If there is one mental trait that unifies naturalists around the world, it is probably being comfortable with not knowing all the answers. To be sure, I think we’re all driven by the impulse to know everything about everything in the natural world. But given how much biodiversity is out there and how complex the interactions among wild plants and animals can be, the best we can hope for is knowing a little bit about a few things.
Indeed, everything I learn seems to open more questions than it answers; the more I know, the less I understand. If you find that frustrating, then studying nature may not be for you. But if incomplete knowledge strikes you mainly as an opportunity to learn more, you’re likely to find the natural world infinitely captivating.
A good example might be a robber fly that has been much on my mind: Cyrtopogon marginalis, sometimes called the shiny-edged bandit. Some aspects of this insect’s lifestyle are quickly grasped; others I think I’m figuring out; still others bewilder me.
C. marginalis is a distinctive insect, and as robber flies go, it’s rather sluggish, docile, and easy to observe. Its range spans the East Coast from New Jersey to New Hampshire, plus the Great Lakes region, with a few scattered records at higher elevations in the Appalachians. Despite its broad distribution, Cyrtopogon marginalis appears to be common nowhere; it’s another on a long list of specialized insects that sparsely inhabit barren habitats.
Like all robber flies, it is a predator, taking aerial prey. But I think every individual I’ve ever seen has been basking complacently in the sun, usually on loose, sandy soil. With one exception, all of my Vineyard records of this species come from the productive fire lanes of Correllus State Forest.
Robber flies are typically hairy, spiny insects, and the shiny-edged bandit elevates this trait to an extreme. All six legs are densely covered with short spines. A row of spines rings the thorax, which otherwise features a row of four or five inexplicable shiny knobs along each side. The abdomen is hairy, with each abdominal segment edged by a band of white hair (I’ll return to this feature later). And the underside of this fly is covered with long, dense, white hair, which, I suspect, offers thermal insulation against the sun-heated sand the species clearly loves to perch on.
The iNaturalist platform contains 58 observations of this species, and the photos associated with the vast majority of those observations show the insect basking on loose, sometimes gravelly sand. Moreover, on the one or two occasions on which I’ve seen females laying eggs, they’ve been doing it in loose, dry sand, in full sun.
So one thing I do know about C. marginalis is that it really, really likes sand. But the question this raises is, very simply, Why? Bare sand in full sun is a hostile, desiccating environment. Why don’t the adults overheat? Why don’t the eggs dry out and fail? Why does this species persistently perch in such an exposed setting, standing out as a target for the convenience of any bird that would like to eat it? This fly has a lifestyle with a multitude of apparent downsides.
A partial answer comes from examining the photos of the species in iNaturalist. A few of these, including one of my own, show an individual holding a prey item, and in all cases that item is a wasp or bee. So I know, or think I know, that this predator is a bee and wasp specialist, and the dense spines are likely protection against defensive stings.
I built on this idea as a result of a recent, momentary misidentification. Cueing on those white hair bands on the abdomen — a bee-like feature — I recently mistook a perched Cyrtopogon for a bee in the genus Colletes. Finally recognizing the mistake, I poked the fly with a finger, and, as this species usually does when disturbed, it weakly flew a couple of meters and perched, once again, on open sand. Botching the initial ID showed me how much this fly can look like Colletes.
Now, one particular Colletes species, C. validus, overlaps strikingly with Cyrtopogon marginalis in its Vineyard distribution, its period of adult activity, and its fondness for sand (the bees nest underground in sandy soil, and male Colletes, especially, spend much of their time patrolling low over the ground in sandy areas, searching for nesting females but easily targetable by Cyrtopogon). Mimicry of bees is a common evolutionary strategy for robber flies that prey on bees, presumably making it easier to get close to potential victims (the huge genus Laphria, for instance, has the common name “bee-like robber flies”). Finally, C. marginalis has a demonstrated fondness for hymenoptera prey.
So my current hypothesis is that, at least on the Vineyard, Cyrtopogon marginalis is optimized as a predator of Colletes bees, quite specifically. In coming years, I hope to spend more time with this fly, watching it for long enough to see it take prey. It might confirm or deny my hypothesis — while undoubtedly raising new questions to explore.