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UPCOMING OPPORTUNITY:
Join us for another
Winter Hummingbird Expedition to Costa Rica
in February 2006

"STEALTH" FLIES ON BIRDS:
THE HIPPOBOSCIDS

Looking at birds through the window is a fun thing to do, and each year tens of millions of Americans invest--incredibly enough--billions of dollars on feeders, bird seed, and associated paraphernalia. People spend this money because they admire birds for their colors, their songs, their ability to fly through the air, and because they like to bring these feathered wonders in for closer view. Folks can discover a great deal from simply observing all these backyard birds with binoculars, camera, or spotting scope, but we can learn only so much from such encounters; there are times when having a living bird in-the-hand reveals far more. Such was the case recently at Hilton Pond Center when an Eastern Towhee we captured gave us interesting insight into its relationship with a stealthy little insect most birders never get a chance to see.

Early one morning, the towhee in question blundered into a mist net we deploy to catch wild birds for banding. From our office window in the Center's old farmhouse we saw the bird hit the net and immediately went outside to retrieve it. The towhee was a young hatch-year male just completing his post-juvenal (AKA "first pre-basic") molt; that is, he had traded nearly all his dingy juvenal plumage for jet-black body feathers that are quite different from the rich brown of towhee females. (See John James Audubon's comparative rendering above right of male and female Eastern Towhees, formerly called "Rufous-sided.") We removed the bird gently from the net and carried him inside to the banding table for a closer look.

All text & photos © Hilton Pond Center

As we bent over our notebook and tools, an insect suddenly sprang from the towhee's plumage and flew directly into our left eyebrow. From experience we knew this must be a hippoboscid fly, so we made a quick grab for a small collecting jar we keep on the desk. When we plunked the vial over our eye, the fly flew in, and we slid on the lid to capture it. From side view (above) we could see the quarter-inch-long insect was very flat from top to bottom with long spindly legs, a configuration that reminds us of the compressed shape and landing gear of the U.S. Air Force F-117 "stealth fighter" (below left). Could it be the creators of the F-117 were part time entomologists who incorporated the hippoboscid fly's unusual shape into their own aeronautical design?

F-117s depend on their radical configuration and engineering components to sneak past enemy radar, while the shape of our newly captured fly serves a slightly different function. Members of the Hippoboscidae (Louse Fly or Flat Fly Family) are ectoparasites that dine on blood of birds and mammals. Some are wingless adult forms called "keds" that infest deer, sheep, and other hoofed creatures and have economic importance. Those that occur on birds, however, usually have--like their hosts--a single pair of functional wings throughout their lives and are "insignificant" to humans. (There are a few records of hippoboscids biting people, but these are probably accidental occurrences when the parasite was simply "testing the waters.") In any case, the flatness of flat flies allows them to slide between feathers or fur to get at the epidermis of the host, where they plunge a hypodermic-like mouthpart through the skin to suck up succulent blood meals.

All text & photos © Hilton Pond Center

The mouthpart, called a "palp," is normally held flat against the fly's body and is hidden by other mouthparts, but when our captive hippoboscid died its muscles relaxed and the needle-shaped structure unfolded for viewing, as shown in the photo above. (Also see lateral view near top of page.) Although the fly's death was unexpected, it was opportune; taking close-up photos of a hyperactive hippoboscid while it's alive is next to impossible.

As often is the case with parasites, many avian hippoboscid flies apparently are host-specific; i.e., different species of birds host different species of hippoboscids. We once netted a Red-shouldered Hawk with a "huge" hippoboscid crawling among its feathers; the parasite was almost half an inch long, while the smaller one pictured here was half that size. We've never encountered a hippoboscid on a hummingbird but have heard from other banders who have; they comment that a hummer hippoboscid is amazingly tiny--certainly to be expected since a large parasite might overwhelm a host hummingbird. Along similar lines, in most cases a single bird of any species seems to carry only a few hippoboscids; "excess" flies apparently disperse, even though there are records of an individual bird plagued by a hundred or more flat flies.

All text & photos © Hilton Pond Center

In top view (above), our flat-bodied hippoboscid from the towhee looks much like any typical member of the Diptera (Fly Order), with large compound eyes, short bristle-like antennae, and two wings held flat against the body when at rest. (The left wing of the fly above is torn, something that must have happened during the handling process; otherwise, the fly probably couldn't have made it from the towhee to our eyebrow.) In hippoboscids, wing veins are heavy but few, and to us the abdomen seems quite short compared to the rest of the body, although it may have shriveled somewhat in the dead specimen. The hippoboscid foot (right) is also nicely adapted to its feather- or fur-clinging lifestyle, with two prominent tarsal claws that provide a sure grip.

After avian hippoboscids mate--which happens only once in many species--the female never lays eggs. Instead a single ovum hatches internally and just one larva at a time develops within the mother's uterus, nurtured by secretions from special glands. The growing larva molts and goes through a total of three instar stages, after which it emerges from the mother fly and usually falls to the ground to form a pupa; when it expupates, the fly's initial task is to find a bird somewhere and take a long, warm drink of blood. Curiously, most hippoboscid species produce only one generation per year, which means flat flies have one of the lowest reproductive rates of any insect group. Again, this may be a mechanism that prevents a closely related hippoboscid family from wiping out its host. Hippoboscids--especially females--are apparently rather long-lived; by inhabiting a warm-blooded bird with lots of insulating feathers, a temperate-zone hippoboscid is not affected by seasonal cold weather that is the bane of many small arthropods.

Based on wing venation and head configuration, the hippoboscid from our Eastern Towhee is apparently a species of Ornithomya, likely the relatively common O. fringillina (formerly O. anchineuria). Interestingly, this fly species has been implicated as a possible vector in the spread of West Nile Virus (WNV). Epidemiologists have reported infestations of O. anchineuria and Icosta americana (another common hippoboscid) on owls and other birds that died and tested positive for WNV. The hippoboscids also tested positive for WNV, but it is not known whether the virus was in the blood that flies had ingested from the host or if the virus had actually infected the flies themselves.

All text & photos © Hilton Pond Center

Testing hippoboscid blood isn't easy, since it is indeed hard to separate the blood meal in its gut from the insect's own blood. In the ventral view of the hippoboscid above, the green matter at the leg bases and joints is blood, or hemolymph. Insect blood contains no red pigment; when you swat a mosquito and it splats red, that's YOUR blood, not the skeeter's. Devoid of hemoglobin, an insect's blood does not carry gases, which instead are transported throughout its body by a branched system of tiny tubes called trachae. Like human blood, insect blood carries nutrients and hormones to the insect's cells and waste products away from cells. The greenish color of hippoboscid blood in the photo above apparently comes from plant pigments eaten by the fly's avian host.

One other interesting aspect of hippoboscid life history deals with "phoresy," a symbiotic relationship in which a non-parasitic organism hitches a ride on another. When a hippoboscid from a bird is examined microscopically, we often find tiny chewing lice (ischnocerans) attached to the fly's abdomen. These wingless lice don't dine on the fly but are merely using this larger winged parasite as a way to get to a bird--which the lice DO bite. Chewing lice typically have narrow bodies that allow them to hide between a bird's feather shafts and avoid being dislodged when the bird takes flight or preens.

Dr. Barry OConnor of the University of Michigan further informs us that "Hippoboscids are also hosts to hyperparasitic mites in the Epidermoptidae. Some of these mites have a bizarre life-cycle modification where immatures and males are parasites on the avian host like other epidermoptids, but adult females are hyperparasites on hippoboscids. [Hyperparasites are, simply stated, parasitic on parasites.] Females are white and rounded (they swell quite a bit on fly hemolymph) and are typically seen near the end of the abdomen. They use highly modified front legs to embed in the fly cuticle."

With this information in mind, we used a 10X macro lens to check the abdomen of our dead hippoboscid fly for mites and found a barely macroscopic yellow object (above left) that Dr. OConnor says definitely is NOT a epidermoptid mite. It might, however, be a syringophilid mite--they're long & yellow and about that size--but live inside feather quills; syringophilid associations with hippoboscids are not well-documented. (On the other hand, the yellow object might just be a piece of tiny fly trash, but 10X magnification is the best we could do without a microscope.)

Like we said, looking at birds through your window is entertaining and informative, but not until you get a bird in the hand and a ectoparasitic fly in your eyebrow does it open up a whole 'nother aspect of avian ecology--from hyperparasites to phoretic chewing lice to hippoboscid stealth flies on Eastern Towhee hosts.

All text & photos © Hilton Pond Center

Thanks to Barry OConnor (Univ. Of Michigan), Peter Adler (Clemson Univ.), Jeff Boettner (Univ. Of Massachusetts), Russ Rogers, and Paul Beuk for sharing hippoboscid info