The parthenogenetic Aphidae except Chermes and Phylloxera are viviparous. In the Cynipidae there is as a rule only one such generation, but many in Aphidae, Coccidae, Psyche and Solenobia. Paedogenesis or the production of ova by the immature animal is rare, and is in Insecta always parthenogenetic. In the larvae of certain Diptera (Heteropeza, Miastor) the cells of the genital rudiment develope into larvae which feed on the parent and burst their way out through the cuticle; and the pupa of a Chironomus (Diptera) produces ova at a very early period which are laid just before or as soon as the imago becomes free. The Strepsiptera are said by Von Siebold to afford another instance of its occurrence.
The Insect at birth is always minute in size. Growth is accompanied by moults or ecdyses, and in many instances by changes, variable in degree, in the structure of the antennae, eyes, mouth-parts, body-somites, and internal organs, making a metamorphosis more or less complete. Accordingly Insecta may be described as Ametabola, Hemimetabola, and Metabola (or Holo-metabola 2 ). In the Ametabola, the young insect or larva resembles its parent in most respects, except in the number of antennal joints, of facets in the eye, absence of wings, and want of maturity of the sexual organs. In the Hemi-metabola, the larva may differ notably from the adult in the structure of the antennae, eyes, mouth-parts, shape and disposition of the body-somites, especially the thorax, as well as in the absence of wings and in internal structure. It is aquatic and possesses special contrivances for respiration. The characters of the adult may be acquired gradually or quickly. In the Metabola, three well-marked stages are distinguishable, larva, nymph or pupa, and imago. The larva may be Campodeiform, or eruciform (see p. 150), with or without limbs; its antennae differ much from those of the adult; its eyes are ocelli; its mouth-parts may be similar (not identical) or very dissimilar to those of the adult.
It passes into a pupa-stage, which becomes the imago in one, rarely in two moults. The pupa is quiescent for the whole or part of its existence (p. 152), and profound changes take place leading to a re-construction more or less complete of organs external as well as internal, together with the development of structures peculiar to the imago (wings, genitalia). In a few instances (Mantispa among Neuroptera, Meloidae among Coleopterd) there is a hyper-metamorphosis. The first larva is Campodeiform, the second more or less eruciform. Much variety in the details of metamorphosis are noticeable.
1 It is not stated if the organs are complete in these instances.
3 These terms are defined here in a sense which differs somewhat from that generally given to
Alternation of Generations is coupled with parthenogenesis, and is known in this case as Heterogamy. Dimorphism is common, and falls under several heads: - (1) sexual when the male and female differ markedly, a form of dimorphism sometimes connected in Lepidoptera with change of locality: (2) seasonal when the imago produced at one time of the year differs from that produced at another - seen in the summer and winter broods of some Lepidoptera: (3) functional, e. g. winged and apterous viviparous Aphidae, the small female or worker in the colonial Hymenoptera (Bees, Wasps, Ants) or the arrested male and female workers and soldiers among the Termitidae (White Ants 1).
Some Coleoptera are phosphorescent, and the light is produced by special organs lodged in the head (Elateridae) or in the abdomen (Lampy-ridae). Some insects live in colonies, e. g. the social Hymenoptera (Bees, many Wasps, Ants), the White Ants, some Lepidopterous larvae. A great variety of structures are found, made for protection during pupation or for rearing the young. Many insects possess the power of emitting sounds, caused by the friction of external parts or of special organs (Cicada among Homopterd), by the motion of the wings in flight, by the them, but accords with Brauer's views as to Classification (infra) and the results of recent researches. Compare the general works cited p. 146. passage of air through the stigmata or through the mouth (Death's Head Moth).
1 Complementary ' kings and queens' are also met with in this family.
The majority of Insecta are terrestrial, some are aquatic in the larval and adult condition, or in one or the other. They feed on vegetable and animal matters living or dead, and some few are parasitic (Sipkonaptera, Mallophaga, &c). The oldest known fossil Insecta are genuine Orthoptera (Blattidae, PhasmidaeyMantidae), genuine Neuroptera (Sialidae in Devonian strata), and Rhynchota Homoptera (Fulgoridae\ which differ from living forms only in genus or even, looking at generalised generic characters, belong to genera still living (Brauer). A Blatta has been found in Silurian, Coleoptera in Carboniferous strata; Hymenoptera, a Lepidopteron, Heterop-tera and Diptera occur in the Solenhofen Slates (Oolitic).