Aphids or greenfly, plant lice (superfamily Aphidoidea) are small plant-feeding insects (1 to 10 mm). Of the 4,000 species of known aphids (distributed in 10 families), around 250 are serious pests for agriculture and forestry as well as an annoyance for gardeners. Natural predators include ladybirds, hoverfly larvae and lacewings.
Aphids have two compound eyes and two ocular tubercles made up of 3 lenses, each of which is located behind and above the compound eyes. They have 2 tarsal segments. Besides, the 5th abdominal segment bears a pair of tubes on the dorsal surface named siphunculi (cornicles), which are upright and point backwardly. A cauda is usually present below and between them on the last abdominal segment.
Similar to related superfamilies, they passively feed on sap of phloem vessels in plants. The plant keeps this sap under high pressure; once a phloem vessel is punctured, it is forced into the food canal. Aphids actively ‘drink’ (suck) from xylem vessels when thirsty.
Some species of ants “farm” aphids, supplying them with leaves to eat, and eating the honeydew that the aphids secrete. Many aphids are host to an endosymbiont bacteria, Buchnera, which synthesizes the essential amino acids that are absent in the phloem that the aphids eat.
For part of all of their life, aphids are often found to be parthenogenetic. Aphids have been known to have what is called telescoping generations. With telescoping generations the female aphid will have a daughter within her who is already parthenogenetically producing its own daughter at the same time. At different times of the year, they can be viviparous or oviparous. During spring and summer, aphids are often parthenogenetic and viviparous and then give birth sexually during autumn. Therefore aphids are said to undergo cyclical parthenogenesis or to have a holocyclical life circle.
With that said, male and female aphids mate in autumn. Sexual females, but also asexual ones, have two sex chromosomes while sexual males only have one.
There is one species of cabbage aphid that reproduces during the summer. They are all females, and can survive up to 41 generations of offspring. If no aphids had died during the summer, there would be more than one and a half billion billion billion aphids (1.5 x 1027) by the end of the season.
Aphids are world-wide, but they are most common in temperate zones.
Most aphids are monophaguous, thereby only feeding upon 1 species of plant).
Aphids probably first appeared 280 millions of years ago, in the Carboniferous. They probably fed on non-flowering plants like Cordaitales or Cycadophyta. The oldest known aphid fossil is one of the species Triassoaphis cubitus from the Triassic. There were relatively few species of aphids at that time, and the number of species only considerably increased since the appearance of angiosperms 160 millions of years ago. This is due to the fact that angiosperms provide an occasion for aphids to get specialized.
Aphids have not always looked like they do nowadays. Organs like the cauda or the siphunculi did not evolve until the Cretaceous.