I recently started working part-time at Songbird Supplies, LLC, located inside Summerland Gardens here in Colorado Springs. One of the perks has been encountering insects I seldom see. Julie, the proprietor of Summerland Gardens, called me over the other day to help her identify a "primordial soup" bug in one of the water gardens she is cultivating. I was surprised and delighted to find the critters in question were "rat-tailed maggots," the larvae of flower flies in the tribes Eristalini and Sericomyiini (subfamily Eristalinae, family Syrphidae). I had seen them maybe one other time previously.
The conundrum faced by folks with water container gardens is this: do I put in "mosquito dunks" to kill the mosquito larvae, or do I refrain so that the syrphid larvae can grow up into pollinator-capable adult flies? There is no right or wrong answer. Syrphid flies are definitely not endangered species, so I might deploy the dunks.
The rat-tailed maggots are named for the posterior spiracles, which are fused into an extensible breathing siphon that connects them to the water surface at all times. Meanwhile, at the other end, they are filtering bacteria and other microbes from organic matter in the water. Larvae of some species exist in rather dense material like saturated manure, which barely qualifies as aquatic, but is too wet for other dung-eating fly larvae. The low oxygen levels in this kind of substrate are no problem because the maggot's spiracles are always connected to the surface.
The length of the "snorkel" varies greatly depending on the genus of flies. Eristalis larvae have very long tubes, as shown in the images here; Chrysogaster falls at the other end of the spectrum, with quite a short siphon.
Different genera occupy different aquatic niches. Mallota and Mylopeta, for example, live in the water that collects in tree holes. Sericomyia larvae live mostly in bog mat pools. Chrysogaster thrive along the edges of ponds and among emergent vegetation. The most commonly-encountered species are probably in the general Eristalis, Eristalinus, Helophilus, and Palpada, since they are most likely to be seen in urban, suburban, and rural areas. They live in putrid and organic-rich standing water. I know they are sometimes found while cleaning rain gutters.
Once the larva reaches maturity, it seeks dry land on which to pupate. Teh pupal capsule is hard, and resembles a tiny mouse in shape.
The "Drone Fly," Eristalis tenax, is the most common adult version of a rat-tailed maggot that one is likely to see. It is a stunning mimic of the honey bee. Likely introduced ages ago from Europe, it is now an established and widespread species across the entire North American continent. It is also a fairly respectable pollinator, visiting wildflowers, and the blossoms of orchard trees and landscape plants.
Here is an interesting piece of trivia: It is speculated that it was rat-tailed maggots that gave rise to the biblical account of "bees" spontaneously appearing from the rotting carcasses of animals. Certainly, the fluid decomposing material in a corpse would be a perfectly acceptable habitat for Drone Fly larvae.
Sources: Carr, John F. 2013. "Rat-tailed Maggots," Bugguide.net.
McAlpine, J.F. (editor). 1987. Manual of Nearctic Diptera Volume 2. Ottawa: Agriculture Canada. Monograph No. 28. pp. 675-1332.
Merritt, Richard W. and Kenneth W. Cummins. 1978. An Introduction to the Aquatic Insects of North America. Dubuque, Iowa: Kendall/Hunt Publishing Company. 441 pp.
Pfiester, Margaret, and Phillip E. Kaufman. 2009. "Rat-tailed Maggot," Featured Creatures. University of Florida Department of Entomology and Nematology.