Susan's Snazzy Arthropod of the Day: Mastotermes darwiniensis

I know some of my dear readers are less than fond of our termite friends, but as I'm pretty sure that my readership is still restricted to acquaintances in the U.S., you don't have to worry about it eating your house. It’s Mastotermes darwiniensis of Australia, the only extant member of the termite family Mastotermitidae. Apparently the common name is the Giant Northern Termite or the Darwin Termite. All the termite biologists I know, however, just call it by its genus name since that is pretty darn specific, it being such a phylogenetic loner.

The thing that makes Mastotermes so cool is its evolutionary significance. Upon its description a hundred or so years ago, it was the clue that made biologists pretty sure that termites are very closely related to cockroaches. This suspicion was confirmed last year with the first comprehensive phylogeny of roaches and termites using molecular data: In the roach family tree, termites are just one branching limb nestled in the tree, and Mastotermes is the branch closest to the roach trunk.
Below are pictures of a generic termite, Mastotermes side-by-side, with a generic cockroach right underneath.


You, a layperson, can easily see some of the striking similarities between Mastotermes and the cockroach. On the extended wings, do you see how Mastotermes and the cockroach both have that extra lobe coming out of the back end of the hindwing? That’s called the anal lobe. It’s found in all cockroaches, and no termites except for Mastotermes. The nerve patterns between Mastotermes and the cockroach are similar as well. Additionally, you can see the differences in the shield behind the head (called the pronotum, which covers the prothorax): on the generic termite it’s pretty small, but much larger in Mastotermes, as large as its head—much closer in size to the cockroach’s greatly expanded pronotum. You may also be able to infer from these drawings that Mastotermes is also quite a bit larger than other termites, closer in size to a roach. Other morphological similarities between Mastotermes and the cockroach that you’re not able to see in these drawings are the 5-segmented tarsi (feet) with pulvilli (adhesive pads), the ovipositor (egg-laying tube) in the females, the oothecae (egg masses), the row of spines along the tibiae (a leg segment), and possession of the same kind of gut microbiota. None of these features are shared by the other termites.

So it seems that both the morphological and molecular data point to Mastotermes representing a “transitional form” in termite evolution from a roach-like ancestor: i.e. it branched off early in termite evolution and still retains many of the ancestral cockroach characteristics. (Note: creationists commonly claim that no transitional forms exist, thus evolution is wrong—despite the overwhelming evidence. Next time you have to talk to one, remember Mastotermes.) Of course Mastotermes is a modern animal, and can’t be confused with the actual transitional form between roaches and termites that lived in the early Cretaceous or before. But since it seems to have kept many of those ancestral features, it’s a pretty good proxy in many respects.

As you probably know, termites are highly social creatures—they live in colonies of extended families where most individuals are altruistic and only a couple individuals reproduce. (Note that “social” in the context of evolutionary biology has a much more specialized meaning than it does in conventional speech.) Though some roaches live in simple family groups, none are truly social like the termites. So if we were interested in learning about the evolutionary steps in between family living in the roaches and true social (“eusocial”) behavior in the termites, it seems like Mastotermes would be the perfect place to look, because of its half-roach, half-termite appearance.

Wrong.

The odd thing about Mastotermes is that while it is morphologically primitive, and has not changed its physical appearance much in many millions of years, its behavior and social structure are highly complex, and as derived as the termites that have evolved most recently of all. Mastotermes builds huge underground nest structures that contain extensive gallery construction and tunnel excavation; it forages far afield from the nest, and has been known to damage structures over a hundred yards away from its colony. Full-grown colonies contain over a million individuals, with rigid caste structures and obligatory sterility for the worker forms. This is a lot like the most-derived, most-recently evolved termites, like the great mound-builders of Africa. In contrast, the most termite-like cockroach and the next-most-primitive termites after Mastotermes all live and eat inside one piece of rotting wood, have very flexible development, do not have obligatory sterility in the worker forms, build no galleries and no tunnels, and are have many fewer group members.

Mastotermes is thus a weird chimera of primitive morphology but derived behavior and development. If it were translated into, say, the primates, it would be a lemur with a big brain, language capabilities, and maybe a car. I think it’s a great example of the complexity of evolution, and shows how even within a single species vastly different evolutionary paths can be taken in different areas of one genome.