Hover flies--also called flower flies, drone flies, and syrphid flies--make up some 6000 species in 200 genera. They live everywhere on Earth except deserts and very high tundra.
They are most versatile as maggots. Most eat aphids and thrips by piercing, lifting, and draining. Species that live in water containing much decaying matter feature an anal breathing tube twice the length of the body; these forty-odd species are called 'rat-tailed maggots' and are capable of hatching and living inside humans. Some live in flowers (grow chamomile organically some time), while most inhabit decaying organic matter. One species, volucella, lives in bumblebee nests.
After two to three weeks and several instars, larvae attach themselves to leaves for pupation. Gradually, the pupa turns the color of the finished fly.
Hover flies are classified as Mullerian mimics because they resemble bees and wasps. Their heads, however, remain fly-like, while each wing--two, not four as in bees--is traversed by a spurious blood vessel. Hover flies can attain a length of 5/8 inch.
Syrphidae are among few insect families capable of hovering and flying backwards. Their flight patterns are rigorous; they dart, slow-curve, ascend, and descend with control that appears artificial. Their wings beat hundreds of times per second, giving some species a characteristic whine.
Hover flies feed mostly on pollen and nectar. Like ants, they also take honeydew from aphids. Most feed on three to six pollen types, while others correspond with only one. Hover flies carry much unappreciated weight in ecosystems; North American tree savannah evolved without honeybees. Hover flies are used agriculturally in New Zealand to pollinate coriander and tansy leaf. Small flowers encourage them into home gardens as well.
Seven generations of hover fly can pass in one year. Eggs, creamy-white and elongated, hatch after 3-9 days near aphid infestations, in mulch, in stagnant water, or in bumblebee nests.
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CV Duke, BSc (Hons). "All About Hover Flies."
Candice Hawkinson. "Hover/Syrphid/Flower Flies."
Texas A&M AgriLife Extension. "Syrphid Fly."
UC IPM Online (UC Irvine). "Syrphid, Flower, or Hover flies."