Hover flies, sometimes called flower flies, belong to the family Syrphidae in the order Diptera (the true flies). Many species are colorfully marked with yellow and black, and could easily be mistaken for bees or wasps as they hover above the heads of blooming flowers. (Bee flies, equally adept at hovering, tend to be rather hairy and resemble small bumblebees. Unlike hover flies, bee flies often rest on bare ground.) About 900 syrphid species are present in North America, of which probably more than 300 species can be found in California according to Powell and Hogue in "California Insects". The Syrphidae are important pollinators, feeding chiefly on pollen. Nectar and honeydew are also consumed.
The habits and forms of the larvae (maggots) are more varied than the adults. The typical maggot is sluglike, colored green, brown, gray or some combination of these. The maggots of many species crawl about exposed on vegetation feeding on aphids. The prey is grasped by the jaws and raised into the air while the body contents are drained. Some species are useful in forensic entomology, being among the species that colonize corpses. Others live in the nests of bees, wasps and ants. A few are injurious to crops as larvae.
Hover flies (and bee flies) are common at the Nature Center. Watch for their activity near aphid colonies - perhaps you will find an adult laying eggs, or even a maggot feeding. And if you come across anything like I found at Dos Picos please let me know. I am still puzzled by what I found.
Ron Lyons (volunteer 1990-1999)