Blue Dasher Dragonflies (Family Libellulidae)

Howdy, BugFans,

OK—it’s April—time to fan the latent flames of those early spring dragonfly fantasies. Not with an exotic dragonfly, or with an early spring species, but with the very common and exquisitely beautiful Blue dasher (Pachydiplax longipennis), a.k.a. the Swift Long-winged skimmer (they aren’t especially long-winged, though that’s what longipennis means) and the Blue pirate.

Blue Dasher

The Blue dasher is in the Skimmer family Libellulidae, the largest and showiest dragonfly family, and it’s the only member of its genus. Its range stretches across North America from British Columbia to Ontario, south (except for the Rockies and Dakotas) to California and Florida, with scenic side-trips into Mexico, the Bahamas, and Belize. According to the Wisconsin Odonata Survey, Blue dashers may have been extending their range north in the state in recent years.

  • Blue dasher

With jade-green eyes, a slight amber wash in their wings, and a “frosted” sky-blue color on their abdomen, the unmistakable males are peacocks of the dragonfly world. Their abdomen starts out dark blue, but as they age, it develops a coating of waxy cells that lighten it; (it’s called pruinescencefrom the Latin pruina, for hoarfrost). Females, which are about the same size as males, have shorter, yellow-striped abdomens, though some females also develop pruinescence. Western males may have blue thoraxes and resemble Eastern pondhawks. Juvenile Blue dashers (bugguide.netcalls them “tweens”) have long, slim abdomens and resemble females in coloring (and some have startling red/grey eyes.  See http://bugguide.net/node/view/598 for the various “plumages.”

Their flight period here in God’s country extends for much of the summer, but most of the BugLady’s pictures were taken in July and August. They have an eight-month flight period on the Gulf Coast, and for reasons that are not understood (available food for naiads? relative salinity?), individuals from early broods are larger than those from later ones.

Male Blue dashers are conspicuous perchers that frequently draw their wings forward (“over their eyes,” said one source) and that will raise their abdomens above horizontal on warm days to minimize the angle of the sun’s rays (but rarely, noted another researcher, to the almost-vertical angle that is assumed by Meadowhawks).

Blue dasher naiads live in the still waters of ponds, ditches, marshes, and occasionally, very slow-moving streams, and because they can tolerate pretty low-oxygen/poor water quality, they are sometimes used as biological indicators. Mature naiads are well-camouflaged, (dorsal) hook free and relatively spine-less.

  • Blue dasher

The naiads hunt passively—lurking leaf axils on stems of aquatic plants and lunging at the delicacies that pass by—which allows them to be undetected by predators. Naiads feed on small critters that live underwater with them (including tiny fish and tadpoles). They are said to have a special fondness for mosquito larvae, which they snag using their extendible mouthparts and a close-up view). Adults feed on small, flying insects that they spot from perches and snatch out of the air, and they are said to consume 10% of their weight in prey daily.

Male Blue dashers are pretty territorial, although that territory is movable. Within his territory the male chases other dragonflies energetically—not solely those of his species, but similarly-colored species like Slaty skimmers, too. He flies out from his perch every minute or less, returning to it if the coast is clear. His pruinose, blue abdomen reflects UV light like crazy, and he raises it in flight as he approaches intruders. The “chase-ee” responds by flying with his abdomen lowered. A territorial male often flies up under the intruding male and forces him to gain altitude and ultimately to leave the territory. Larger male Blue dashers tend to have and keep better territories than smaller males.

He guards her to buy her time to oviposit—preventing rival males from abducting her and removing his sperm – and also to maintain the integrity of his territory. If she strays out of his territory while ovipositing, she’s on her own. She’s in danger from frogs as she oviposits in the shallows.

The Odonata Central website says that because Blue dashers are so common and abundant, they are “one of the most well studied dragonflies in North America.” The BugLady read the abstract of a paper called “Kinematics of a territorial defense maneuver by the dragonfly Pachydiplax longipennis (Odonata: Anisoptera: Libellulidae”—with its yaw turns, stroke amplitude, horizontal and vertical flight velocity, and phase flapping, there was maybe a little too much aerodynamics to fit comfortably into a BOTW. When she checked the definition of “yaw,” she came across this lovely website How Things Fly – Roll, Pitch, and Yaw. And here’s a less technical but very fine article on a similar topic: Entomologists Amazed by How Dragonflies Dance. Enjoy.

Ah, Dragonflies.

 
The BugLady