Everyone knows that evolution can take one basic design for a plant or animal and diversify it into scores, or even hundreds, of specialized life forms. Evident anywhere you look carefully in the natural world, this principle tends to be more evident in predatory groups, which rely on taking a relatively scarce resource, than in herbivorous groups, which often have essentially unlimited food supplies and hence have less incentive to specialize. Perhaps the most vivid illustration of how a group of predators divides up the universe of potential prey items comes from the robber flies, family Asilidae.
As a group, robber flies date back at least 110 million years; there are specimens trapped in amber of that vintage, clearly showing the piercing mouthparts, fuzzy legs, and robust thorax that characterize this family. That’s ample time for diversity to evolve; the family Asilidae now numbers somewhere around 7,500 known species.
Only a small portion of that diversity occurs on the Vineyard. Working from photographs taken in the field, I’ve been able to identify about 15 species of robber flies here. I’m sure I’ve missed as many as I’ve found, but still, this is not a hugely diverse family on the Island.
Even among the few species I know something about, our robber flies show an amazing degree of specialization. In one way or another, most of these insects focus their attention on a particular subset of the available prey. This accomplishes two things. First, it reduces the amount of competition among robber fly species. And second, it means each species has been able to optimize its habitat preference, its hunting habits, and probably even the chemical composition of its venom for subduing a particular kind of victim.
Asilus sericeus, for example, a large, reddish-brown robber fly that is fairly common on the Vineyard, seems to prey exclusively on butterflies. This robber’s behavior is shaped by its prey preference: the way to find Asilus during its midsummer flight period is to poke around milkweed patches. Milkweed blossoms are irresistible to butterflies, and by perching on a stem or leaf near a cluster of milkweed flowers, Asilus has access to a steady supply of victims.
Other robber flies specialize in taking bees and wasps. The fact that these stinging insects have well-developed self-defense capabilities may reduce the amount of competition these robbers face — who wants to tangle with a wasp? But the two members I’ve found of the genus Proctacanthus appear to prey exclusively on bees and wasps (taking stinging and nonstinging types with equal enthusiasm). The genus Laphria (I’ve found two species in this genus as well) specializes even more tightly, preferring bees in particular. Our Laphria species have even evolved beelike, black-and-yellow color schemes that closely resemble the coloration of bumblebees, an adaptation that must surely make it easier for these predators to bushwhack their favorite prey.
Perhaps the most bizarre example of specialized targeting comes from the genus Tipulogaster, a group that is oddly barren of leg bristles by robber fly standards. I’ve only seen this genus a couple of times, but in both instances, the robber fly was enmeshed in a spider web. When the spider responded to what it surely thought was its own dinner, Tipulogaster quickly turned the tables, capturing and eating the spider. The robbers then proved able to escape from the web; perhaps the dearth of leg bristles is evolution’s way of making this genus better able to disentangle itself from spider silk. I don’t know if this is the sole strategy Tipulogaster uses to hunt, but offering itself to a would-be predator appears to be part of its repertoire.
One form of specialization that may not be immediately obvious to humans is exploitation of a particular vertical stratum. Most robbers seem to prefer waiting in ambush on vegetation a few feet off the ground, a strategy that presumably ensures a fairly steady supply of potential victims flying past. But one robber fly, Cyrtopogon marginata, evidently forages from ground level. Legs that are bristly even by robber fly standards may insulate this species from excessively hot, sun-baked sand. And all the attacks by this species that I’ve witnessed involved short, hopping flights to capture a smaller fly in the air, but within a few inches of the ground. Focusing on ground-perching or low-flying prey, Cyrtopogon has evolved to sample a totally different suite of victims than other robbers.
The universe of potential robber fly prey is further subdivided by size (small robbers like the very common Dioctria hyalinipennis specialize, reasonably enough, in small, soft-bodied prey like leafhoppers), seasonality (Ommatidia tibialis matures weeks before any other robbers are on the wing), and habitat (Efferia albibarbis appears to occur exclusively on upper beaches and dunes, where other robbers are scarce). Whatever the mechanism, each of our robber flies has carved out a subset of prey it can exploit efficiently and with only limited competition from related species.