|An ovipositing River Jewelwing (Calopteryx |
aequabilis) on the South Fork of the John Day
River, Oregon. Only the distal two-thirds of the
wings are above the water surface.
This behavior seems counterintuitive for a group of insects with relatively long, narrow wings unprotected by a rigid covering while at rest (as is the case with beetles and true bugs), but some species oviposit in this way frequently or even exclusively (for example certain bluets [Enallagma], jewelwings [Calopteryx], and rubyspots [Hetaerina]). It is infrequent in some other genera, but it could be more common than is realized. I’ve only observed it a few times, although it isn’t very conspicuous—damselflies crawling down a plant stem to get under the surface don’t exactly draw your attention, and once submerged they are pretty difficult to spot.
|A completely submerged Appalachian Jewelwing|
(Calopteryx angustipennis) ovipositing in
Wilson Creek, North Carolina.
Considering that odonates abandon their water-breathing gills when they emerge, how do the adults get their oxygen when they return to the depths? A thin envelope of air clings to the body and wings which gives them a silvery appearance while they’re under water. Body movements likely force the air in and out of tracheal openings facilitating respiration.
There are of course pros and cons associated with submerged oviposition. Potential advantages are that the female (or pair, if that’s the case) is free from harassment by males looking to copulate, and the risk of desiccation to the eggs is reduced in habitat where the water level can fall substantially during the flying season—the lower she goes, the longer the eggs will stay wet. There is a risk of drowning for the female when she is unable to break free of the surface tension—possibly because immersion in cool water for a time saps her strength, but she could also be rescued by a male who goes into tandem with her with the intent of copulating. The eager mate might provide enough lifting force to break her free of the surface, or else he could “tow” her across the surface to something like a plant stem where she can climb out (imagine an ultralight aircraft towing a water skier).
Considering that odonates spend most of their life in water (as nymphs), it doesn’t seem all that unnatural for them to return to the depths to complete their life cycle and produce the next generation. Watch for this inconspicuous behavior. It probably happens more frequently than we know—and right under our noses (well, right under the water).