On the left is Syrphus ribesii. Adult body length is 9 - 13 mm. Adults are commonly seen flying from April to October, and is abundant in the U. K. It is found in most habitats, and the adults visit flowers of many types.
It overwinters as a larva in leaf litter. The larva eats aphids.
Episyrphus balteatus, commonly known as the marmalade fly, and is one of the most common hoverflies in the U. K. Adult body length 7 - 11 mm, and wing length 6 - 10 mm. Flies from May to October, but most commonly seen in July and August. It overwinters in the U. K. as an adult, but may not survive very cold winters.
Adult males are territorial, and within the territory will display to females and chase off any other males.
The female lays her eggs on plants where there are aphids.
The larva eats aphids from a wide variety of plants, but seems to have a preference for aphids located lower down on the plant. A single larva will eat around 200 aphids before it pupates. Uaually it pupates near the aphid colony.
From egg to adult takes around a month. There are usually 2 generations per year.
Adults feed on nectar, and their favourite flowers are hogweed, cow parsley, ragwort and thistles.