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Among the various predators, species that are strongly specific aphid-feeders (including ladybirds and hoverflies) can be distinguished from opportunist and carnivorous ones (ground beetles, spiders for instance) that consume other prey as well. Among the aphid-feeding predators, different degrees of specificity can be defined. Some predators are monophagous, targeting a single species, whereas others, like ladybirds, show preferences for several aphid species.
Some predators have their preferred hunting environments: humid or xeric for example or different vegetation levels (herbaceous, shrubby, degree of tree cover and so on).
In Syrphidae and Cecidomyidae (gall midges), notably, only the larva is predatory on aphids, as the adult is flower-dwelling. In ladybirds, both larvae and adults hunt for aphids. The bugs (Heteroptera) like earwigs (Dermaptera) often have a mixed diet, both phytophagous and carnivorous. 
The amount of aphids consumed varies with the species’ energy needs. An older larva of 7-spotted ladybird can devour 100 aphids/day whereas a Symnus larva (small ladybird) eats no more than 8/day.

Various different predators are illustrated here.

Chrysopa perla (larva) Chrysopidae	Deraeocoris ruber Miridae	Meconema thalassinum Tettigoniidae	Xysticus cristalus Thomisidae

Classification of principal predators