Wild Side: Bug hunting

Searching Sengie for long-legged flies.

This highly specialized long-legged fly (family Dolichopodidae) is common on algae mats in Vineyard saltmarshes. — Matt Pelikan

On a recent canoe outing up into channels in a salt marsh adjacent to Sengekontacket Pond, I was delighted to find a batch of small flies, perhaps three-eighths of an inch long, dashing madly about on mats of algae. This is the kind of thing that gets me curious: Clearly these were specialized insects, adapted to the salty conditions and varying water levels of the marsh.

I managed some poor photos with my phone as my wife and I ate a picnic lunch in the boat, bobbing gently in a channel. At the time, I badly misidentified the flies, assigning them to the wrong family. But on reviewing the photos later, and sending them out to some friends for help, I corrected the error. These were long-legged flies, members of the family Dolichopodidae.

About two years ago, I wrote about some tiny, elegant members of this family (bit.ly/LLFlies). The most common and accessible ones are abundant denizens of dry, leafy sites, including yards and gardens. The flies in the marsh were somewhat similar in appearance, but markedly different in behavior and habitat preference. I needed to learn more.

And so last Sunday, I sealed my camera in a plastic bag, launched my kayak at Pecoy Point, and paddled along Sengie’s inland shoreline to where we had found the flies. It was a challenging commute: A stiff northeast wind piled up waves in the mouth of Majors Cove and then along the Felix Neck shoreline. I was safe enough, paddling close to shore in water only a few feet deep, but I had to take the waves nearly broadside, shipping water and corkscrewing along like an inebriated porpoise.

I had planned this expedition near low tide, figuring that since the flies liked mud and mats of algae, low tide would produce lots of habitat. This was a bad idea. Low water simply exposed the vertical sides of the channel without adding much habitat, and it made it harder for even my shallow-draft boat to penetrate the marsh or get close to the flies. So I waited, as the tide rose and greenhead flies carved up my brisket.

Even with more water in the channel, photographing the flies was difficult. They were in nearly constant motion, and very wary. Wading to approach within the roughly 12-inch working distance of my close-up rig seemed unwise; the mud was full of sharp mussel shells, and the muck was so deep and glutinous that I seriously feared sinking entirely, to emerge 5,000 years from now as a mysterious but well-preserved Bog Man in somebody’s archaeology project. Shooting from the boat was not much better, requiring contortion and ingenuity to keep the boat, the camera, and myself all upright and in close proximity while I lined up shots. And the feeble display on my primitive camera is almost useless in bright sun: I was shooting blind.

But the good thing about digital photography is that you can keep blasting away, confident that you’ll eventually end up with a few good shots, if only by accident. And the flies were plentiful. I drained one camera battery and started on another, finally quitting with nearly 300 photos on the memory card. The results were dreadful: frame after frame of blurry insects, or of blurry algae that insects had just flown away from. But in the mix were a half-dozen or so acceptable shots. Success!

In short, it was fun.

The flies worked out to be members of the subfamily Hydrophorinae, an odd and largely aquatic branch of Dolichopodidae. Identifying them to species based on my photos is out of the question, but I think they’re in the genus Hydrophorus, a sizable group with anywhere from 50 to 120 members worldwide, depending on whose taxonomy you follow. Clearly this group has not been adequately studied!

The legs on these beasts are even more exaggerated than those on terrestrial long-legged flies. In a striking instance of what biologists call “convergent evolution,” the marsh flies closely resemble water striders, aquatic bugs that have evolved to skim across the surface tension of the water. The flies zipped across water as easily as they darted across mats of algae. As is the case throughout the family Dolichopodidae, these flies were predators: I couldn’t identify what tiny arthropods they were preying on, but from time to time I’d notice that one was munching on something it had caught.

After two visits, I know just the most basic outline of how these remarkable flies survive in their harsh environment. Their larvae, research suggests, are probably predatory as well, likely living in the soil of the marsh and dining on — what? I surely don’t know. But somehow these remarkable insects have evolved methods to survive saline conditions, baking summer heat, a high density and diversity of predators, and periodic inundation. I’m happy to have made their acquaintance.