The great majority of Insects, as is the case with most of the higher animals, consist of male and female individuals; but there occur some striking exceptions to this rule, as seen in the Social Insects. In those organised communities which are formed by Bees, Ants, and Termites, by far the greater number of the individuals which compose the colony are either undeveloped females, or are of no fully developed sex. This is the case with the workers amongst Bees, and the workers and soldiers amongst Ants and Termites. And, these sterile individuals, or "neuters," as they are commonly called, are not necessarily all alike in structure and external appearance. Amongst the Bees, all the neuters resemble one another; but amongst Ants and Termites they are often divided into "castes," which have different functions to perform in the general polity, and differ from one another greatly in their character.
The organs of the two sexes are in no case united in the same individual, or, in other words, there are no hermaphrodite insects. (In some abnormal cases amongst Bees, Lepidoptera, etc, hermaphrodite individuals have been observed.) As has been noticed, however, before, asexual reproduction is by no means unknown amongst the Insecta, and the attendant phenomena are often of extreme interest. (See Introduction.)
The great majority of Insects, during their adult condition, are terrestrial or aerial in their habits, but in many cases, even of these, the larvae are aquatic. Many other insects live habitually during all stages of their existence in fresh water. A few insects inhabit salt water (either the sea itself or inland salt waters) during the whole or a portion of their existence. (This is the case with two or three Beetles of the families Hydrophilidae and Dytiscidae, some Hemipterous Insects, and the larvae of various Diptera.) Lastly, many insects live parasitically upon the bodies of Birds or Mammals, or upon other Insects.
The most ancient remains of Insects at present known to us are from the Devonian rocks of North America. Here occur several forms apparently belonging to the Neuroptera (or Pseudo-neuroptera), In the Carboniferous rocks the remains of Insects are more abundant, and we find examples of several orders (such as the Cokoptera, Orthoptera, and Neuroptera). The orders Hymen-optera and Lepidoptera are not certainly known to occur till the Secondary period is reached; and in the Tertiary rocks we find representatives of almost all the existing orders. Amber, which is a fossil resin, has long been known to contain many insects in its interior (in certain specimens); and all of these appear to belong to extinct species, though amber, geologically speaking, is not an ancient product.