It's good to know about the helpful allies as many of these species resemble miniature yellow jackets. A good number of the species in the Syrphidae family have black and yellow bands on their abdomens. The interesting thing about these flies is they can hover, and can actually fly backwards, hence the name "hover fly." Many of the species in this family will land on your exposed skin to lap up salts and minerals in the heat of summer. They also use flowers as sources of nectar.
The good news is that they lay eggs that subsequently turn into larvae (or maggots), and these maggots have a strong penchant to devour aphids. So a responsible hover fly lays her eggs where aphids are or are likely to be soon. The maggot upon hatching roves about the plant and seeks out aphids, clamps onto the aphid, and they then drains the aphid of their body fluids.
The adults do not sting or bite and are termed stingless flies (by the entomological community). These insects undergo complete metamorphosis with 3 larval instars. They hatch out of their whitish 1mm eggs, into legless larvae, and many species have a longitudinal stripe on their dorsal surface. Larvae vary from 1-13 mm (1/32-1/2") in length, and pupae are pear shaped, and green to dark brown in color. Pupation occurs on plants or on the soil surface. A single syrphid fly larva can consume hundreds of aphids in a month. They consume the body contents and discard the empty skin on the soil or plant tissue. Some species do not devour aphids, but feed on fungi, but the majority of the species eat other insects.
Entomologists break up the Syrphidae into 3 subfamilies Brachyopini, Microdontinae, and Syrphinae. These groups are based on characters of the tribes, and subtribes that make up subfamilies. This is family that you might not want to try to key out, as there are 800 species and 100 genera in our area, and 200 genera and 6000 species worldwide. Further, one keys these out in the adult form by their wing vein patterns and the antenna forms. Therefore, the fly needs to be keyed out in a lethal manner. You can't distinguish wing patterns while they are hovering around. Interesting? Well certainly in an academic sense, but from a practical point of view, perhaps its best simply to appreciate their contribution in any ecosystem or agricultural setting.
In a recent visit to the Landisville flower trial gardens, plants were mobbed by hover flies, and at times I had up to seven of them resting on my arms. They were continually doing their helicopter imitation and then zooming off to get some nectar on one of the flowers in the trials. Alyssum, marigolds, petunias, and cosmos are some of their preferred flowers, but many species are utilized for nectar. In about 3 days their larvae totally destroyed a colony of aphids in the greenhouse on a black pearl pepper plant. They had some help from Aphidus spp. (tiny parasitic wasps), who also destroy aphids, but these wasps lay an egg in the aphid which then eats it's way out of the aphid leaving a hollow exoskeleton with a distinct hole in the abdomen. So far it has been a very busy season for these aphid-destroying insects, and there are very few aphids at the flower trials. There is little need for insecticides when the population density of predators and parasitoids is so high. We did not purchase and import these fellows; they simply showed up and did the job on their own. Know how to recognize the beneficial insects; they will help your garden flourish!