Mysterious Assassin at the Natural History Museum

The assassin moved slowly through the small lobby.  Reaching the sunlight that streamed through a set of locked glass doors, she (he?) paused.  For the moment, she was alone in this area open only to staff and volunteers of the Smithsonian’s National Museum of Natural History.  Later, we would wonder how she made it into this inner sanctum.  Did someone unknowingly give her access?  Certainly, not knowingly, because she was a dedicated and skilled assassin, sporting a fear-inducing piece of armor and able to dispatch her victims with the stab of her long, curved weapon.

As soon as I opened the stairway door to the lobby, I knew something was amiss.  It took me a moment to register what was definitely out of place here.  Ignorant of her lethal nature, I innocently approached her.  She went still at my approach and, when I dropped a tissue next to her, she crawled onto it.  Her size alone gave me pause.  Gingerly holding the tissue at one corner and then another as she moved inexorably up toward my hand, I hurried through a corridor, past museum offices, then into the public lobby, and finally out into the open air and the noises of Constitution Avenue.  I deposited the tissue in the grass behind a bush and the assassin slowly, very slowly walked off.

This is the large Arilus cristatus, a member of the group of bugs known as Assassin Bugs.  She bears the popular name of Wheel Bug in recognition of the extension of her thorax, a striking cogwheel structure.  This crest (cristatus is Latin for “crest”) is unique among U.S. insect species, but its function remains largely a mystery.  As zoologist Bob Thomas observes

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, Loyola Center for Environmental Communication, Loyola University of New Orleans, November 11, 2009.)

Her prey are a mixture of insect heroes and villains, among them caterpillars, Japanese Beetle larvae, wasps, lady bugs, and honey bees.  In the nymph stage, she feasts on aphids.

Her modus operandi is quite dramatic.  With that long, curved, red proboscis, she injects her victims with a paralyzing mixture of enzymes that quickly dissolves the internal organs which are then sucked out.

Much of what I’ve read emphasizes how slowly this insect moves, whether walking or flying.  Coming in at between some 1 and 1 1/4th inches long, it’s probably just as well for the rest of us that the adults are not in a hurry.

Even more disconcerting, particularly given my maneuver through the museum’s corridors with the Wheel Bug-bearing tissue, is that, when disturbed, she is known to use her sharp proboscis on humans, delivering a deeply painful and enduring bite.  As described in the online Wheel Bug profile prepared by the Entomology and Nematology Department of the University of Florida:

This bite has been described variously as worse than stings from bees, wasps, or hornets. Barber (1919) and Hall (1924) described in detail the effects of such bites. In general, initial pain often is followed by numbness for several days. The afflicted area often becomes reddened and hot to the touch, but later may become white and hardened at the puncture area. Occasionally, a hard core may slough off, leaving a small hole at the puncture site. Healing time varies but usually takes two weeks.

(The two citations in the preceding description of the effect of a Wheel Bug’s bite are: G.W. Barber, On the bite of Arilus cristatus, Journal of Economic Entomology, Volume 12, 1919; and M.C. Hall, Lesions due to the bite of the wheel-bug, Arilus cristatus (Hemiptera; Reduviidae), Journal of the Washington Academy of Science, Volume 14, 1924.)

Wheel Bugs have a single generation during the year, largely spending the spring as nymphs, the summer as adults, and the winter as eggs (which, incidentally, look much like a compact collection of bullet cartridges).

Now, I have to admit that, not knowing what insect this was when I encountered it on the sunlit floor, my instinctive action of removing her to the great outdoors was perhaps somewhat misguided.  Afterwards, before heading to the literature, I did wonder if she could have been some rare, tropical insect escaped from the museum’s Entomology Department and if what I’d done was such a good thing.  Was this an invasive that would soon disturb the ecology of the Washington, D.C. area?  Luckily, no.  I assume she came in from the grassy area outside the glass doors; though, how this slow-moving assassin accomplished that trick remains a mystery.