The tail of some species of the genus Ephemera is furnished with three long, jointed, hairy bristles. We learn from Reaumur with respect to one, that though in the female these are all equal in length, yet in the male there is only a rudiment of the third. On the belly near the anus these males have four fleshy appendages, the posterior ones setaceous and long, and the anterior pair filiform and shorter. They are supposed to represent the anal forceps of other insects[888]. In Ephemera vulgata, described by De Geer, both sexes have three bristles, but those of the male are the longest; and he describes the forceps as consisting of only a pair of jointed pieces, forming a bow not unlike the forceps of an earwig[889].
v. All the differences I have hitherto noticed between the sexes of insects occur in their bodily structure; but there are others of a somewhat higher description observable in their character. You may smile at the idea of character in beings so minute; but if you recollect what I formerly related to you when treating upon the societies of insects, you will allow that something of this kind does take place amongst them. In general the males are more fitted for locomotion and more locomotive; and the females, on the contrary, are necessarily more stationary. And this for an obvious reason:—the law is, that the male shall seek the female, and therefore he is peculiarly gifted for this purpose, both in his organs of sensation and motion: while his partner in many cases has very simple antennæ, he has very complex ones; and while she has either no wings or only rudiments of them, he is amply provided with them. Again: amongst the insects that suck the blood of man or beast, such as the gnat (Culex) or horse-flies (Tabanidæ), it is the female alone that is bloodthirsty, the males contenting themselves with the nectar of flowers[890]. But the difference of character in the sexes is most conspicuous, at least it has been more noticed, in those that live in societies, and is quite the reverse of what takes place in the human species. While the females and workers (which are now generally considered as sterile females, in which the ovaries are not developed) are laborious and active, diligent and skilful, wise and prudent, courageous and warlike;—the males, on the contrary, take no part in promoting the common weal, except merely a sexual one. Though till a certain period they are supported at the expense of the community, they take no part in its labours, either in collecting and forming the public stores, or in feeding and attending the young. They are idle, cowardly, and inactive; have neither art nor skill of any kind, and are unprovided with the usual offensive weapons of their species. These observations in their full force apply particularly to the hive-bee, and partially to the other social insects; amongst which, if you consult my former communications, there are some exceptions to this slothful character in the males[891].
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.
In the case, however, of the earlier or later exclusion of the imago, another agent has probably some influence. Buffon found that, other circumstances being alike, the silkworm-moths placed in a northern, lived longer than those exposed to a southern aspect: whence it appears that the stimulus of heat shortens the lives of insects, and consequently that cold tends to lengthen them.
It must be observed too, that as the death of the female insect does not take place until all the eggs are excluded, the term of her life, though usually short in the majority of species, which lay their whole number at once, is proportionably long in those which, like the queen-bee, have a longer period assigned them for this important office. Huber affirms, that he had certain proofs that she was engaged for two years in laying eggs, all impregnated by a single sexual union[898]; and in the females of most insects that live in society, several months are required to mature the last eggs that are in the ovary. There is one tribe of insects, however, the females of which are affirmed to survive this operation: I mean Dorthesia Bosc; after which they even moult, though not so often as before[899].
I formerly related to you the singular fact, that the drones in a beehive at a certain period are without mercy slaughtered by the workers[900]. A fact the reverse of this is recorded by Morier with respect to the locusts: he affirms that the female, when she has done laying her eggs, is surrounded and killed by the males. He says that he never himself witnessed this extraordinary circumstance; but that he heard it from such authority that he gave full credit to it[901]. It is a fact, however, that seems to require further evidence to entitle it to such credit. These are instances in which, by a law of nature, the life of these insects is shortened by violence. It does not appear to have been ascertained how long those drones live that, under particular circumstances, as stated in a former letter[902], are exempted from the usual slaughter.
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