The best character by which the New Zealand aphides are to be recognised is to be found in the pair of longer or shorter horn-like processes, or “cornicles,” projecting from the upper surface of the abdomen; in some species, however, the “cornicles” are reduced and inconspicuous (_e.g._, Woolly Aphis), or altogether absent (_e.g._, Grape Phylloxera). The “cornicles” are frequently called “honey-tubes,” since for many years it was thought that they secreted the honey-dew; it has been shown, however, that the honey-dew is secreted from the rectum, and that the function of the “cornicles” is to secrete a waxy protective substance, which may take the form of a powder or woolly threads. The wings, when present, are membranous, the front pair being much larger than the hind ones, and when not in use usually close roof-like over the body. [Illustration: FIG. 8. (1) Life History of an Aphis: A, egg; B, C, and F, wingless females; D, winged female; E, male; G, section of head and plant tissue to show method of attack. (2) Life History of a Leaf-Hopper: H, eggs under bark of twig; I, first stage hopper; J, later stage hopper with developing wings; K, adult from above; L, adult from side. (3) Life History of a White Fly: M, egg; N, first stage larva; O, pupal stage under scale covering; P, adult. (4) An adult Thrips.] In their life-histories and habits aphides present many variations, sometimes of considerable complexity, but fundamentally the processes are as follows:--Eggs are laid on the host plant during the autumn, and give rise to wingless females in the spring; these females (being asexual or parthenogenetic, since they reproduce without being fertilised) are viviparous, producing living forms similar to themselves. Some of these forms remain wingless, while others may develop wings, upon which a wider dispersal of the species depends, but in both cases such females are asexual and viviparous. Several such generations may develop until the autumn, when males and females appear, the latter being oviparous, producing the over-wintering eggs when fertilised by the males. Very often, however, the life-cycle is considerably complicated by the winged forms flying to other host plants and establishing there colonies differing in many respects from the parent stock; from these secondary hosts there is a return migration to the original species of plant. Again, the migrations may be restricted to different parts of the same plant, from the leaves or branches to the roots, for example. Most aphides are readily controlled by means of insecticides, such as nicotine-sulphate, or kerosene-emulsion. They are also very often held in check by natural enemies, such as aphis-lions, hover-flies, ladybirds, and numerous forms of hymenoptera. The following species are some of the commoner aphides met with in New Zealand:--