I was walking my dog around our local pond when I noticed two people, a man and a woman, peering down at something on the path. I stopped well behind them to give them space. At the same time, I tried to see what it was that they were observing. At first, I could not see anything. Then, suddenly, I noticed a black dot moving on the path. Now I knew what had captivated their attention.
They were looking at a Tenebrionid darkling beetle (Eleodes longicollis, family Tenebrionidae). This beetle is black, completely flightless, and about 26 mm long (a little longer than one inch). As they would get close to the beetle, it would do a headstand, lifting its butt up into the air and standing very still. After a while, sensing no movement, the beetle would lower its butt down and start to move along the path, Fascinated, the people would move closer, casting their shadows on the beetle, which would stop and raise its butt up again. Clearly, the people were engrossed in watching this behavior.
I thought of coming up to them and telling them a few facts about these beetles.
I wanted to tell them that these beetles have defensive secretions, made up of quinones and hydrocarbons, that they squirt out of glands near their anus at predators who try to eat them. This is a great defense against predatory birds and small mammals. However, it doesn’t work against the carnivorous Grasshopper Mouse (Onychomys leucogaster). Experienced mice come up to a beetle that is doing a headstand defensive display, grab the beetle with their front paws, turn the beetle around, and stick the beetle’s butt into the sand. With the beetle’s defenses immobilized, the mouse chews off the beetle’s head, and then proceeds to eat at leisure down to the level of the defensive glands. Some mice go on killing sprees. Sometimes you can find a row of 10 or more beetles stuck with their butts into the sand, positioned like telephone poles, all with their heads chewed off but otherwise uneaten.
The defense doesn’t work against Striped Skunks (Mephitis mephitis) either. When a skunk comes across a beetle doing its headstand display, the skunk first sniffs the butt end of the beetle. If the skunk detects the odor of defensive secretions, he then starts to roll the beetle around in the sand with his front paws. Meanwhile, the beetle is releasing its defensive secretions. Periodically, the skunk will stop the rolling and take a small taste of the beetle. If there are still defensive secretions present, the skunk will shake his head vigorously several times and then continue rolling. Once the beetle has exhausted its supply of secretions, the skunk proceeds to eat it.
However, the beetle’s secretions work well against Harvester Ants (Pogonomyrmex rugosus). The ants have a disk-shaped area around their nest opening, onto which foraging workers drop seeds and bits of vegetation that they pick up anywhere within about 100 meters (300 feet) of the colony. Other workers emerge from the nest, pick up these seeds and droppings, and take them down inside. The Tenebrionid beetles come into these disk-shaped areas and feed on the seeds and vegetation that the ants have brought. If the ants try to sting or otherwise attack them, the beetles spray minute quantities of defensive secretions, with great accuracy, at the approaching ants, causing them to immediately stop, curl up into a C-shape, and start cleaning themselves. On days when the ants are particularly aggressive, you can see a beetle leisurely feeding while perhaps 10 or more ants are frantically cleaning themselves nearby.
I wanted to tell these people all of that. But I didn’t. Instead, I waited until the beetle left the path completely and was lost in the surrounding vegetation, and then I walked past them, merely saying Hello.
Sometimes, there is a place for lots of information. Other times, there is a place for simply watching and appreciating what nature has to offer.
Con,
Aren't there also some Carabid beetles who mimic Tenebrionids by standing on their heads, or all all the "head standers" Tenebrionids? Also there are some local Tenebrionids with grooved elytrae and some with smooth, shiny elytrae?
Posted by: Doug Von Gausig | June 13, 2007 at 08:51 AM