Sunday, July 1, 2012

Orange Bluet, with water mites

A tiny orange bluet, Enallagma signatum, peers at your narrator from its resting spot on a lily pad. There are lots of these handsome insects at my local patch wetland, but one must get there towards day's end to see them. For some reason, the orange/yellow-colored damselflies seem to be largely crepuscular, or active towards evening. I saw plenty of orange bluets on the evening that I made these images; a visit at midday generally produces none. If anyone has an explanation for the dusk-active behavior of this species and its orange ilk, I'd love to hear it.

A male orange bluet shares space on this fragrant water-lily leaf with the exuviae, or shed exoskeleton, of a damselfly larva. I'm guessing the shed is from an orange bluet nymph. The predatory nymphs, or larval stage, of damselflies and dragonflies live a completely aquatic existence. Depending on the species, they may remain under the water as a nymph for anywhere from a few months to several years in the case of certain large dragonflies. When the fateful evening arrives, the nymph crawls from the water and clambers onto a plant or some other structure, and undergoes a remarkable transformation to the winged adult. I once was fortunate to stumble into a wandering glider bursting from its exuviae, and photo-documented the process HERE.

A male orange bluet is truly tiny, measuring not much more than an inch from stem to stern. Luckily they are much "tamer" than most larger dragonflies, and by slowly wading up to their lily pad perches, I am able to get my lens pretty close. Still, their minuscule size taxes the limits of my Nikon's 105 mm macro lens, and handholding the rig and making clear crisp shots is a challenge.

As I scoped the animal through my lens, I noticed it carried some baggage. Those little globules on the underside of the bluet's thorax are water mites, in the genus Arrenurus (presumably). These mites are parasitic on damselflies, and can occur on their hosts commonly. Studies of water mite/damselfly infestations have found anywhere from 10% to over 50% of a given damselfly population can be parasitized by mites. The mites are not benign; their purposes for attaching to a damselfly are twofold. One, the mite taps directly into its host and extracts bodily fluids, and the greater the number of mites that are present, the greater the reduction in fitness of the damselfly. Two, the damselflies essentially serve as winged taxi cabs, hauling the water mites to new areas that can be colonized. Flighted insects, and birds, play an enormous role in the dispersal of other organisms, whether they want to or not.

1 comment:

zippiknits said...

We have blue damsel flies here - in Central Coastal California. they have the same fierce little look, and the same tame curiosity and fearless demeanor. Great photos!