Assassin bug is ready for war with armor on its back

By Jeff Mitton

Forty miles east of Trinidad the land rises steeply to Mesa de Maya, 15 miles long and five miles wide, just four miles north of the New Mexico border.

The mesa stands 700 feet above the plain to the west, and this relief coaxes sufficient rain from the sky to support a dense piñon-juniper woodland. From the top of the mesa the Spanish Peaks and Sangre de Cristo are visible 70 miles to the west, and 25 miles to the northwest streams flow into Purgatoire Canyon, 1,000 feet lower.

Much of the mesa is covered by a ranch run by three generations of the Louden family. Willard Louden, patriarch of the ranch and renowned western landscape painter, hosted me for several days at his ranch. He considers himself to be a steward of the land -- he runs some cows on the ranch, but he also manages the land to sustain pronghorn, mule deer, elk, coyotes, turkeys, eagles and hawks.



On the first morning, Willard brought me to a large pool fed by an ephemeral stream and cold springs a few miles west of the mesa. The road to the pool wound around several shallow depressions devoid of plants. These were buffalo wallows, remnants of the times and places that buffaloes rolled on their backs to crush biting flies and pack protective mud into their coats. Today, the spring pool is a verdant oasis embedded in an arid landscape.

I was scrambling around the limestone rim 30 feet above the pool when a large insect moved in a nearby bush, catching my attention. I walked around the bush to get a better look at the bug, but it maneuvered stealthily, constantly moving to the other side of the bush. I was as determined as the bug, so we continued our slow dance around the bush for many minutes. When I finally wore it down and got a good look at it, I realized it was a wheel bug, the first I had ever seen in the field. They are well camouflaged and very shy.

The wheel bug, Arilus cristatus, is the largest of the assassin bugs, reaching 1.5 inches in length. Its armament is the most arresting feature of this distinctive bug—it appears to have a bicycle sprocket or a cog protruding from its back. The head, thorax and sprocket are black, the wings are light and dark brown and the abdomen is striped in light and dark grey. The head is long and narrow with bulging brown eyes, long red antennae, and long mouthparts forming a hollow tube that serves as dagger, hypodermic needle and straw.

Wheel bugs are predators of insects. They pin their prey down with their forelegs, impale it with their beak and inject it with enzymes that anesthetize it and dissolve its tissues. When the tissues are the consistency of soup, the wheel bug sucks all the fluids and discards the empty shell.

The sprocket is defensive armor to ward off predators. Some assassin bugs also have elaborate armor, but none as distinctive as the sprocket on the wheel bug.

I saw two more wheel bugs later the same day, up on the mesa. They have membranous wings that flail noisily in slow flight.

Coloradans love their mountains and the Colorado Plateau to the west, but generally eschew the eastern plains. I found Mesa de Maya to be a wild place managed by a wise rancher, an enchanting, wooded platform commanding sweeping vistas in all directions.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at theUniversity of Colorado. This column, Mitton's 200th, originally appeared in the Boulder Camera.

November 2010