The adults of the Aleyrodidae (whiteflies) are small winged insects usually yellow-white in colour; some species bear grey marks on the wings or are darker, even brown-black. Immatures may be misidentified as aphids or scale insects, but the "vasiform orifice" will help in separating whiteflies from other groups in slide mounted and red-stained puparia. All instars secrete wax, in the shape of powder, curls, threads or as vitreous layers. They are called "whiteflies" due to their general whitish appearance. They are mostly bisexual, but several species or "strains" show both arrhenotokous and thelytokous parthenogenesis, usually in relation to insemination. The post-embryonic development is neometabolic, consisting of four larval instars, pupae (immotile and not-feeding) and adults. Whiteflies share with other Sternorrhyncha the piercing-sucking mouthparts and the specialized "filter chamber" mid gut. They suck plant sap, killing their host plants in heavy infestations and excreting abundant liquid faeces as honeydew drops. These drops cover infested plants, which then blacken because of colonization by sooty mold fungi. The blackened plants or products are untradeable due to the sooty mold. Whiteflies may also cause physiological changes and transmit viruses. In the Mediterranean Region whiteflies live mostly on woody perennial plants. Natural or classical biological control programmes have successfully controlled current or potential pest species by the introduction of effective natural enemies. Aleurocanthus spiniferus Quaintance is currently of major concern to citriculture in the Mediterranean Region because it was recently introduced and no indigenous natural enemies seem to control it. As in the past, unwanted, introduced whiteflies may become key pests of citrus in the Mediterranean Region.