Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms. To form a multicellular organism, these cells need to identify and attach to the other cells.
Only a dozen or so unicellular species have cells that can be seen individually with the naked eye. The rest of the nearly two million visible species are multicellular. In particular all species of animals, land plants and filamentous fungi are multicellular, as are many algae. Some organisms are partially uni- and multicellular, like Dictyostelium.
Multicellular organisms —like plants, animals and brown algae— arise from a single cell and generate a many-celled organism. Pluricellular organisms are the result of many-celled individuals joining together trough colony formation, filament formation or aggregation. Pluricellularity has evolved independently in Volvox and some flagellated green algae.
Aphids, also known as plant lice and in Britain and the Commonwealth as greenflies, blackflies or whiteflies, (not to be confused with "jumping plant lice" or true whiteflies) are small sap-sucking insects, and members of the superfamily Aphidoidea. Aphids are among the most destructive insect pests on cultivated plants in temperate regions. The damage they do to plants has made them enemies of farmers and gardeners the world over, but from a zoological standpoint they are a very successful group of organisms. Their success is in part due to the asexual reproduction capability of some species.
About 4,400 species of 10 families are known. Historically, many fewer families were recognised, as most species were included in the family Aphididae. Around 250 species are serious pests for agriculture and forestry as well as an annoyance for gardeners. They vary in length from 1 to 10 millimetres (0.04 to 0.39 in).
Natural enemies include predatory ladybirds, hoverfly larvae, parasitic wasps, aphid midge larvae, crab spiders, lacewings and entomopathogenic fungi like Lecanicillium lecanii and the Entomophthorales.