Here Be Dragons
Most AJMers would agree that one of the best parts of our jobs is spending time out in nature, getting the chance to see wildlife up close. We often focus on vertebrates, but it's hard to forget that the animal world is mostly insects when we visit a site as diverse and full of bugs as a wetland.
This field season, we relocated numerous amphibians from wetlands that were slated for construction and observed many insects in the process, from grasshoppers (which are surprisingly easy to mistake for tiny boreal chorus frogs) to pesky mosquitoes. Some of my favourites are dragonflies. They are incredibly fast, agile fliers with excellent eyesight that makes them formidable predators, especially of mosquitoes! They catch as much as 97% of the prey they chase (Olberg et al. 2000). Because they can hover, make sharp turns, and even fly backwards, they are often compared to helicopters or fighter jets.
They perform an even more acrobatic flight maneuver during mating and egg laying called a tandem flight: the male grasps the female's head with special structures on the tip of his abdomen and the two fly together. This is a way of warding off romantic rivals; the pair may stay attached like this even while the female lays eggs (Latty 2006).
As I was scouring a mostly dry wetland for amphibians to salvage before construction, I found a group of tandem-flying dragonflies all intently focused on the same small tussock of grass. They were a species of meadowhawk, small dragonflies that are often bright red and hunt smaller insects over pastures and meadows. (As they are often difficult to identify to species without collecting and dissecting them, I'm not sure which species these were.) In this short video I took, you can see five tandem-flying pairs and one lone male. The ruby-red males are in the front of each tandem pair and the yellow-brown females behind them. The females sometimes appear to dip or shake their abdomens (both while flying and while perched). This is how they lay eggs: they simply let them drop to the ground! If you look closely, you can spot some of these eggs as they fall.
It seemed odd to me that the dragonflies wouldn't spread out across the entire wetland to avoid competition for food between their offspring (or the risk of another dragonfly nymph eating their offspring—like the adults, dragonfly nymphs are also voracious predators.) It seemed even odder that they were laying eggs where there was no surface water even though their young (called nymphs) are fully aquatic. After some research, I found out that meadowhawks often lay eggs out of water, including in spots that will only temporarily hold water. When the snow melts in spring, the wetland will flood and the hatching eggs will be underwater. But why the preference for that single, unremarkable patch of plants?
There is no concrete answer, but I was delighted to find a scientific paper remarking on the same phenomenon published in 1987 by a German researcher who was visiting the Calgary area (Schmidt 1987). They described the sites where egg laying pairs clustered as "random"—though I personally wouldn't rule out some factor invisible to humans but vital and obvious to dragonflies—and suggested that this clustering was a way for mating pairs to avoid interference by unpaired males (i.e., trying to steal mates).
A more recent study on a different meadowhawk species suggested another reason: by laying eggs in groups, a dragonfly pair is less likely to be eaten by frogs than a pair laying eggs on its own (McMillan 2000). This is called the dilution effect, and is also seen in sheep or deer living in herds, fish swimming in schools, and birds forming flocks. If a predator attacks, it's more likely to attack another group member than you in particular. It's not the friendliest way to think of your neighbours, but laying eggs is one of the riskiest behaviours for a dragonfly. (The dragonflies in this case didn't know that AJM biologists had already relocated most of their predators out of the area!)
Another possibility is simply that the dragonflies assume that if one pair has chosen to lay eggs somewhere, it must be a good place to lay eggs. It might save them the time of examining the habitat closely. In other words, maybe they're just there because everyone else is, so it must be cool! And so far, no one has examined whether laying eggs in groups affects the eggs and nymphs themselves, as opposed to their parents.
I greatly enjoyed watching these dragonflies and it was a pleasant surprise to find that someone else had noticed the same behaviour and written about it so many years ago. I hope future scientists will keep noticing odd little biological puzzles like these and investigating them.
By AJM Environmental Scientist, Laura Southcott, PhD, MSc, HBSc
photo by Laura Southcott
References
Latty TM. 2006. "Flexible mate guarding tactics in the dragonfly Sympetrum internum (Odonata: Libellulidae)." Journal of Insect Behavior 19: 469-477.
McMillan VE. 2000. "Aggregating behavior during oviposition in the dragonfly Sympetrum vicinum (Hagen) (Odonata: Libellulidae)." The American Midland Naturalist 144(1): 11-18.
Olberg RM, Worthington AH, Venator KR. 2000. "Prey pursuit and interception in dragonflies." Journal of Comparative Physiology A 186: 155-162.
Schmidt E. 1987. "Notes on a peculiar reproductive behaviour and on habitat recognition in Sympetrum internum Montgomery (Anisoptera: Libellulidae)." Notulae Odonatologicae 2(9): 144-147.