Broad-headed bugs are fairly large insects, with big individuals approaching an inch in length. The common name for the family comes from the shape of the head, which may not seem conspicuously broad unless you look at other kinds of bugs. In alydidae, the head and the prominent eyes on either side can be nearly as wide as the body of the insect, dramatically wider than the very small heads typical of most bug families.
These bugs have leathery, membranous wings, generally folded over the insect’s back, and long antennae. They are active and agile insects, spending a good portion of their time scrambling actively over vegetation, presumably in search of suitable food.
Broad-headed bugs seem rather unevenly distributed on Martha’s Vineyard, probably reflecting this family’s preference for dry, sandy soils and for certain plants in the legume family that occur on such soils. And it doesn’t appear that many species in this family, which is a small one to start with, occur here (so far, I’ve only found two).
Our alydids both appear, quite suddenly, in early June, going from absent to quite common in the span of just a couple of days. Presumably the emergence of adults coincides with the maturation of the plants these bugs prefer to feed on. Once present, broad-headed bugs persist in gradually declining numbers through the summer.
Like most herbivorous insects, members of alydidae have fairly strong preferences for certain kinds of plants. In the case of this family, it’s the members of the bean family (Fabaceae) that are the favorite. Bush clovers, in the genus Lespedeza, are said to be especially preferred as a food plant by broad-headed bugs, and the presence of a good quantity of Lespedeza seems to be as good an indicator as any of promising habitat for finding these bugs.
Where conditions are favorable, broad-headed bugs can be numerous. Alydus eurinus, for example, an all-black species with chubby rear thighs armed with a row of spines, can be plentiful along fire lanes in Correllus State Forest. Sometimes while I’m walking through the low vegetation along one of the lanes, one or more of these bugs is disturbed into flight with every step I take, suggesting a density of at least one bug per square meter.
Somewhat less common, though also widely distributed on the Vineyard, Megalotomus quinquespinosus is an elegant reddish bug distinguished by a pale band on each antenna. Like alydus, this species has robust thighs armed with spines. The species is sometimes known as the lupine bug; while we don’t have any naturally occurring lupine on the Vineyard, that plant is another member of the legume family, and this bug seems to associate with the same plants that alydus does.
Like all of its reasonably close relatives, a broad-headed bug comes equipped with a long, jointed proboscis. Normally carried folded under the insect’s body, this appendage can be extended for feeding; the bug inserts the proboscis into developing seeds, stems, or leaves, sucking nutritious juices out for food.
While the proboscis looks daunting, a broad-headed bug either can’t or won’t use it in self-defense; I’ve never read or heard of anyone being bitten by one of these bugs. (Assassin bugs, however, not too distantly related and quite similar in appearance, can give you a hearty jab if you bother them. So one wants to know fairly precisely what kind of bug one is dealing with before handling it.)
Rather, broad-headed bugs rely on other tactics for defense. Their vision seems good, and despite their clunky bodies, they’re capable of brisk flight. So detecting and simply fleeing potential predators is their first line of defense. These bugs also bear a superficial resemblance to certain kinds of wasps, which may deter some other would-be predators from attacking them.
And finally, in common with their fairly close relatives the stink bugs, broad-headed bugs possess glands that produce foul-smelling liquid (butyric acid is said to be the main ingredient in the recipe). When really riled up, a broad-headed bug exudes a noxious squirt of this stuff, which is certainly enough to keep me from eating one.
The nymphs, or immature stages, of broad-headed bugs look uncannily like ants, a resemblance that I’ve seen noted but never adequately explained. The resemblance seems too close to my eye to be coincidental, and I imagine that resembling ants must in some way help these insects avoid becoming someone else’s dinner.
Under some conditions, broad-headed bugs can be agricultural pests (mainly, it seems, when species that favor grasses invade rice fields). But while the feeding habits of our alydids surely don’t benefit plants, even at the fairly high densities they can achieve, these bugs don’t seem to harm their food plants. And our two species seem pretty much restricted to their preferred natural habitat.
These bugs, then, can be considered benign neighbors, doing nobody any real harm as they live out their lives on the Vineyard sandplain.