Dragonfly Swarms: A Summer Delight

Wednesday, July 28, 2010

Nature Notes
by Bob Thomas

One of the fun summer events is the formation of dragonfly swarms throughout our area, and they are especially enjoyable when close to home, as in our backyards. There are many more dragonflies (called mosquito hawks by many) in summer, in part because they have been breeding successfully so the population builds. Many adults have arrived by migrating back to our neighborhoods, thus supplementing the home-grown numbers.

Naturalists believe dragonflies may aggregate in suitable feeding and breeding grounds that give them an advantage. They are known to prefer ecological conditions that may be present where we find swarms: wind-breaks (fences in our yards and dense vegetation in the wild), sunny openings (in the forest), moist meadows, abundant food (flowerbeds), and the like. Swarms may give them more access to prey (more dragonfly activity stirring up food items) and protection from predation by their larger numbers.

With the current popularity of ponds in yards, the ability to breed in protected habitats is a bonus.

The swarms may consist of numerous species or just one species. Be aware that dragonflies are sexually and age-cohort dimorphic, so what appear as multiple species swarms may be several varieties of one species.

As they dart about your yard, watch them closely.

Some species are territorial, so their shooting about may be in a specific area and their activities are mostly protecting their space.

Feeding occupies much of their time. Many use hawking, flying rapidly and catching their prey on the wing. Small prey are usually caught in the mouth and quickly dispatched. Larger prey require the use of the dragonflies’ legs and they may take an hour to consume their catch after landing on a perch. Many naturalists believe dragonflies use their legs to catch airborne prey the way we use a seine to catch shrimp. This has never been verified, but it is plausible. A similarly functioning apparatus or behavior is common in other animals that catch their prey on the wing, such as bats using their uropatagium (skin between their hind legs that incorporates their tail) like a catcher’s mitt and caprimulgid birds (night hawks, whip-poor-wills) that have hair-like bristle feathers around the mouth.

Dragonflies that perch keep a sharp eye out for approaching prey. Just like flycatchers, they sally forth, snatch the prey, and return to the perch to feed.

Other dragonfly species feed by gleening. They hover over plants and capture unwary insects.

It is not unusual to see dragonflies cattle-egreting, following larger animals through prey-rich habitats and hawking critters flushed by their passage.

Have you noticed dragonfly pairs flying about connected tail-to-head? The common explanation is that the dragonflies are mating. This is not true, but it is a behavior related to mating.

When dragonflies are ready to mate, males have to prepare for the magic moment via a process called self insemination. Male genital equipment is at the distal tip of their abdomen and their accessory organ, a place they insert their semen to make it available to the female, is near the base of the thorax. The male curls his abdomen forward and fills the accessory organ with semen. He is now loaded and ready to mate. He then locates a receptive female; she lands, the male alights on top of her and bends his tail under his body and grabs the rear of her head with his claspers that reside on the tip of his abdomen, then they fly off in tandem with the male in front. Mating has not yet occurred.

When ready for insemination, the pair lands, and the female curls her abdomen forward and under the male until the tip of her abdomen meets his accessory organ. In doing so, their bodies form a copulation, or mating, wheel which is vaguely heart-shaped. When this is accomplished, the pair unfold and fly away to find a suitable place to lay eggs.

In some species, they quickly separate and the male may leave. In others, mating is followed by guarding that allows the male to protect the female from other suitors, thus protecting his sireship. For those species that use contact guarding, the male maintains his grip on the female’s head during egg laying. For those species using non-contact guarding, the male releases the female, but stays near her so he can thwart the advances of other males. Noncontact guarding is the norm for territorial dragonflies. After mating, the female lays eggs within her mate’s territory, so he simply stands guard over her as he guards his territory.

Another often observed dragonfly behavior is obelisking, where it perches with the abdomen held vertically. The intent is to avoid overheating from sunlight by exposing a minimal amount of the body to its rays. It may also perch in the shade or occasionally dip into the water to cool down.

If too cool, it will bask by sitting fully exposed to the sun or perform wing whirring (vibrating the wings to generate heat).

I suggest you use dragonfly swarms as an opportunity to develop or sharpen your skills in identifying the array of species we have along the coast. You’ll be able to entertain your friends with a whole new vocabulary and a bevy of fanciful names.

A few resources:

Berger, Cynthia. 2004. Wilde guide: Dragonflies. Stackpole Books. ISBN 978-0-8117-2971-0.

Dunkle, Sidney W. 2000. Dragonflies Through Binoculars. A Field Guide to Dragonflies of North America. Oxford Press, New York. ISBN 978-0-19-511268-9.

Nikula, Blair and Jackie Sones. 2002. Stokes Beginner’s Guide to Dragonflies and Damselflies. Little, Brown and Co.,

                                      

Black saddlebags male, Tramea lacerata,                                     Widow skimmer male, Libellula luctosa,
Johnson Bayou, Cameron Parish, La.                                            Lake Thoreau near Hattiesburg, Ms.
Photo by James Beck.                                                                Photo by Bob Thomas.

                                       

Blue dasher male, Pachydiplax longipennis,                                  Blue dasher female, Pachydiplax
Bayou Vermilion District, Lafayette, La.                                        longipennis
, Broussard, La.
Photo by James Beck.                                                               Photo by James Beck.


                                      

Obelisking four spotted pennant male,                                         Golden-winged skimmer female, Libellula
Brachymesia gravida, New Orleans                                              auripennis, St. John the Baptist Parish, La.,
City Park.                                                                                  along I-10.
Photo by Bob Thomas.                                                            Photo by Bob Thomas.