Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/158

 146 INSECTS been considered as only an extension of the chitinous integument. According to their position and function, they act variously, as do those of higher animals, and have received similar names. Their number is often enormous, and when we consider the great powers of flight, or of locomotion by other means, possessed by many insects, it is not difficult to understand that their strength must be proportionately great. Lyonet s celebrated treatise on the anatomy of Coss^ls remains a masterpiece of research on this subject, and in England Lubbock s recent memoir on the subcutaneous muscles of Pygxra bucephala is equally remarkable, and should be studied by those desiring minute information on the complex muscular system. Generative System. In all insects the sexes are separate. True hermaphrodites do not exist, though individual mon strosities, in which the form, coloration, and even internal organization of both sexes are combined, are not rare. The external organs are placed at or near the extremity of the abdomen, and are usually accompanied by secondary or accessory appendages often of most complex structure, serving to ensure complete contact during the sexual act, and probably also to some extent excitatory. In the dragon-flies, however, the intromittent organ of the male is in the under side of the second abdominal segment, which explains the extraordinary position of the sexes when coupled. In the male the testes are very varied in form, ordinarily separated, but sometimes united into one mass, each of the two halves of which has its special duct. But the separate form is by far the most usual. As in higher animals, there are the usual parts, the ductus ejaculatorius, thsvesiculx seminales, and the vasa deferentia, the conditions of which vary infinitely in different insects. Whether the intromittent organ is always traversed by an inner canal or not is a little doubtful. Ordinarily such is no doubt the case, but in others it would appear probable that the ductus ejaculatorius does not end absolutely in the organ, and that a groove on the surface of the latter receives the sperm. Some such arrangement must certainly exist in dragon-flies, in which the testes and the opening of the duct have no direct connexion with the intromittent organ. In the female the ovaries occupy much of the abdomen that is not taken up by the intestinal canal. Each consists of a very varying number of tubes, branching off externally, in which the eggs are contained ; these eggs are conveyed by oviducts, and before extrusion receive the fertilizing fluid stored in the spermatheca, which latter may be simple or compound; they pass out by the vagina. In close connexion with these parts in the female is the poison gland and sting found in some insects. In the gravid female of Termes the ovaries become enormously distended, so that the entire insect may be said to consist of little else than eggs. The rudiments of the sexual organs may be detected in the larva when in a very young state, and the sex of the future perfect insect determined, a sufficient answer to those who assume that sex can be controlled by the nutriment furnished to the larva. It was formerly con sidered that, pairing once effected, the male died almost immediately, and the female followed after having deposited her eggs. Recent observations go to prove that this is to a large extent erroneous, that pairing may be effected several times by both sexes (the female laying her eggs intermittently), in effect that polygamy and polyandry exist. There are certain anomalous conditions of the generative system that may be conveniently noticed here, under different headings. Neuters or Workers. In bees, wasps, and ants, and also in Termes (or white ants), the majority of the members of a colony is made up of individuals which as a rule have no reproductive powers. In the first three, these are aborted females, and it has been proved (at any rate for ants) that occasionally these workers lay eggs, which, how ever, always produce males, the production of a queen depending apparently upon special feeding in the larval stage. In Termes the conditions are different. There are both workers and soldiers, both incapable of reproduction, but not exclusively consisting of aborted females, since both sexes are represented. Also in Termes there are what have been termed complementary males and females, distinct from the pair that were once supposed to be the exclu sive founders of a new colony ; of these the females lay comparatively few eggs, their ovaries not acquiring the extraordinary development of those of the true queen. Vivijjarous Insects. Oviparous generation is the rule in insects ; but there are certain departures from the rule. In the Aphides it is well known that both the oviparous and viviparous exist in the same species. In Lepidojrfera there is a well-authenticated instance of an Australian insect closely allied to the clothes-moth bringing forth larva) already hatched. A similar condition is asserted to exist in a species of cockroach. In Coleoptera, Schiodte has noticed that two species of Staphylinidx, living in the nests of white ants in Brazil are viviparous, as is like wise Oreina in Chrysomelidx ; so also are the Strepsi- ptera. In Diptera flesh-flies of the genus Sarcophaga are known to be viviparous. But the most extraordinary instance is in certain minute flies, to be noticed below, of which the Iarva3 produce living Iarva3. Alternation of Generations, Parthenogenesis or Agamo- genesis. In the bee, ant, many gall-flies, some Lepidoptera and (as is now known) also some Coleoptera, and insects of other orders, females are capable of producing fertile eggs without any contact with the male, and the produce of these eggs is frequently male. This property varies considerably in details. In the case of the bee or the ant, it would appear that one impregnation suffices for the life of the queen (which may last for seven or eight years), but the power of producing females does not probably extend beyond the immediate influence of the impregnation. In some hymenopterous gall-flies a true alternation probably sometimes occurs, combined with dimorphism ; but absolute parthenogensis, in which females are produced generation after generation, is the common condition in many lepidopterous insects. This process is effected by internal budding. In the Aphides the conditions are still more remarkable, owing to the existence -of both winged and apterous forms of both sexes, and of both oviparous and viviparous generation ; but it is not proved that the same individual insect is capable of producing both forms. In the case of the minute fly (Miastor metroloas) mentioned above, the production of larvae from larvae is continued throughout the winter and spring, until in June the brood goes through its ordinary metamorphosis, and results in mature males and females, and so the cycle recommences. Of all the marvels in the history of insects, this is the most astonishing ; no wonder that the assertions of H. Wagner (the discoverer) were met with incredulity from the best physiologists until abundantly confirmed by others, and in other species. Metamorphoses. Hundreds of volumes have been written on this fascinating subject, one or more of which are in almost every library; hence there is no necessity for giving more than a rudimentary outline here. All true insects may be said to undergo a metamorphosis. Such a condition is absent in the small groups known as Thysanura and Collembola; and, although these are here retained amongst Insecta as a matter of con venience, the writer is disposed to agree with Lubbock that they are outside the pale of true insects. Metamor phosis may be broadly grouped into two main divisions