A Bug Story in Three Acts.
As the BugLady was walking through the woods at the Ozaukee Washington Land Trust Huiras Lake property in late spring, she kept scaring up skittish little “wasps” that hop-scotched from shrub to shrub ahead of her. When she finally caught up with them (too briefly), she found not wasps, but flies in the wasp/bee-mimicking syrphid/flower/hover fly family Syrphidae.
Act I – A Short Tutorial on Syrphid flies
It’s a large and widespread fly family whose members range from small-and-delicate to large-and-clunky honeybee mimics called drone flies. Adults feed on nectar and pollen, mostly from yellow or white flowers, and on aphid “honeydew,” and they’re considered important pollinators despite the fact that they’re not designed to carry as much pollen as either wild bees or honeybees can. Overall, they’re exquisite, and good for your garden (unless their larvae are among those that eat flower bulbs). While they are excellent flyers, adult syrphids tend to be homebodies, not straying far from good larval habitat. Intriguing side note—according to Wikipedia, “The orchid species Epipactis veratrifolia mimics alarm pheromones of aphids to attract hoverflies for pollination.”
Syrphid larvae occur in a variety of habitats, including aquatic. While some species are scavengers, feeding on rotting organic materials (even within ant nests), others (despite the fact that syrphid larvae are headless and legless) are carnivores, acting as beneficial biological controls on aphids, leafhoppers, and other pest insects. The most famous syrphid is the Rat-tailed Maggot, the aquatic larva of the drone fly Eristalis tenax, which lives in shallow, murky water, breathing, as both larva and pupa, via a long respiratory “tail” that projects from its posterior and stretches up through the water’s surface film (the tail has earned them the nickname mousies). One source noted that some larvae use those tubes to puncture reed stems underwater and tap into the air chambers within the stem.
Act II – Meanwhile, back at Huiras Lake
So, the small bees turned out to be syrphid flies in the genus Temnostoma (from the Greek words temno and soma—cut body). There are only 10 Temnostoma species in North America, but the BugLady’s not going to venture past genus on this one—four, look-alike species live in the northeastern quadrat of North America. A European key to the genus described them as dwelling in “humid deciduous forests, where they bask on leaves and twigs and visit flowers of understory herbs.” Check.
Temnostomas are territorial and will attempt to chase larger insects. Males hang out near flowers, waiting for females; the females multitask, flying from flower to flower, feeding as they mate, with the males in tow.
Their larvae are saprophagous, living and feeding within decaying wood and possibly spending two winters as larvae. A report on saproxylic flies of old growth forests of Finland and Russia describe the larvae as having “huge hooks on the [barrel-shaped] thorax with which they tunnel into firm sapwood of fallen trees and branches.”
Act III – The Scientific Principle du jour – Mimicry:
Just how good does a mimic have to be, anyway (subtitle—is it more important that the guy who plays Jimmy Hoffa in the movie look like Hoffa, or act like Hoffa?)?
Temnostoma is, at first glance, a wasp mimic, probably of the mason wasps (picture included). Look twice and you note that Temnostoma has two wings to a wasp’s four, and its antennae are short. But some syrphids don’t stop with appearance, they mimic wasp behavior, too—raising their dark-colored, front legs so they look like long antennae (a feature that’s apparently important to discriminating birds) (some syrphids do have long antennae, and they don’t do the leg-raising behavior), mock-stinging when handled, wing-wagging, eschewing the direct flight of syrphids for a wasp’s erratic patterns, and even holding their wings out in a wasp-like “V” at rest.
Have scientists studied this? Indeed, they have! The syrphids’ ploy falls in the category of Batesian mimicry, in which a harmless/palatable organism protects itself by disguising as a harmful one. A mimic may not only look, but may also sound, smell, and/or behave like its model. Adding behavioral prompts to a good visual resemblance seems like a no-brainer, but in a study of 57 syrphid species, Carleton College researchers found that behavioral copycats were not that common (only 6 of their test species, three of them Temnostoma, practiced behavioral mimicry). They questioned whether a poor visual match might be overcome by a good set of behaviors.
What did they find?
- That to the human eye, many syrphid that are bee mimics are pretty darn good copies of their models (“high fidelity mimics”).
- That the syrphids who develop behavioral mimicry are wasp mimics—bees don’t have the characteristic twitches that wasps have.
- That syrphids do not develop behavioral mimicry to compensate for poor visual mimicry; the good behavioral mimics were more likely to be good (to the human eye, at least) visual/morphological matches, too.
- Importantly, they considered what they called “the eye of the beholder”—would an insect that fooled a human also fool a serious avian predator, or do we behold them differently?
- And they concluded by wondering why behavioral mimicry isn’t more common in visual mimics.
In a blog entry entitled “Good mimics have the costumes and the acting skills,” one of the co-authors includes two, nice videos of the flies in action: Katatrepsis—Good mimics have the costumes and the acting skills.
The Eye of the Beholder? Another research team attempted to evaluate wasp-mimicking syrphids through the eyes of a pigeon (with a nod to the fact that pigeons do not prey on syrphid flies). After pigeons were trained to recognize (peck at) images of wasps and images of non-mimetic flies, images of syrphid flies were added, with the various morphological features simplified and optimized.
What did the pigeons tell them?
- Some mimics are “imperfect” mimics—Close, but no cigar. If imperfect mimics get away with it because they look realistic to their predators, what are their predators basing their decisions on?
- The length of the antennae (or of what the pigeons thought were antennae) was important, but pigeons used it in combination with other characteristics, like the presence and number of stripes or color blocks, length of the abdomen, and width of the head.
- Based on information from the pigeons, scientists were able to predict fairly accurately which syrphids would be considered “waspish.” While the pigeons and the scientists agreed about the authenticity of many syrphid fly “costumes,” there was one syrphid fly that the humans thought was a good mimic, but that the pigeons saw right through. And vice-versa.
Like all good studies, this one answered some questions and raised more. Read all about it at Proceedings of the Royal Society B—The key mimetic features of hoverflies through avian eyes.
And when syrphid flies appear on the daisies at the beginning of June, dawdle a bit and enjoy them.