Hosts or Prey
Mostly slow-moving, soft-bodied insects such as mealybugs and especially aphids
Adults are robust to slender flies 1/8 to 1 inch (4–25 mm) long, varying by species. The broad head is about the width of the abdomen or wider and has large eyes with distinct antennae. The body of many adults is black with bands or stripes of orange, yellow, or white, resembling stinging bees or wasps. Some species are mostly brown, metallic blue or green, yellow, or combinations of these or other colors. For example, adults of ant-predaceous Microdon species are blackish to brown or bright to dark greenish. The hairy, stout, black and brown adults of Eristalis tenax are commonly named the drone fly because they resemble male honey bees, which are called drones.
Eggs are oblong, slightly curved, gray to white, and about 1/25 inch (1 mm) or less in length. The underside is flattened and the top and sides are convex. Eggs commonly occur singly near food for the emerging larvae.
Larvae of syrphids differ in appearance according to the environment where they feed. Larvae of most species are maggotlike without true legs and taper towards the head. Coloration is commonly brown, greenish, pink, or whitish. Body contents visible through the thin integument (covering) can cause larval coloration to vary according to the color of what they eat. Mature larvae of most species are 1/5 to 4/5 inch (5–20 mm) long.
Larvae of aquatic species (e.g., Eristalis tenax) are called rat-tailed maggots because they have a thin, respiratory (breathing) appendage (anal spiracles) at the rear that can be up to several times the length of the body. Species that occur in ant nests and prey on ant brood are domelike, round, and commonly brown with meshlike or netlike markings and resemble mollusks.
Fly larvae lack true legs, the three pairs of abdominal appendages. Larvae of aphid feeders and most other predaceous syrphids lack prolegs, which are fleshy, leglike appendages on the abdomen, but the creviced, segmented body can give the appearance of having appendages. Larvae of species that feed on decaying organic matter and microbes (e.g., Eristalis species) have six pairs of distinct prolegs.
Pupation occurs within the integument of the last instar, which hardens and becomes teardrop-shaped in most species. Pupae of ant predators (e.g., Microdon species) are domelike and round, resembling the larvae.
Adults of many species resemble stinging bees and wasps, but bees and wasps (Hymenoptera) have two pairs of wings. Flies (Diptera) have one pair of wings. Wing markings distinguish nearly all species of Syrphidae from adults of other flies. Between the third and fourth longitudinal veins and nearly parallel with them syrphids have a false or spurious vein; this streak of discoloration is a thickening in the wing that can be readily mistaken for a vein, but under magnification can be discriminated.
Eggs of brown lacewings are about the same color, shape, and size as those of syrphids and both can occur together. Brown lacewing eggs have a distinct, short disk or knob projecting from one end; this feature (an opening for sperm) is inconspicuous on syrphid eggs, resembling a discolored spot.
Larvae of some species resemble those of aphid flies (Chamaemyiidae) and predaceous midges (Cecidomyiidae); these larvae also prey on aphids, mealybugs, and other soft-bodied arthropods and can occur together with syrphids. Body size and the pair of breathing tubes at the rear end generally distinguish these groups.
Certain caterpillars and the larvae of many types of flies resemble syrphid larvae, but observing predaceous behavior can distinguish them. In comparison with other flies in the suborder Cyclorrhapha, syrphid larvae are distinguished by
- Each body segment having 12 spines in definite positions that help to identify the species.
- The pair of breathing tubes at the rear end of second and third instars touch (are fused).
Syrphids have four life stages: egg, larva, pupa, and adult. After hatching, larvae develop through three, increasingly larger instars. The last instar commonly migrates before pupating and may remain inactive for an extended period (as a prepupa) before pupating into an adult. After mating, females seek to lay eggs in habitat that is suitable for development of their larvae.
Pupation of most species occurs on host plants or on the ground in litter or topsoil. Pupae generally require dampness or relatively high humidity to successfully mature into adults. Especially under dry conditions the mature larvae tend to migrate away from the host plants to seek moist litter or loose soil where they move downward up to several inches deep to pupate.
For syrphids that prey on aphids or mealybugs, egg to adult development time during the growing season is commonly 2 to 4 weeks. These species can have 5 to 7 generations per year. Overwintering can be as adults in protected locations or prepupae, especially in colder locations. In mild coastal areas syrphids may be active throughout the year. Ant predators commonly have 1 generation per year, as with Microdon piperi that feeds on brood of various carpenter ants, Camponotus species