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aphid or plant louse, tiny, usually green, soft-bodied, pear-shaped insect injurious to vegetation. It is also called greenfly and blight. Aphids are mostly under 1/4 in. (6 mm) long. Some are wingless; others have two pairs of transparent or colored wings, the front pair longer than the hind pair. In typical aphids (family Aphididae), two tubes called cornicles project from the rear of the abdomen and exude protective substances. Aphids feed by inserting their beaks into stems, leaves, or roots, and sucking the plant juices. Usually they gather in large colonies.
The life cycle of aphids is complex and varies in different species. In a typical life cycle, several generations of wingless females, which reproduce asexually (see parthenogenesis) and bear live offspring, are followed by a generation of winged females, which bears a sexually reproducing, egg-laying generation of males and females. Mating usually occurs in fall, and the eggs are laid in crevices of the twigs of the host plant; the first generation of wingless females hatches in spring. Different host plants and different parts of the plant may be used at different stages of the life cycle.
Some aphids (e.g., the woolly apple aphid) secrete long strands of waxy material from wax glands, forming a conspicuous woolly coating for their colonies. Gall-making aphids live in galls, or swellings of plant tissue, formed by the plant as a reaction to substances secreted by the insects; galls of different aphid species are easily identified (e.g., the cockscomb gall of elm leaves). One group of aphids lives only on conifers (e.g., the eastern spruce gall aphid).
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