By John Barrat
Ants dominate the earth’s ecosystems and many are voracious predators that use their mandibles and sheer numbers to pin down and tear apart most other insects. Remarkably, certain groups of beetles have adapted to exploit ants by actually living inside their nests. Yet co-existence with the biting mandibles of ants for millennia has had a dramatic impact on the bodies of these beetles. In two recent papers Terry Erwin and his intern, Lauren Amundson, of the Smithsonian’s National Museum of Natural History have named seven new species of ant-nest-living beetles, known to scientists as the Pseudomorphini or false form beetles. Here Dr. Erwin answers a few questions about these unusual creatures.
Q: Why are the beetles you have recently named called ‘false form’ beetles?
Erwin: Because they have a flattened cockroach appearance and don’t look at all like other beetles. While the whole rest of the carabid beetle family are kind of elegant, long legged, and slender with legs and antennae that stick out, this tribe of beetles—the Pseudomorphini—look like cockroaches. Like cockroaches they are swift and agile runners. They also fly and most of the 1700 specimens in our collection are males that have flown into black-light traps at night. But we know very little else about the life history and behavior of these beetles.
Q. Why is that?
Erwin: Because they live with ants and inside ant nests. For example, we are pretty sure one of the beetles just described and named, Guyanemorpha spectabilis, live with ants that nest in trees in French Guiana. Ants in South America are aggressive and they sting and bite and are just nasty critters. To learn more about the behavior and habits of this species, one would have to tear apart an ant nest while suspended from a tree and that is just not something many carabidologists have yet opted to do.
Ant scientists do this all the time with ground-dwelling ants. They dig a hole in the ground and channel down right next to the side of the cavities and tunnels the ants create to observe their activities. This has just not been done yet to study the Pseudomorphini.
Q: How do these beetles survive in the nests of voracious ants?
Erwin: They have developed the ability to tuck everything in. Their bodies have notches on the underside into which they can fold their legs up so they resemble something like a flat turtle withdrawn into its shell. They can also tuck their antenna into a special notch that directs it below the body so their antennae are not exposed where ants can grab them and chew them off. Like most beetles they also have a very tough outer shell. These beetles can run around inside an ant nest and if they are molested by ants they just tuck everything in until the ants go away.
Other beetle species that live with ants have some bizarre structures. Some have what are called trichomes, hairs that come out of a gland and which exude a chemical that the ants like. The liquid sort of mollifies the ants. Ants don’t bother those beetles because they like to lick the liquid that comes out along the tricomes.
Q: Your description of G. spectabilis reveals it is about half-and-inch long. Isn’t that large to be navigating the narrow tunnels of ant nests?
Erwin: No, there are some very BIG ants in the tropics. Paraponera species for example are one-and-a-half times bigger than this beetle and their nest entrances are 2 to 5 inches wide.
In addition to its size G. spectabilis also has a remarkable color pattern on its elytra [wing sheaths]. All other species of this tribe in the Western Hemisphere are dull brown, dark reddish or blackish with little or no color. Who knows what the color pattern is for? These beetles are nocturnal so what does a color pattern of any bug do for nocturnal predators? Perhaps they sit in the day on some kind of tree bark that is variegated and the color pattern serves as camouflage in some way.
Q: These beetles lay their larvae inside the ant nests?
Erwin: Yes. An egg cannot protect itself and the ants would destroy it so these beetles hatch their eggs inside the female. [The term for this is “ovoviviparous.”] The live larvae are laid by the female inside the ant nest and the larvae have very short legs so they are not able to walk. They have a swollen body and a little bitty head and mouth parts and they have these special setae (hairs) all over the forebody that we are guessing exudes some kind of chemical that either tells the ants “leave me alone” or tells the ants “feed me.”
These larvae just don’t crawl around looking for their own food; they have to be fed by the ants and they actually look like ant larvae. The ants feed their own larvae of course and these beetle larvae are hidden among the rest of the ant larvae and they get fed, as well.
One advantage to this arrangement is that not many outside predators go into ant nests to eat things so the beetle larvae are kind of protected from predators by being with the ants.
Q: You’ve just named 7 new species of false form beetles. Are there more out there?
Erwin: Oh yes. This carabid family has 40,000 plus described species and every time we do a revision like this of the Pseudomorphini every single species in that group is a new species. So there could be 80,000 to 100,000 species living out there now most of which are undescribed. One of my jobs as sort of a senior carabidologist at the Smithsonian is to get young people interested in beetle groups that have not been worked on and the Pseudomorphini have pretty much not been touched at all since about 1925.
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