II. _Age._ There is less diversity in the duration of the lives of insects in their perfect than in their larva or pupa state. Some, like several species of _Ephemeræ_, live only a few hours; some never even see the sun[892]: others, as flies, moths, and butterflies, and indeed the majority of insects, a few days or weeks; and a comparatively small number, such as some of the larger _Coleoptera_, _Orthoptera_, &c., six, nine, twelve, or fifteen months--a period beyond which the life of perfect insects rarely extends. Some, however, certainly enjoy a longer existence in the perfect state. Mr. Baker kept one of the darkling beetles (_Blaps Mortisaga_) alive under a glass upwards of three years. The rose-beetle (_Cetonia aurata_), Rösel informs us he fed with fruit and moist white bread for as long a period[893]. Esper kept our most common water-beetle (_Dytiscus marginalis_) in water in a large glass vessel, feeding it with meat, for three years and a half[894]. With regard to the _Arachnida_, from the very slow growth of _Scorpio europæus_, Rösel suspects that it must live two or three years; and Audebert is stated to have kept a spider for several[895]. In this respect insects follow a law very different from that which obtains amongst vertebrate animals. In these the duration of their life is in proportion to the term of their growth: those which attain to maturity the latest, in almost every case living the longest. In insects, on the contrary, we often meet with the very reverse of this rule. Thus the larva of the great goat-moth (_Cossus ligniperda_) is three years, that of the cabbage-butterfly (_Pieris Brassicæ_) not three months, in attaining maturity; yet the perfect insects live equally long. _Melolontha vulgaris_, which in its first state lives four years, as a beetle lives only eight or ten days[896]. And some _Ephemeræ_, whose larvæ have been two years in acquiring their full size, live only an hour; while the flesh-fly, whose larva has attained to maturity in three or four days, will exist several weeks.
There is yet another anomaly in the duration of the life of perfect insects. This is not, as in larger animals, a fixed period liable to be shortened only by accident or disease, and incapable of being prolonged; but an indeterminate one, whose duration is dependent on the earlier or later fulfilment of a particular animal function--that of propagation. The general law is, that a few days, or at most weeks, after the union of the sexes, both perish, the female having first deposited her eggs. If, therefore, this union takes place immediately after the disclosure of the insect from the pupa, their existence in the perfect state will not exceed a few _days_ or _weeks_, or in some cases _hours_, as in that of the _Ephemera_, and likewise of the _Phalænæ Attaci_ L. &c., which fall down dead immediately after oviposition[897]. But if by any means it be put off or prevented, their life may be protracted to three or four times that period. Gleditsch asserts, that by keeping apart the sexes of a grasshopper, their lives were prolonged to eight or nine weeks, instead of two or three, their ordinary length; and under similar circumstances _Ephemeræ_, which usually perish in a day, have been kept alive seven or eight. It is in consequence of this very curious fact, which has not received from physiologists the attention that it merits, that many butterflies and other insects, which, when excluded from the pupa in summer, perish in less than a month, live through the winter, if excluded late in the autumn, and the union of the sexes does not ensue. It is probable that the great age to which Baker's _Blaps_, Rösel's _Cetonia_, and Esper's _Dytiscus_ attained, was owing to their being virgins when taken, and subsequently kept from any sexual intercourse. A parallel case happens in the vegetable kingdom:--if annual plants are kept from seeding, they will become biennial; as, likewise, if they are sown too late in the year to produce seeds.