X-rated dobsonflies

About a month ago, I got some really strange insects to my blacklighted sheet while out doing moths one night. Long, gray bugs with giant wings and feathery antennae, I wrote about them in this post. They turned out to be fishflies, a group of insects I’d previously never heard of.

This weekend I found their cousins, the dobsonflies. I had actually heard of dobsonflies before, because one of the reviews I’d read for my new macro lens, prior to buying it, had had a close-up photo of a dobsonfly head. Still, I’d never actually seen one myself. There were four of them, three females and a male, attracted to bright white security lights on the side of a large building out near where Blackburnian’s mom lives. These happen to be Eastern Dobsonflies, Corydalus cornutus, the only species to occur in eastern North America. The other three North American species are found primarily in the southwest.

In the photo above there’s a male and a female. The male is the guy with the giant mandibles. The females were all mostly quite low to the ground and easy to reach. The male was another story, he was up near the top of the wall and I had to stand on a box and reach up with Blackburnian’s shoe to gently knock it down to a level where I could reach it. He was the one I really wanted, though, because he’s so impressive. Males reach about 4 inches (10 cm) long, nearly a quarter of which is the mandibles. Females are only slightly smaller at just over 3 inches (7.5 cm).

I plucked one of each of them up, pinching their wings together over their back. When I snagged their wings they both reared their heads back, trying to grab me with their mandibles, or at the very least startle me. I knew that their wings were long enough that I was out of reach, so I didn’t buy into it, despite the ferocious appearance. While the female might be able to give you a pinch, the mandibles of the male aren’t really built for biting and can’t really do much for you. Most sources I read say that the adults don’t eat; however, one said that an older research paper indicated they would feed from honey water solutions or have been found at fermented baits (like what I’d put out for moths in cold weather). Either way, they’re not hunting their food.

The male’s mandibles are, if anything, even more ferocious-looking than those of the female. As Blackburnian described them, the creatures look prehistoric. Given that they’re not used in eating, what are those huge mandibles actually used for? A commenter to my fishflies post suggested that they have a role in mating. Searching the net turns up little additional information. Many sites simply say the long pincers are used to grasp females during mating. One suggests their impressive length has developed through sexual selection, used to impress the ladies. Another indicates the mandibles are used in male-male confrontations, such as in competing for females (whether they actually fence with them or simply show them off depends on the species).

This site mentions a 1952 paper that describes the mating process: “As part of the premating ritual, males place their elongated jaws on the wings of the females perpendicular to the axis of the female’s wings. The male’s jaws also function in jousting with rival males. However, males were not observed to grasp the females as reported in older literature.” This second site has a slightly different description: “The male uses the mandibles during the mating process when capturing, prodding, and caressing the female, and they are also used when males fight one another. Prior to mating the male will flutter his wings, and both males and females will touch antennae.”

I had the opportunity to find out first-hand.

I actually missed capturing the first stage on camera. When I’d finished taking photos of the two of them I plucked them up, one in each hand, and carried them off to the garden where I placed them on a low brick wall. Not even giving it much thought, I’d placed them down nearly side by side. They didn’t take off upon release (they never do), but instead the male turned his head and I could almost hear the thought run through his mind: “Oho! A female! What luck!”

He dispensed with the foreplay, there was no touching of antennae as described on the pages I read. There was no showing off of mandibles to the female. In fact, the female seemed completely disinterested through the entire session. She just sat there, not moving, her antennae folded over her back, reminding me of how a mammal who is feeling unamused will lay its ears flat. I bet if I looked closely I could’ve seen her rolling her eyes.

The first thing the male did upon making his discovery was (as the first linked site said) turn perpendicular to the female and lay his long mandibles across her back. But he didn’t just rest them there, he appeared to actually put pressure on the lower half of her abdomen and then slid them backwards along her wings to the wingtips. He did this several times, and I presume the purpose was to squeeze out the spermatophores of any competing males. Dobsonflies transfer sperm in the form of gelatinous balls, so it would be fairly easy for a male to remove any left by a previous male, in order to ensure that his own sperm are the ones to fertilize her eggs. A lot of species, both invertebrates and vertebrates, will employ a similar strategy.

I finally decided it might be worth getting my camera to record. When I returned, he was reaching under the female’s “skirt” with his mandibles. I couldn’t tell exactly what he was doing – perhaps checking for the presence of a pre-existing spermatophore? Removing it, if there was one?

Once he felt satisfied with whatever he was checking for, he started fluttering his wings and curling his abdomen around toward the female. He tried to tuck it under her wings to make contact, but seemed to be having trouble for some reason. Actually, it looked to me that his abdomen just wasn’t long enough to do what he wanted it to do (isn’t this a common feeling with males?). He seemed reluctant to remove his mandibles from under her wings, which appeared to be the main problem, to me.

After making a few attempts from the left, he decided to try from the other side. He shuffled over and swung around to face the other way. Again he checked under her wings first.

Then tried from the other side.

This time he removed his mandibles from under her wings so he could straighten out and reach better. Success! During all this he was fluttering his wings like crazy and generally obscuring the female from view. From what I could see, she continued to simply sit there. It seemed funny that the mating of something with such a long, flexible abdomen would be of the facing-opposite-directions sort, rather than the male above (or below), facing the same direction, and simply curving his abdomen down to meet the female’s.

It didn’t take long, perhaps ten or fifteen seconds, and then he turned himself around again to face the female.

Finally, he rested his mandibles across her abdomen and they both sat there, still. I checked on them a few times as I went about photographing the rest of the moths I’d caught, and they stayed like that for at least ten minutes. Eventually when I went back the female had crawled off the top of the brick into the shade of the side, but the male remained where he’d been. I assume that this was the male mate-guarding the female until the spermatophore “took”, to prevent other males from removing his sperm before they had a chance to fertilize the eggs, despite what I’d read on one site suggesting it hadn’t been observed in the species.

I thought this whole thing was so neat. You know, it’s one thing to just simply see a species (cool as this species happened to be), but it’s quite another to observe a behaviour. Something I’m often guilty of, I find I tend to breeze by, looking at but not really watching what I’m seeing. I’ll identify the birds, for instance, often simply by ear, but I won’t really pay attention to what they’re up to unless there’s some commotion or something to draw my attention. It really is something I should do more – stop and smell the roses, as they say.