LONG-HORNED BEETLES

Beetles are the most diverse animals in the world, with surely over a million species, although only about 350,000 have been described to date. Any collection of tropical insects contains undescribed species, and beetles often make up a good part of this, even though they have been favorites with collectors ever since specimens were first collected for museums.

One explanation for the great diversity of beetles is their long association with angiosperm plants. It appears that a taxonomic group that is associated with these very diverse vascular plants has an unlimited array of niches available for speciation, furnished by both the many different species and their many parts—leaves, stems, roots, flowers, fruits—on which to specialize.

Long-horned beetles, family Cerambycidae, are strongly associated with these plants and one of the most diverse beetle families, with about 35,000 species known to date (1,200 or so in North America). Almost all of them have larvae that bore in plant stems. They are distinguished by—guess what—their long antennae as well as C-shaped eyes that curve around the antennal bases. They range in length from a few millimeters to over 17 centimeters and because of their size, beauty, and those looong antennae, are among the most charismatic and well-liked of beetles.

The family is common in the Pacific Northwest, and some of the species are striking. A large black and white one, the cottonwood borer Plectrodera scalator, breeds in cottonwood trees, so look for it along rivers with associated riparian forest. Originally distributed east of the Rockies, the species has become established in the Pacific Northwest.

The eggs of Plectrodera are laid in the fall, and the larvae bore into the bases of cottonwoods and willows. They pupate beneath the bark and emerge as adults in 2-3 years. The larvae may girdle and kill trees and are considered a pest in some areas.

The larvae of Monochamus scutellatus, the white-spotted sawyer, bore into pines and spruces, and the adults are attracted to trees that have been burned in forest fires. Apparently the burned trees release chemicals that attract the beetles from long distances. The species is considered an economic pest because the boring larvae make the wood, which might otherwise be harvested, unsightly, as well as allowing access to fungi. Wood-boring beetles cause the timber industry to lose millions of dollars annually.

The elderberry borer Desmocerus auripennis lays its eggs in elderberry bushes. The larvae bore in the stems just like their larger relatives do in trees. The locust borer Megacyllene robiniae is also native to eastern North America but has spread into the West with the planting of black locust trees. This and the cottonwood borer are spectacular species, and it’s a bit disappointing to find out that they came from somewhere else!

Check patches of milkweed for milkweed beetles, Tetraopes femoratus. A related eastern species is shown here. The larvae of these beetles live in milkweed stems, and both larvae and adults are distasteful because of the chemicals ingested with the milkweed tissue. The bright red coloration is surely an aposematic advertisement of this.

A very large number of species of cerambycids are flower visitors, eating the pollen and probably in some cases effecting pollination even as they destroy the reproductive efforts of the plant. Lepturobosca chrysocoma, Pseudogaurotina cressoni, and Xestoleptura behrensii are examples of this group. Many are brightly colored and tapered behind, perhaps giving them some resemblance to bees or wasps, especially in flight. By that mimicry, they fool some avian predators that normally leave stinging hymenopterans alone.

Dennis Paulson

KINGS OF THE INSECT JUNGLE

Many insects are predators on other insects. Dragonflies and damselflies (order Odonata) come to mind immediately, as all of them eat smaller insects and spiders. But put them up against robber flies (order Diptera, family Asilidae), and they have not only met their match but been bested with ease.

Robber flies eat dragonflies and damselflies regularly, but there are almost no records of odonates turning the tables. One type of predator is clearly superior to the other. I have seen robber flies take insects from most orders, including their own. Size is no limit, as an inch-long robber fly can latch onto a flying dragonfly three times its size and bring it down to the ground instantly with a paralyzing bite. Presumably if the fly was captured, it could do the same thing to its captor.

The two wings of a robber fly are narrow but strong, and they propel their owner through the air with an audible—sometimes impressively loud—buzz. Their flights are usually short, and when you hear that buzz you can often find its source resting on a branch, rock or the ground. They usually perch right out in the open, again like a dragonfly, where they can see potential prey. They have relatively large, forward-pointing eyes as befits a predator.

The thick, tubular proboscis injects venom that is both proteolytic and neurotoxic. The neurotoxin paralyzes the prey almost immediately, and the proteolytic enzymes digest the innards into a liquid soup that the fly sucks out. The proboscis is strong and sharp enough to penetrate the hard cuticle of a beetle.

Many insects are poisonous and distasteful and brightly colored to advertise their unpalatibility. This adaptation must be against birds, because robber flies freely feed on such insects, as do dragonflies.

Robber flies are very bristly. The legs have long, sharp spines to hold onto the prey, much as in dragonflies. The face has a dense coat of bristles, called the mystax, presumably to protect it from the legs and mandibles of struggling prey (but it’s a sure thing that they don’t struggle for long).

Many robber flies are sleek and pointed at the rear, the jet fighters of the insect world. Others are fat and fuzzy, very effective mimics of bumble bees but just as effective as predators. They have been called aggressive mimics, mimicking their favored prey species to get close enough to make a kill.

Fly larvae are legless and look like maggots, and robber flies are no exception. Slim and pointed at both ends, at least some of them feed on the larvae of other insects, usually in rotting organic material such as logs and dead trees or in the soil. Surprisingly little is known about the larval life of this group, however.

With over 7000 species in the world, robber flies are diverse on all the continents. They are relatively uncommon in the wet western lowlands of the Pacific Northwest, just as many groups of insects are less common in our cool, cloudy summer climate. Head across the Cascades to see a lot more of them in the dry, open areas that they prefer.

Robber flies are easily observable, as they are fairly tame, but capturing one in an insect net and looking at it closely allows you to appreciate its adaptations even more. Be cautious, however, as their bite can be painful. I know enough about their adaptations that I have never allowed one to bite me!

Dennis Paulson