Every so often around the BiodiversityWorks office, where I hold my real job, somebody finds something cool, and we all flock outdoors to see it. Most recently, my colleagues Rich Couse and Robyn Graygor were setting up for a native plant sale and called me to look at a bizarre-looking, inchlong insect on one of the plants. I was outdoors, camera in hand, in a moment.
Given its long, thin body and even longer, thinner legs, we initially thought the insect was an immature walking stick, a type of insect that as far as I know has never been found on Martha’s Vineyard. (Oddly, both Cape Cod and Nantucket have them.) But that couldn’t be right. While it superficially resembled a walking stick, it had robust, modified forelegs that it held out in front of itself — all wrong for a walking stick.
When I took a look at my photographs, those forelegs proved to be raptorial ones, shaped and spiked like the grasping front legs of a praying mantis. And tucked below the insect’s face was a long, dagger-like beak. What Rich and Robyn had found was clearly an assassin bug of some kind. Given the astonishingly thin legs, I quickly zeroed in on a subfamily called Emesinae, the aptly named thread-legged bugs, and from there to the genus Barce. While not a walking stick, it still appeared to be a first for the Vineyard.
Barce, with its extreme anatomy, is not a typical assassin bug, or member of the family Reduviidae. But as the common name of the family suggests, this is a group of uniformly predatory insects, all with anatomical and behavioral adaptations evolved to make killing easy. These are fascinating insects, and worth a closer look.
I know of 10 species of assassin bugs that occur on the Vineyard, and while this is probably an undercount, it may not fall too far from reality. Reduviidae is a huge taxonomic family, with about 7,000 species worldwide, but most of that diversity is concentrated in the tropics; fewer than 200 assassin bug species are known from the U.S. and Canada.
In common with other bugs, and also groups such as grasshoppers and katydids, assassin bugs exhibit partial metamorphosis. That is, they hatch from the egg already resembling their adult form, progressing by sequential molts through several larger and more developed forms until they reach adulthood. (In contrast, think of the complete metamorphosis of a larval butterfly, or caterpillar, into an adult.) So would-be assassin bug observers should expect to find any given species in multiple sizes and stages of maturation.
One locally common and quite typical member of the family is Zelus luridus, the pale green assassin bug. Despite its common name, this insect appears in various blackish and brownish forms as well as green. But it is always marked by prominent spines on either side of the thorax. Like all members of this family, Z. luridus is equipped with a prominent, pointed beak with which it impales prey, injecting them with paralyzing toxins and digestive enzymes. Its coloration makes it hard to spot, but if you look with care, Zelus can often be found waiting in ambush on leaves or near flowers.
Perhaps even more common are the ambush bugs of the genus Phymata. With compressed bodies and brown-and-yellow coloration, ambush bugs typically lurk well-concealed on flowers (often goldenrod), jabbing bees, beetles, flies, and other insects as they seek pollen and nectar. Species-level identification of Phymata from photographs is perilous, and to my knowledge nobody has made a serious local study of this genus. So it’s not clear how many Phymata species occur here. But collectively, Phymata can be found nearly anywhere on the Island, and the genus must be a significant player in regulating populations of the many species it preys on.
But the Vineyard’s apparently underwhelming diversity of assassin bugs looks even worse when you realize that most of our known species are rarely reported. Of the 10 species represented among observations on the iNaturalist platform, seven are documented just by single observations. I’ve found, for example, just a single Acholla multispinosa, hunting on a jade plant on our front porch; and likewise just one masked hunter (Reduvius personatus), an introduced species attracted to the porch light of our Oak Bluffs home.
It’s hard to say whether this paucity of records reflects actual scarcity, elusiveness, or a lack of interest among observers. Habits don’t seem to vary much within Reduviidae, so the fact that a few species are encountered frequently suggests to me that rarely observed species are truly uncommon. I have no idea whatsoever why this family, aside from a few common types, is so poorly represented in our insect-rich habitats. But low diversity and inexplicably uneven abundance seem to be characteristics of this family on Martha’s Vineyard.
