Monday, January 24, 2011

Returning to the Depths: Submerged Oviposition

I suspect a lot of people don’t realize that some odonates deposit their eggs while submerged. I don’t mean that they simply stick the end of their abdomen below the water surface to lay their eggs (which is also done), but sometimes they completely submerge themselves.

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. 

The behavior is almost completely restricted to damselflies (suborder Zygoptera) which all oviposit endophytically—inserting their eggs into plant material instead of just dropping them from the end of the abdomen. It is rare among the dragonflies (suborder Anisoptera), probably because it’s difficult for them to break the surface tension with their broad wings sticking out to the sides. Damselflies must be better able to pierce the water’s surface with their wings folded together over the abdomen.

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.
Philip Corbet, in his monumental Dragonflies: Behavior and Ecology of Odonata (1999), summarized what was known about underwater oviposition up to that time. According to him the duration of uninterrupted submerged oviposition is often at least 30 minutes and frequently close to an hour. Apparently the record goes to the Marsh Bluet (Enallagma ebrium) which is known to go submerged for an incredible five hours! Descending to at least 10 centimeters is common, but some are known to go as far as a meter below the surface.

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).

A pair of Northern Spreadwings (Lestes disjunctus) ovipositing in tandem at Frater Lake, Washington.
They are several inches below the surface (note the meniscus near the top right corner). Spreadwings
typically perch with the wings spread open—including when ovipositing, but note that both are
keeping their wings tightly closed in this case.

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).

3 comments:

  1. Great post and super photographs - very cool blog! I'll definitely keep an eye open for this type of behaviour the next time I'm out at the pond.

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  2. Wow, great photos and fascinating info! I'm in the Midwest (MN) but will keep an eye out for this behavior around here. I must confess I really don't know one odonate from another in the wild (although I can tell a damsel from a dragon), I just enjoy seeing them! I'm following your blog now to learn more :)

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  3. I knew Zygoptera oviposited endophytically but did not realize they submerged themselves completely to do this. I wonder if this represents the primitive condition for the order and dropping by Anisoptera represents a more derived condition.

    That second photo is beautiful.

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