Fly Day Friday: The Good Mosquito

Believe it or not, there is a mosquito species I look forward to seeing every year. You may think I'm crazy, but hear me out. There are at least three reasons to love elephant mosquitoes in the genus Toxorhynchites. They are pretty, beneficial in the larval stage, and do not bite as adults.

A male Elephant Mosquito

Elephant mosquitoes are also known as giant treehole mosquitoes, so you can sense a theme here. They are indeed large for mosquitoes, averaging about seven millimeters in body length. That does not include the mouthparts and antennae. Their long legs make them appear larger still. Why the name "elephant mosquito?" It may be a reference to the long, upcurved palps of the males, which suggest elephant tusks.

Notice the tusk-like palps (part of the mouthparts), and bushy antennae on this male.

There is no denying the beauty of our single North American species, Toxorhynchites rutilus. They are iridescent blue and purple, with silver and gold highlights, and white tips on the hind legs. It seems that no one photo captures all the colors all at once. Despite their brilliance, they are surprisingly cryptic in the dimming light of dusk, when they are most active (though they are considered day-fliers).

As their other name indicates, elephant mosquitoes breed mostly in water-filled treeholes. The larval stage, at least in later instars, is predatory on other aquatic insects, including....wait for it....the larvae of other mosquitoes. Yes, you heard that correctly, they are a natural biological control of the mosquito species that pose a threat to human health. The larvae are reddish in color, with a short anal siphon (bearing the spiracle that permits intake of air at the water surface), and a broad, black head with strong mandibles. There are four larval instars, an instar being the interval between molts.

The female Elephant Mosquito has straight palps, and simple bristle-like antennae.

The adult elephant mosquito has a distinctive proboscis, bent strongly downward near the middle of its length. Instead of using their beaks to draw blood from other animals, elephant mosquitoes of both sexes feed only on flower nectar. As flower visitors, they are also pollinators of wildflowers. Some other species in the genus may also feed on fruit juices, and honeydew (the sugary liquid waste of aphids, scale insects, and various planthoppers).

The female again, showing that long,sharply-curved proboscis.

Here in northeast Kansas, on the Missouri border, I have found Toxorhynchites rutilus only in late autumn, and only on the flowers of White Snakeroot. I have only seen them in our yard, in fact, but I am not usually exploring elsewhere as sunset approaches. The species ranges from the eastern half of Texas and Oklahoma diagonally northeast to southern Pennsylvania, Connecticut, and Delaware.

Can you tell which sex this one is?

There are roughly eighty species of Toxorhynchites, occuring mostly in tropical regions around the globe. Due to their unique appetite for other mosquitoes, they have been employed as biological controls for container-breeding mosquitoes in Japan, southeast Asia, the Caribbean, and in some cities in the United States.

Are you a new fan of elephant mosquitoes yet? If not, please see the resources and links below for additional information on the biology of these remarkable flies. Mosquitoes, like most insects, and people, defy neat categorizing as "bad" or "good." Entomology is a great place to learn lessons like that.

So handsome!

Sources: Alomar, Abdullah A. and Barry W. Alto. 2022. "Elephant Mosquito Toxorhynchites rutilus Coquillett, 1896 (Insecta: Diptera: Culicidae)," Ask IFAS EENY-787/IN1380. University of Florida.
Coin, Patrick, et al. 2004. "Species Toxorhynchites rutilus - Elephant Mosquito," Bugguide.net
Donald, Claire L., Padet Siriyasatien, and Alain Kohl. 2020. "Toxorhynchites Species: A Review of Current Knowledge," Insects 11(11): 747.
McAlister, Erica. 2017. The Secret Life of Flies. Buffalo: Firefly Books. 248 pp.
Ricciuti, Ed. 2019. "Meet the Mosquito With a Big Appetite - for Other Mosquitoes," Entomology Today.

Strange Flies on a Fish Carcass

Spring has been dragging her heels here in northeast Kansas, USA, but my partner and I managed a brief outing to take advantage of a certifiably warm and sunny day this past Saturday. Finally, there were a few insects to be seen, even if they were mostly non-biting midges, a few other flies, one butterfly, and several Eastern Boxelder Bugs. Heidi spotted the remains of a large fish just off the trail, and I noticed some small, unusual flies dashing over the head of the dead piscine. I managed a few images, and saw that I had something unfamiliar and pretty interesting.

One female (top, left) and two male waltzing flies

The location of our short hike was the Mill Creek Streamway Park in Johnson County, Kansas, north of Shawnee Mission Park. We were on Nelson Island in the Kansas River for most of the time, and that is where we found the fish remains. The overstory was mostly cottonwood trees, the understory just beginning to green up.

A blow fly and at least two waltzing flies

There were the obligatory blow flies on the fish, too. I suspect the deep metallic blue insects were either Phormia or Protophormia. Without collecting specimens, fly identification is problematic. It comes down to patterns of setae, the color of thoracid spiracles, and other miniscule characters that are not usually visible in mere photographs of wild, mostly moving targets.

The smaller flies were another matter. They were so unique that once I returned home I was immediately able to identify them to species. More importantly, I learned about their bizarre behavior. About twice the size of your average kitchen-inhabiting "fruit fly," Drosophila sp., these flies were slender and long-legged, the better for running around and chasing each other, as was apparent in the field.

Male waltzing fly

Surprisingly, my hunch that they were members of the "skipper flies" family Piophilidae was correct. The most famous members of the family are the cheese skippers, named for the "jumping" maggots that infest rancid cheese, "skipping" away from danger by grasping their rear end in their mouths and suddenly releasing it, catapulting themselves vertically and horizontally. As a whole, the larvae of most species feed on decomposing fatty tissues in carrion, often after the corpse begins drying out.

Female (top) and male waltzing flies

The flies I had been observing were "waltzing flies" of the species Prochyliza xanthostoma. Their most distinguishing aspect is the sexual dimorphism between males and females. Males have greatly elongated heads, with a torpedo-shaped face and remarkably long atennae originating from the tip of the cone. Females are more....demure, or at least less comical in appareance, with a head of more normal dimensions.

Why the long face on the males? Stephen A. Marshall, in his epic tome Flies: The Natural History and Diversity of Diptera, explains:

"One of my favorite piophilines is Prochyliza xanthostoma, a Nearctic species that appears in abundance in early spring when melting snow exposes the carcasses of winter-killed animals. Males of P. xanthostoma have long antennae and a greatly elongated head that is used in ritual male-male battles for territory, during which these striking insects stand on their hind legs and engage in vigorous head-butting contests."

Dr. Erica McAlister, in her fascinating book The Secret Life of Flies, also notes the ferocity of these sparring matches:

"The males in another of the group, the waltzing fly, Prochyliza xanthostoma, have quite wonderful heads - almost conical in shape with very large and thick antennae - and they feed on animal carcasses. As their name suggests the males dance to woo the females. But they also have to defend their territories (carcasses) from other males and can have the most amazing fist fights - they really reign down punches on each other.

A female ignoring two competing males

I am now regretting not to have taken a video or two while I was there, though Heidi had long since made her way far ahead of me on the trail. Should I encounter waltzing flies again, I won't make that error of omission. Meanwhile, you might want to search for them. The species occurs over most of North America, especially the eastern half of the continent. If you are sufficiently intrigued, this Wikipedia article goes into extraordinary detail about the behavior and biology of the species.

Fly Day Friday: Biting Midges, No-See-Ums, Punkies

The list of aliases for the biting midges of the fly family Ceratopogonidae is seemingly endless, but most are apt descriptions of how tiny these insects are. I suppose I could emphasize their diminutive size by not illustrating this post at all, and leaving their appearance to your imagination....These flies certainly stretch the limits of my Canon PowerShot SX50 (and more recently SX70), so I encourage you to visit other photo resources to see the insects' true majesty.

Adult female Culicoides sp.from Colorado, USA.

Were it not for the few species of biting midges in the genus Culicoides that habitually bite humans, we might know little about them at all, aside from those that bite livestock. The majority of ceratopogonids apparently feed mostly on the hemolymph (blood) of other insects, believe it or not.

Biting midges feeding on the hemolymph (blood) of an Arizona Sister butterfly in the Huachucha Mountains of Arizona, USA

Only the females take blood meals, and in some species the females have atrophied mouthparts and do not feed on blood. Both sexes feed on nectar and other sugary liquids as well, to fuel their flights. There are at least 39 genera, with over 600 species, of Ceratopoginidae in North America north of Mexico.

© Judy Semroc

I was prompted to finally do a post about no-see-ums, in part, by an e-mail I received from my friend Judy Semroc, an accomplished wildlife photographer. She sent me images of peculiar "larvae" around the nostrils of a snapping turtle. It wasn't until the final close-up image that I was able to discern that the creatures were actually adult female biting midges, some so engorged with blood that they looked more like mites.

© Judy Semrock

Not all ceratopogonids are that small. They collectively range from one to six millimeters in body length. They are not always easy to tell apart from other families of non-biting midges, but with a little practice one can often discern the "long face" that includes a short proboscis and associated palps.

This biting midge has killed a non-biting midge in Wisconsin, USA

No-see-ums can be more than a mere nuisance. Some are known to transmit disease-causing microbes to humans and livestock. They are a principal vector of bluetongue, a virus which afflicts cattle and sheep.

In this specimen from Colorado, USA, you can see the proboscis and palps.

There is, however, at least one redeeming member of the family. A species in the genus Forcipomyia is the sole pollinator of cacao trees, the plant from which we get chocolate. Unfortunately, when tropical forests are turned into cacao plantations, the tiny flies lose the humid conditions and moist soil needed to complete their life cycle.

The life cycle of ceratopogonids usually revolves around aquatic or semi-aquatic habitats, or excessively moist conditions in otherwise dry habitats. Larvae may live under bark of decaying logs, in mosses, treeholes, sap flows, or soil. There, most feed on floral debris, algae, or fungi. Larvae in fully aquatic habitats are mostly or exclusively predatory on other small organisms.

Male biting midges, like this one from Illinois, USA, often fold down the setae on their antennae, giving them the appearance of having a mustache.

Adult male midges of many species form aerial swarms over a prominent object or landmark, to attract the attention of females. The plumose (feather-like) antennae of males function as "ears," tuned to the wingbeat frequency of approaching females. Mating usually takes place while airborne, though some species alight to copulate.

Clinohelea bimaculata, Leavenworth, Kansas, USA

Another encounter that fueled my curiosity about ceratopogonids happened just yesterday. I spotted what I initially thought was a small, skinny mirid plant bug on the surface of a leaf in our back yard. I had serious difficulty getting an in-focus image with any detail whatsoever. Reviewing the images it became clear it was not even a true bug. On a hunch I explored Bugguide.net for ornate punkies. Lo-and-behold, I found a match: Clinohelea bimaculata. It will be a state record for Kansas for both Bugguide and iNaturalist, provided I have identified it correctly.

Oh, my, I think it has prehensile "toes" (tarsal claws)!

The preceding is at best a snapshot of a highly diverse and complicated family of flies. Much is known, especially about species of economic and public health importance, but there remains much to learn.

Sources: McAlister, Erica. 2017. The Secret Life of Flies. Richmond Hill, Ontario: Firefly Books. 248 pp.
McAlpine, J.F., et al. 1981. Manual of Nearctic Diptera, Volume 1. Ottawa, Ontario: Agriculture Canada. 674 pp.
Bugguide.net
a href=http://campus.belmont.edu/cienews/cie.html>The Ceratopogonid Web Page.

A Funny Story

More content will be coming to this blog, more regularly, in October, but I wanted to share a humorous experience from our last bioblitz at Ute Valley Park in Colorado Springs. It was Saturday morning, September 16, and I'd already been up earlier than I usually am, so I will blame my assumptions on being a little tired.

I happened to spot an interesting syrphid fly, Ferdinandea sp., right in the middle of the trail. The park gets a lot of foot and mountain bike traffic, so I hurried to take pictures. My camera was not focusing well on this mostly overcast day, so I maneuvered to the side of the trail and kept trying....

Sure enough, I heard a trail runner pounding down the trail, and then stopping abruptly. I thought that was quite considerate and polite, but I hurried even faster to get a respectable image. I said "thank you for stopping" to the obliging jogger, but there was no reply. I finished shooting, and turned to continue on the trail. When I looked up, this was the "trail runner."

Naturally, when I related this story to others back at base camp, everyone had to offer their own punchlines. "But what did the buck say?" in reference to the song "The Fox" (What Does the Fox Say? "No, the buck stopped there," punned another individual. Yeah, yeah, hilarious.

I do not intend to be quite so assuming in the future, lest the next trail runner be a bobcat, mountain lion, coyote, or bear. Even our urban neighborhoods here can be dangerous to those who are not aware of their surroundings. Happy trails, folks.

Louse Flies? Are You "Ked"-ing Me?

Until recently, insect diversity here along the Front Range of Colorado this spring has been suppressed by cool, wet weather. How amazing, then, that I should see not one, but two specimens of an insect I had seen only once or twice in all my years. I am speaking of "louse flies" in the family Hippoboscidae.

Back at the end of December in 2009, I volunteered to do a guest blog for my good friend Dave Small after he sent me an e-mail inquiring whether I might know the identity of an insect in an image he attached with his message. I didn’t quite understand the story at first, like why two hunters were involved, and why the insect was adhered to hard candy. The image was clear enough, though, that I could tell what the creature was, and that this was not a tall tale evolved from heavy drinking. Here is the rest of that piece.

Deer and related animals play host to a variety of parasitic invertebrates, including the familiar, slow-moving ticks. Such blood-feeding creatures are quick to abandon a deceased host, however, and so animals harvested during the hunt often yield some strange, living cargo during inspection and dressing.

The specimen discovered by these two hunters is an insect known as a “louse fly,” among the most peculiar of insect parasites. They are much more nimble than ticks, dodging attempts to catch them as they skirt through a host’s fur. Their flattened shape, top to bottom, and talon-like claws enhance their ability to slip between hairs and grip the hide of the host, avoiding licking and biting efforts to dislodge them.

Members of the fly family Hippoboscidae, most known louse flies are actually parasites of birds. The most infamous member of the family, though, is the “sheep ked,” well-known to those who manage flocks of ovine livestock. The adult flies are wingless, but they find each other easily for mating when the host animals crowd together.

Back to the specimen the hunters found, and collected by sticking it with a piece of hard candy. That would probably be a “deer ked,” Lipoptena cervi. Here is a terrific image of a related species, Lipoptena depressa from northern California. Both species have a life history that is typical of the entire family of louse flies, but truly mind-blowing compared to other insects.

Female louse flies do not lay eggs. They grow one offspring at a time, within their bodies, much like mammals. A single larva develops inside the equivalent of a uterus, feeding from a “milk” gland. When mature, it is “born alive,” dropping to the ground where it immediately buries itself and pupates. An adult fly emerges weeks later. Deer ked are winged, at least initially, but females that have found a host will break off their wings, the better to maneuver through the dense, coarse hair of their host.

Wingless deer ked after two days in captivity

While keds are known to transmit certain diseases from host to host, they are for the most part not economically important. Consequently, little is known about them. Kudos to our hunter friends for bringing this one into the spotlight.

The first specimen I found this year landed on my while I was hiking in Aiken Canyon Preserve off of highway 115, about fifteen miles northeast of Penrose, Colorado on Memorial Day (May 30). I recognized it fast enough to grab it while it was tangled up in some of my sparse arm hair.

After chilling the tiny, 3 millimeter creature in the fridge for a few minutes, I dumped it out into a casserole dish to photograph it. Well, it was apparently completely unaffected by the cold and flew off after I fired off only a couple of legible shots. About 30-45 seconds later, from out of nowhere, it landed on my chest. I guess it pays to be a mammal to keep these flies under some degree of control. A couple of days later, I found the fly had self-broken its wings, such that they were mere stubs. Despite that, the insect was still highly mobile and most of my shots were as blurry as when it had flight capability.

The next encounter was at a blacklight set up to attract moths in Cheyenne Mountain State Park on the night of June 4. I routinely scrutinize the most minute of creatures in the event they may be of interest, and much to my surprise there was another "ked." It, too, was fully winged but, perhaps mesmerized by the ultraviolet light, sat motionless and allowed me to take multiple images....most of which are still not optimal because the blame bug is so miniscule.

Want to find your own louse flies? You might volunteer to go hunting game mammals and birds with your friends to get a chance at shaking one loose. Otherwise, good luck. These insects are common, but so seldom stray from hosts that it is sheer serendipity to stumble across one. More than likely, the fly will find you.

Bite of the Black Fly

It happened at the zoo. I was parked innocently opposite the African Elephant yard, with the window rolled down on this nice late afternoon of September 19, 2015, waiting for Heidi to get off work. I noticed a tiny fly had flown into the car, but was truly shocked to see it was a female black fly, likely a species of Simulium....and she wanted my blood.

She got it, too, because I so rarely see members of the family Simuliidae, let alone adult females. I was able to get some respectable images of the 3-4 millimeter vampire precisely because she was occupied pumping blood out of my knuckle. What was she doing here, though?

I was under the mistaken impression that black flies, also known as "buffalo gnats" and, here in Colorado at least, "turkey gnats," seldom venture far from fast-flowing streams and rivers. That is because they spend the egg, larva, and pupa stage of their lives in aquatic torrents. It occurred to me that the elephant yard does have a waterfall, and I would bet that black fly larvae live on the very edge of that artificial cataract.

Well, I have since learned that female black flies are perfectly capable of flying miles in search of a blood meal. There are some records in Canada of black flies migrating over ninety (90) miles from where they grew up as larvae.

Only the female black fly bites. Like mosquitoes, she needs the protein for the proper development of her eggs; and she can lay several hundred eggs in her two- or three-week adult lifespan. Unlike mosquitoes, black flies do not have beak-like mouthparts to extract blood from our capillaries. Black flies slice you open with knife-like mouthparts, then lap up what spills out.

Depending on the species, the black flies lay their eggs on vegetation or other objects in the water, under the water, or scatter them on the water's surface. Interestingly, freshly-laid eggs apparently produce a pheromone (scent) that attracts other adult female flies of the same species, and stimulates them to lay their eggs in the vicinity.

The larvae that hatch spin silken pads on the surface of stones or vegetation in the middle of flowing water. They then anchor themselves to the silk pads with special hooks on the rear of the abdomen. The larvae feed in a unique manner, by deploying a pair of "cephalic fans" that intercept organic particulates from the current. Larvae molt 7-11 times.

Black fly larvae in Arizona

At the end of its larval life, the creature spins a silken bag in which it will pupate. The tapered rear of the bag points upstream into the current, while the wider, open portion projects downstream. The larva molts one final time to reveal a resting stage with branching gills that may reach beyond the lip of the silken bag.

Black fly pupae in Arizona © Tony Palmer

The adult fly emerges in a few days or so, rising to the water surface in an air bubble and floating to an emergent object it can climb onto and finish expanding its wings and hard its exoskeleton. The total time from egg to adult takes roughly 3-4 weeks and is heavily influenced by water temperature. The colder the water, the longer the life cycle. There can be three or four generations per year; winter is typically spent in a dormant larva stage.

Male black fly (note huge eyes meeting at top of head)

There are about forty species of black flies in Colorado, in three genera: Simulium, Prosimulium, and Metacnephia. Different species live at different elevations, and on different sides of the Continental Divide. Surprisingly, the majority are not pests of people or livestock, preferring to feed on birds and other wildlife.

The species that do afflict horses, cattle, poultry, and people can cause severe distress, and may carry diseases. I will spare you the agonizing details of suffering incurred by victims of black fly attacks, as there is no end to the resources where you can learn such information if you are so inclined.

Suffice it to say that it pays to be prepared with an excellent insect repellent if you plan to be in black fly territory. Prevention is always the best tactic for battling *any* bloodsuckers. Take care.

Female black fly on Feb. 24, 2014

Sources: Bechinski, Edward John, and Marc J. Klowden. 2005. Black Flies - Biology and Control. Division of Entomology, University of Idaho (available online as a PDF).
Cranshaw, W.S., F.B. Pearis, and B. Kondratieff. 2013. Biting Flies. Colorado State University Extension. Fact Sheet 5.582.
Kuhn, Dwight. "Black Flies - Life Cycle," Kuhn Photo.

The Stable Fly

Of all the biting flies we have in North America, one of the most annoying has to be the Stable Fly, Stomoxys calcitrans. They are especially abundant around farms, ranches, zoos, and other places where large mammals are kept. Unfortunately, they will also bite people when livestock is not close at hand.

Like any notorious villain worth their salt, the Stable Fly has it aliases: "beach fly," "dog fly," and "lawn-mower fly" among them. The insect has also fled local jurisdiction. It is apparently native to Eurasia and Africa, with speculation that it probably came to the New World in colonial times, maybe in ship's ballast.

The Stable Fly is easily dismissed as a House Fly under cursory examination. Both flies are in the family Muscidae; and both are about the same size, the Stable Fly measuring 5-7 millimeters. Each species is mostly gray, with black "pinstripes" down the back of the thorax. The Stable Fly differs mostly in having a slender, black, slightly curved beak tucked under its "chin."

Proboscis (painfully) deployed!

While it is only the female mosquito, black fly, deer fly, and horse fly that sucks blood, both genders of the Stable Fly can bite. This is not a painless event, either. An immediate, sharp sensation occurs when the fly plunges its piercing mouthparts into your skin. Shoo it away and it returns instantly, and repeatedly. This persistence is perhaps the main source of our aggravation.

Commencing feeding

Almost full!

The adult fly is only one quarter of the life history of the species of course, with eggs, larvae, and pupae making up the other three stages in its metamorphosis. The female fly deposits her eggs singly, or in clusters of 25-30, in wet, decaying fibrous organic matter. Typical breeding material includes horse manure, silage, rotting hay, grass clippings, and partially composted livestock bedding.

Female full of eggs

The eggs hatch in one to fourteen days. The maggots that emerge take anywhere from 11-30 days on average to mature. The interval is largely determined by temperature, humidity, and food quality and quantity. The hotter and more humid the substrate, the faster the maggots develop. The maggot molts twice after hatching, and may be up to twelve millimeters in length by the time it enters the pupal stage.

The pupa represents the larva's third molt, the shed exoskeleton of which forms a hard, oval, dark brown "capsule" around the pupa itself. The pupal stage typically lasts six to twenty days. The adult fly then bursts out of its capsule by more or less inflating the front of its head.

Stable Flies will also sip flower nectar

The determination of Stable Flies in their feeding behavior naturally induces stress in its victims, and this can take a toll on livestock. A mere twenty flies on a cow can result in decreased milk production. Mild anemia and weight loss can also be a result of high numbers of feeding Stable Flies.

Were it such that general malaise was the only negative effect of Stable Fly populations, it would possibly be tolerable. Unfortunately, Stomoxys calictrans can also carry a variety of diseases. Most of these are of limited effect in the U.S., thankfully, but they cannot be dismissed entirely. Here, the fly can transmit anthrax, which affects livestock, pets, and people. Anthrax exhibits a variety of symptoms, the worst of which include lesions of the lungs or brain.

The next time you visit a local farm, ranch, or zoo, you might want to consider applying that DEET-based insect repellent to help fend-off attacks of Stable Flies. It will make your experience much more enjoyable if it is bite-free.

Sources: Cumming, Jeffrey M. 2006. "Diptera Associated With Livestock Dung," North American Dipterists Society.
Newberry, J. 2003. "Stomoxys calcitrans" (online), Animal Diversity Web.

The Maggot on the Rosebush: Aphid-eating Flower Fly Larvae

Gardeners cannot always be faulted for not understanding who their insect friends are. Entomologists have done a relatively poor job of advising the public on what garden insects are pests, and which are beneficial. It is especially problematic when you consider metamorphosis. You do not always know which larva corresponds to which adult, and whether both life stages are helpful or harmful. Case in point, larvae of flower flies (family Syrphidae), also known as "hover flies."

The overwhelming majority of flower flies in the subfamily Syrphinae are aphid predators in their larval stages. What you might easily mistake for just another flower- or foliage-munching caterpillar on your rosebush is probably one of these allies in your war on pests.

Killing an aphid!

Not that you would even notice them, as they can be incredibly cryptic, disguised as a discolored patch on the edge of a leaf, or a shed pollen cone on a pine tree....

Flower fly larva on pine needle

Female syrphid flies lay eggs in or near aphid colonies. The tiny larvae that hatch feed and grow, with three instars before pupation. An instar is the interval between molts. Despite its elastic appearance, the larva still has an exoskeleton that must be shed in order for the insect to grow larger. The final molt results in the pupa stage, a convex pear-shaped lump attached to a leaf, stem, or other substrate. The adult fly eventually emerges from the capsule-like pupa.

A little Allograpta sp. flower fly

Most adult syrphids strongly resemble wasps or bees, to the degree that they frequently fool entomologists. Like their hymenopteran models, the flies are capable pollinators of various flowers, visiting blossoms for nectar and picking up pollen in the process.

Adult female Syrphus sp. laying eggs in aphid colony

Flower flies can be recognized by their hovering behavior (few bees are capable of hovering, oddly enough), enormous eyes that meet at the top of the head in males, and nearly meet in females, and only one pair of wings to a bee's two pairs. Flower flies also have very short antennae in most cases, whereas bees and wasps have longer, thicker antennae, often "elbowed" where the first, long segment is connected to the remaining shorter segments.

A tiny Toxomerus sp. pollinating a flower

Lastly, while bees and wasps have chewing mandibles and sometimes a tongue-like arrangement of other mouthparts, flower flies have an extendable...."arm" tipped with a sponge-like pad. This organ is retracted under the fly's "chin" when not in use.

Adult Eupeodes or Syrphus sp.

Syrphid flies are among the most abundant of garden insects, especially conspicuous in spring and fall when aphid populations are at their peak. Watch for them and be careful not to accidentally kill their larvae when you are contemplating taking aphid control into your own hands. Between syrphid larvae, lady beetle larvae, and lacewing larvae, you may not have to do anything to keep aphids from reaching destructive numbers.

Another one bites the dust!

Sources: There are several excellent online resources about syrphids.
Rotheray, Graham E. 1993. "Colour Guide to Hoverfly Larvae in Britain and Europe," Dipterists Digest. No. 9. Sheffield, England: Derek Whitely and the Royal Museum of Scotland. 160 pp.
Flower Fly Survey of Los Angeles County
University of California Integrated Pest Management website on syrphids.
Texas A & M University website