of a single species, even of tl1e smallest insects, that ea~h can commit, when required, more ravages than the elephant t. Next to locusts, the aphides, perhaps, exert the greatest power over the vegetable world, and, ]ike them, arc sometimes s? numerous as to darken the air. '"fhe multiplication of these ~Ittle creatures is without para11e1, and almost every plant has Its .peculiar species. Reaumur has proved, that in five generatiOns one aphis may be the progenitor of 5,904,900,000 descendants; and it is supposed that in one year there may be "' Kirby and Spence, vol. i.1 p. 250. t Wilcke1 Amron. Acad.1 chap. ii. |