EXTERNAL SEXUAL ORGANS.—The terminal segments of the body are usually very highly modified in connexion with the external sexual organs, and this modification occurs in such a great variety of forms as to render it impossible to give any general account thereof, or of the organs themselves. Some of these segments—or parts of the segments, for it may be dorsal plates or ventral plates, or both—may be withdrawn into the interior, and changed in shape, or may be doubled over, so that the true termination of the body may be concealed. The comparative anatomy of all these parts is especially complex in the males, and has been as yet but little elucidated, and as the various terms made use of by descriptive entomologists are of an unsatisfactory nature we may be excused from enumerating them. We may, however, mention that when a terminal chamber is found, with which both the alimentary canal and the sexual organs are connected, it is called a cloaca, as in other animals. PARTHENOGENESIS. There are undoubted cases in Insects of the occurrence of parthenogenesis, that is, the production of young by a female without concurrence of a male. This phenomenon is usually limited to a small number of generations, as in the case of the Aphididae, or even to a single generation, as occurs in the alternation of generations of many Cynipidae, a parthenogenetic alternating with a sexual generation. There are, however, a few species of Insects of which no male is known (in Tenthredinidae, Cynipidae, Coccidae), and these must be looked on as perpetually parthenogenetic. It is a curious fact that the result of parthenogenesis in some species is the production of only one sex, which in some Insects is female, in others male; the phenomenon in the former case is called by Taschenberg[66] Thelyotoky, in the latter case Arrhenotoky; Deuterotoky being applied to the cases in which two sexes are produced. In some forms of {142}parthenogenesis the young are produced alive instead of in the form of eggs. A very rare kind of parthenogenesis, called paedogenesis, has been found to exist in two or three species of Diptera, young being produced by the immature Insect, either larva or pupa. GLANDS. Insects are provided with a variety of glands, some of which we have alluded to in describing the alimentary canal and the organs of sex; but in addition to these there are others in connexion with the outer integument; they may be either single cells, as described by Miall in _Dicranota_ larva,[67] or groups of cells, isolated in tubes, or pouches. The minute structure of Insect glands has been to some extent described by Leydig;[68] they appear to be essentially of a simple nature, but their special functions are very problematic, it being difficult to obtain sufficient of their products for satisfactory examination. {143}CHAPTER V DEVELOPMENT