Wheel Bug, Arilus cristatus

Wednesday, November 11, 2009

Nature Notes
by Bob Thomas

One of the most easily recognized insects in our region is the wheel bug, Arilus cristatus. Adults are a little over an inch long, grey to brown, and have a gear-shaped half wheel on their thorax. I’ve always called them Rotary bugs since the structure is reminiscent of the Rotary International emblem.

As in all members of the true bugs, the Suborder Heteroptera of the Order Hemiptera, and especially the assassin bug Family Reduviidae, wheel bugs have a rigid, three-segmented piercing proboscis that easily dispatches their prey. They inject enzymes that immobilize their prey within 30 seconds and the fluid literally digests the prey’s internal organs, then they suck the nourishing liquids out using their proboscis as a straw. They are known to provide humans with a very painful bite, so they should be handled carefully if at all.

Wheel bugs are slow walkers and slow fliers. They have wing covers and wings characteristic of the Heteroptera: forewings that are thick and protective to the front and somewhat membranous toward the rear (called hemelytra, as opposed to the completely hardened forewings/covers [elytra] of beetles). The second pair of wings (hindwings) are entirely membranous and propel the insect during flight.

They are voracious predators of small prey, especially caterpillars, beetles, other bugs, wasps, and the like. They are known to feed on leaf miners (i.e., larvae of certain species that burrow in leaves) by piercing the animal in its burrow inside the leaf. Nymphs love aphids and other small insects. Adults and nymphs tend to lurk on any vegetation that is rich in their prey. Although they are diurnal, they are often attracted to lights in the evenings, undoubtedly in search of other insects that are also attracted by the lights. Since many of their prey species are considered pests by humans, wheel bugs are considered beneficial, but they may also eat highly valued honey bees and ladybugs.

When approached by a potential predator, wheel bugs may rely on crypsis (protective coloration), bite the intruder, or evert small red scent sacs located near the anus that emit a disconcerting odor. If all else fails, they may fly away, if they are capable of moving fast enough.

Wheel bugs lay their eggs in the fall. The eggs resemble a cluster of dark bottles with light caps and number 42-182. The eggs over-winter and hatch into tiny red nymphs in early spring. These molt five times before becoming adults. They are red with black legs and have no wheel on the back. Summer months are spent feeding and enjoying a wheel bug’s joie d’ vivre, before mating, laying eggs, and dying.

Wheel bugs, like many reduviids, produce a clicking sound by rubbing the tip of the beak over tiny ridges on the lower surface of the anterior portion of the thorax (the prosternum). The sound’s purpose is probably communication among individual wheel bugs.

From the “odd” department, there have been few publications about the natural history of wheel bugs since the 1940s. Naturalists, take note. These are cool bugs and worthy of further observation.

There are no firm opinions on the purpose of the gear-like structure on the wheel bug’s thorax. It may serve them in species recognition, may help potential predators recognize them as dangerous, or the teeth on the gears may make them less palatable or more uncomfortable to eat. Since they have a ravenous appetite for agriculturally damaging insects, maybe the wheels are indeed a Rotary symbol of “service above self.”

                                             

Wheel bug, Arilus cristatus, at the Louisiana                                       Note the gear-shaped crest, the long,
Nature Center. This is a short-lived voracious                                      tubular head, and the large piercing
predator of insects. Adults are most commonly                                   proboscis that is folded under the head
found from mid- to late summer.                                                       when not in use.
Photo by Bob Thomas.                                                                    Photo by Bob Thomas.