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While sitting at our desk and occasionally glancing out the window at Hilton Pond, we thought we noticed a movement along the trunk of an Eastern Red Cedar. The tree supports one of our pull-string hummingbird traps, so we were hopeful a bird was about to enter and be captured for banding. We continued to watch but, alas, saw nothing fly into or away from the trap, so we went back to work crunching data into the computer. A few minutes later, we saw something move again. This time we trained our binoculars on the spot and finally managed to pick out something that resembled a tiny robot working its way up the trunk. It was a Wheel Bug, one of the slowest insects we've ever encountered.
The Wheel Bug, Arilus cristatus, does indeed look like a mechanical device (above), from its long legs that extend in jerky, robot-like motions to the large cog-like structure on its back that gives rise to its common name. Wheel Bugs are True Bugs classified in the Hemiptera, an order that includes such diverse insects as Stink Bugs, Water Striders, and Bed Bugs. Although the generic term "bug" is bestowed on all sorts of insects, it is scientifically accurate only when applied to True Bugs. (Lightningbugs, Ladybugs, and June Bugs, for example, are actually beetles.) The most distinctive trait of hemipterans (meaning "half-winged") is two pairs of wings in which the front half of the anterior pair is leathery and the back half is shiny and membranous like that of a fly or bee (right); the hindwings are completely membranous. Most of the True Bugs are capable flyers, although a Wheel Bug in flight always reminds us of one of those slow-moving ultralight airplanes--including the loud buzzing sound made by bug and aircraft.
Another characteristic of True Bugs is their mouthparts, which form a long, narrow beak that extends posteriorly beneath the body. The outer segmented portion of the beak protects two mandibles and two maxillae--all used to pierce the bug's food item--and serves both as a conduit for saliva and liquid food the Wheel Bug draws in. Many True Bugs use their mouthparts to feed on the juices of plants--including some commercially important crops--while others prey upon pestiferous insects that are economically significant. A few such as Bed Bugs bite humans, birds, or wild mammals and suck their blood, and some True Bugs even carry diseases. True Bugs are widely distributed as a group; the Wheel Bug occurs primarily in the eastern and central U.S., but populations occur as far west as California and into Central America.
The Wheel Bug has some of the best-developed mouthparts of any True Bug. Its formidable beak (below left) arises at the anterior end of its long tubular head and unfolds forward. When it encounters a prey item--usually some adult insect or caterpillar--it typically lunges forward in its own slow way, grabs onto the prey with its front legs, and buries its hypodermic beak into some soft body part of the hapless prey. The Wheel Bug then injects enzyme-laden saliva--which immobilizes the prey within 30 seconds and turns its parts into porridge--after which the predatory bug sucks out all the victim's bodily fluids. This activity, of course, kills the prey item, which is why the Wheel Bug is classified in the Reduviidae--the Assassin Bug Family. It's worth noting that Wheel Bugs aren't all that particular about where they stick their beaks--which is fair warning that humans should use appropriate care when handling one. Some folk have allergic reactions to the bite, while others simply say a Wheel Bug nibble hurts ten times more than a hornet sting and takes weeks or months to heal.
Visible in the photo at above left is the Wheel Bug's bulbous compound eye. Curiously, there is an ocellus--simple eye--just posterior to it. These multiple eyes undoubtedly serve the bug well as it patrols looking for a succulent caterpillar to drain. The head of the Wheel Bug also bears two long, jointed antennae that it waves around slowly, testing the air. It may be smelling around for another Wheel Bug--something that isn't all that hard to do because a compound they exude is discernible even by the weak noses of humans. The Wheel Bug's odor--which is produced by orange-red scent sacs that it everts from its anus, especially when disturbed--isn't as potent as that given off by its cousin the Stink Bug, but the aroma is still strong enough to make an impression on a potential predator.
After a compatible pair of Wheel Bugs finds each other in autumn, the female lays a cluster of 40-200 tiny brownish bottle-shaped eggs on a twig and--sometimes after attacking and eating her mate--eventually dies. Each fertile egg hatches the following spring into an eighth-inch-long wingless red nymph (above right) that undergoes five molts and finally metamorphoses into the inch-long adult by summer's end. Wheel Bugs are predatory throughout their lives, with nymphs eating tiny caterpillars and insects and adults sometimes consuming agricultural pests larger than themselves. (HINT: Don't kill Wheel Bugs; they're beneficial insects.)
No one seems to know whether the gear-like structure atop the Wheel Bug's pronotum has a specific function. It may play a role in species recognition--both males and females have 'em--or like the red color of the nymphal Wheel Bug it may alert potential predators that these slow-moving critters taste at least as bad as they smell. Whatever its function, we doubt hapless insects at Hilton Pond Center realize the cogs on a Wheel Bug's back are a sure sign this assassin of the insect world has just arrived to do them in.
All text & photos © Hilton Pond Center
"This Week at Hilton Pond" is written and photographed by Bill Hilton Jr., executive director of Hilton Pond Center for Piedmont Natural History.You may wish to consult our Index of all nature topics covered since February 2000. You can also use the on-line Search Engine at the bottom of this page.
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SPECIES BANDED THIS WEEK:
* = New species for 2003
WEEKLY BANDING TOTAL
YEARLY BANDING TOTAL
BANDING GRAND TOTAL
(since 28 June 1982)
NOTABLE RECAPTURES THIS WEEK
(with original banding date, sex, and current age)
None this week
None this week
OTHER SIGHTINGS OF INTEREST
--Two of the birds banded this week were juvenile Red-shouldered Hawks--both going after songbirds already snared in hummingbird nets just outside the office windows at Hilton Pond Center. These two hawks were the second and third of their species caught in hummer nets within the past month, bringing the yearly total for red-shouldereds to three. In all, we've only banded eight at the Center since 1982. After the third hawk of the year was banded on 5 Sep, it flew to the top of a nearby pine and sat there for several minutes. In the photo we snapped just as it departed the tree (above), the forward-pointing flange of a lock-on band is visible on the bird's left leg.
--Ruby-throated Hummingbirds are still few and far between as fall approaches. As of 7 Sep we had banded only 100 in 2003, which is only 87% of our average for that date. If the pace continues we could have our worst totals for any full field season at Hilton Pond Center since we began banding hummers in 1984.
In 2004, informative and entertaining hummingbird banding presentations are already scheduled for North Carolina, Virginia, Michigan & Kentucky/Tennessee.
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