Trickery and Sex in the Firefly World

This is not news in the scientific world, but it’s news to me—I ran across this in some reading I have to do for a seminar tomorrow. Further evidence that bugs are awesome:

Fireflies, lightning bugs, or Lampyrids (if you were the nerdy type). Who didn’t spend innumerable summer evenings in childhood chasing after these beetles, empty pickle jar in hand? But you might not have known of the high drama and Shakespearian trickery that was going on among these insects right there in your own childhood backyard.

There’s multiple species and genera of Lampyrids in North America, each with its own characteristic flashing pattern. Males and females of the same species locate each other for mating using these unique signals. However, females of the genus Photuris have evolved a deadly trap: they lure males of other genera by imitating their own females’ flashes. When the males come a-courtin’, the Photuris females pounce, and devour them with gusto.

You may wonder how the Photuris females ever mate, if they’re always on the prowl for dinner during mating time. Turns out that male Photuris capitalize on their female’s predilection for firefly meat by imitating the flashing patterns of males of other genera—the very insects that the female is trying to lure to their death! Some Photuris males go even farther in their imitation, by flashing not only the right pattern, but also at the right time of night, in the right location, to be even more convincing as the potential prey.

In some species of Photuris this imitative behavior has gone so far that the species has completely lost the ability to produce its own unique flashing pattern, instead conducting all its business using signals stolen from others.

I’m unaware of any papers that detail what happens next, after the flash exchange—how does the male Photuris end up as the mate, and not the dinner of the female? I’ll do some more searching because this has piqued my interest, and let you know if I find an answer.

The Black Widow Spins Her Deadly Web... Well, Sometimes.

I always get disproportionately excited when I read about a new discovery showing behavioral complexity in invertebrates. Of course, now that we understand the incredible intricacy of honeybees' language, these sorts of things shouldn’t surprise me too much. But I still love this stuff, wherever it pops up.

Black widows alter their web architecture to be better insect traps as they get hungrier. A neat little paper in this month’s issue of Animal Behavior [76(3):823-829] describes how this happens.

When a black widow spider is well-fed, it uses that energy with gusto—spinning out lots of thread, but making a chaotic, disordered cloud of a web near the opening to its hideout. This kind of web is called a ‘tangle-based’ web, and for all that thread, isn’t really that sticky.

However, take a spider who hasn’t been fed for a week, and watch her spin a web. That jumble becomes a well-engineered, efficient killing machine. Instead of making a jumble of undifferentiated, generic thread, the spider uses three different kinds of silk structures to spin a very specific trap (using less thread overall):
(This image is taken from the original paper)



SH is the silk sheet, a flat plane on which the spider can easily maneuver, supported by a network of threads (ST). Anchoring the web to the substrate are the very sticky gumfooted threads (GF), which are kept under constant tension.

The researchers set up homes for 112 juvenile black widow females, fed half of them every day for a week, and the other half nothing for a week. After the members of each group spun their characteristic webs, the webs were imaged to quantify the structural differences between them. Then the researchers saw what happened when half of each group was then moved to the other group’s webs, and then fed.

Under most metrics used, the researchers found that it was indeed much easier for black widows of both types to catch prey when the spider was using a hungry spider’s three-thread design, rather than the chaotic mass of thread of a fed spider. It seemed that the silk sheet made it easier for the spider to sprint out towards its prey. The prey may have alerted the spider to its presence by touching the gumfooted threads and sending out vibrations, and perhaps was slowed down by those threads’ especial stickiness.

Why would black widows switch between a poor web and a good web, though? Wouldn’t it be better to just maintain the efficient functionality of the three-thread design whether the spider was hungry or satiated? This is especially head-scratching when you consider that it uses up more silk to make the jumbled webs of the full spiders. The spider would be using up energy both to modify the web as well as to make the loads of silk needed to the tangle-based web.

Consider this though: the authors point out that many spiders are prone to killing more prey than they can eat, and just leaving the extra prey rolled up in silk on their webs. It’s also been documented that some spiders may just eat themselves to death if able to catch too many prey. Perhaps the switch to the poorly-trapping tangle-based web is a smart move by the black widows, at the very least saving them the energy of pursuing and killing more prey than they can use, at the most sparing them a fate like Monty Python’s Mr. Creosote. The authors also suggest that the tangle of web of a fed spider might serve as a predator defense in those times in which it doesn’t need the web to serve as a food trap.

I think the behavioral plasticity that can be built into such a small animal is really fascinating, and the adaptability of the webs of these spiders is just another example of that. So three cheers for the black widow (one for each part of her web)!

And one more that we aren’t one of the flies who get caught by her web when she’s hungry…

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.

Susan's Snazzy Arthropod of the Day: Zonocerus elegans

This is Zonocerus elegans, or the "elegant grasshopper." Native to southern Africa, it was commonly found in the area in Mozambique where I used to live. Its species name, though, refers only to its coloration, not to its locomotion: it has got to be the most ungraceful, clumsy excuse for a grasshopper I've ever seen. When it hops, it rarely ever seems to land how one might expect it wanted to; instead of hopping in a manner that would allow it to proceed in a straight, efficient path, it tends to sort of slowly flop, landing sideways or backwards or sometimes not even on its feet. Maladaptive? Nah, you don't have to be an effective escape artist if you're as nasty-tasting as these guys are. They have a predilection for eating poisonous plants and sequestering the toxins as a predator defense. Additionally, they secrete a foul-smelling yellow goo when handled, and taste accordingly. Most things won't touch them, and they often can be a pest in agricultural settings.
Unfortunately for Z. elegans, though, some humans like funny-tasting stuff. The Pedi people of South Africa, for example, traditionally enjoyed them as a relish with porridge. I'm not certain how they dealt with the sequestered poisons; perhaps the concentrations are generally too low to affect humans, or the poisonous substances (generally cardiac glycosides or pyrrolizadine alkaloids, even cannabinoids) are degraded upon cooking.
In any case, other than their zesty flavor, I'm sure the Pedi also enjoyed how darn easy they are to catch. I would have liked to have been shown the proper way to catch them and cook them up. Nowadays, though, Z. elegans doesn't have as much to worry about. With entomophagy and other traditional diets on the wane in southern Africa, this pretty insect must be breathing a little grasshopper sigh of relief as it flounders off into the sunset, uneaten.


Photo by Lambert Smith, from insecta.co.za. Used with permission.

Kneeling at the pipes: a poem by Marge Piercy (dedicated to Amy W. and her roommates)

Princely cockroach, inheritor,
I used to stain the kitchen wall with your brothers,
flood you right down the basin.
I squashed you underfoot, making faces.
I repent.
I am relieved to hear somebody
will survive our noises.
Thoughtlessly I judged you dirty
while dropping poisons and freeways and bombs
on the melted landscape.
I want to bribe you
to memorize certain poems.
My generation too craves posterity.
Accept this dish of well aged meat.
In the warrens of our rotting citites
where those small eggs
round as earth wait,
spread the Word.