The sexes are separate. Hermaphroditism when it occurs is due only to malformation. The male differs in size, shape, etc. from the female. The genital rudiments develope into a series of ovarial tubes s. ovarioles, or testes. The latter are very variable in number and shape, and the same statement is true also of the ovarial tubes. The anterior extremities of the ovarioles are prolonged into filaments, the filaments belonging to one side unite, and the common filament thus formed is attached to the dorsal aspect of the abdomen or thorax. There are two oviducts and two vasa deferentia. The latter sometimes dilate to form vesiculae seminales. In the Ephemeridae the two male or female ducts open separately, the former on the ninth abdominal somite in connection with one or two copulatory organs, the latter between the seventh and eighth somites, on the ventral aspect. The male ducts are similarly arranged in Dermaptera, but in the genus Forficula the terminal portion of one duct alone persists. In other Insecta the external genital apertures are single, and there is an azygos portion invaginated from the integument constituting the vagina in the female, or ductus ejaculatorius in the male, the terminal portion of which is generally an intromittent organ.
Accessory female organs in connection with the vagina are the bursa copulatrix and seminal receptacle, and other glands by which the ova are fixed to some foreign object or enveloped in a cocoon or case. The male possesses glands which unite the spermatozoa into spermatophores as well as others of unknown function. The female aperture lies either between the seventh and eighth or in the eighth somite of the abdomen; the male aperture in the ninth or tenth. In Lepidoptera the oviduct opens on the same papilla as the anus, but the entrance to the bursa copulatrix is in its normal position on the eighth somite. The two structures are connected internally (see p. 160). The male copulatory organ in Odonata is placed upon the second abdominal somite, and therefore far in front of the sexual aperture. For the ovipositor see p. too.
1 Two branchiate Lepidopterous larvae are known.
The ovum is invested in a tough shell or chorion produced by the epithelium of the ovarian tube. This chorion is pierced by at least one pore or micropyle through which the spermatozoon enters as the ovum passes down the vagina. A fold of the blastoderm over the ventral plate gives origin in the majority of Insecta to two embryonic membranes, an outer serous envelope and an inner amnion.
A few Insecta are viviparous. The Tachinae, some Oestridae among Diptera, some Staphylinidae among Coleoptera, the Strepsiptera (note p. 511) produce larvae; the Pupipara among Diptera pupae which are nourished in the larval condition by a special gland; and certain generations of Aphidae among Homoptera, young which closely resemble the parent. Parthenogenesis occurs normally among certain groups, and then the female possesses either complete sexual organs (Psyche, Solenobia (two sp.) among Lepidoptera; Coc-cidae among Homoptera; certain Cynipidae1, and Tenthredinidae among Hymenoptera; Gastrophysa Raphani among Coleoptera1), or the ovaria and ducts are perfect but accessory organs wanting (Aphidae among Homoptera). Voluntary parthenogenesis occurs in the queen Bees, Humble Bees, and Wasps, among Hymenoptera; occasional parthenogenesis among Lepidoptera, as also in the worker Bee, Wasp and Ant which are dimorphic females with rudimentary ovaria and ducts. The offspring produced are of the male sex in the Bees, Wasps (and Ants?); of the female in Psyche, Solenobia, Coccidae, Aphidae and Cynipidae, but males make their appearance at last. In some Tenthredinidae the offspring belong to the male sex, in others to the female, whilst in others again the brood is mixed male and female.