Great spreadwings have spread from the southwest to northern New England
A large damselfly hung placidly from a forsythia branch in my back yard, so I approached within three feet to get a good view. On the next day it had moved to a spirea, and once again it was tolerant of my close approach. I startled it several times while trying to capture a good macro shot, but each time it fluttered a few feet away and then returned to perch on the same twig. Others describe the same behavior—great damselflies, unlike most dragonflies and damselflies, do not flee when people approach.
A female great spreadwing basks in the warm sun. Photo by Jeff Mitton.
Damselflies are close relatives of dragonflies, but these groups can be separated by the way they hold their wings when they are perching. Dragonflies hold their wings straight out from the body, like the wings of an airplane. Many damselflies hold their wings together, sticking straight up above the body, but a group of damselflies called spreadwings hold their wings as in the accompanying photo.
The great damselfly, Archilestes grandis, is the largest damselfly in North America, with body lengths that reach 2.4 inches and wingspans of 3 inches. Perhaps the easiest way to identify this species is by the conspicuous stripe of yellow on each side of the thorax. A simple way to tell males from females is eye color—males have bright lapis blue eyes, while the eyes of females are lighter blue and pale lavender.
Adult spreadwings are active through the summer and into the warm days of late fall. In fall, females lay their eggs inside plant tissues, quite often in the leaf petioles (stalks of leaves) on branches of sycamore hanging over ponds and irrigation ditches. They can also utilize the stems of herbaceous perennials and twigs of willow and elm. Eggs can overwinter in the plants and when larvae begin to develop in spring, they drop from the plant into the water. Larvae (nymphs) that drop onto land can wriggle short distances to the safety of the water. Both nymphs and adults are insectivores.
A salient feature of the natural history of great spreadwings is the recent expansion of their geographic range. Historically they were a southwestern species, with a few curious, isolated populations in Central and Southern America. But in the last hundred years they have expanded their distribution further north and east. They reached Ohio in 1927, Pennsylvania in 1935, Washington, DC in 1949, New Jersey in the 960's, New York in 1992, Michigan in 2005, and they have now reached northern New England. This is a remarkable journey for a species that started out on the Colorado Plateau.
Two hypotheses have been proposed to explain the great spreadwing's march north and east: climate change and a continual proliferation of man made ponds and reservoirs.
Observations collected by dedicated citizen naturalists but summarized by professional biologists show that, within the last 20 years, butterflies and damselflies have extended their ranges to the north in Utah and in Massachusetts. In Britain, 37 butterfly species have been monitored since 1980, and 34 of them have expanded their ranges 45 miles to the north, slightly more than a mile per year.
Since the early 1900s, farmers, ranchers, equestrians, sportsmen, golfers, private citizens and industrial companies have increased the number of ponds, reservoirs, holding ponds and irrigation ditches across the country. Recent studies have documented that great spreadwings are more tolerant of polluted water than are the majority of dragonflies and damselflies. Consequently, great spreadwings have used clean lakes and ponds and polluted industrial sites as stepping stones to increase their geographic range.
It seems to me that each of the hypotheses for great spreadwing's range expansion has considerable merit. I think it is likely that a combination of climate warming and proliferation of water bodies explains how a southwestern damselfly now finds itself in Michigan and Vermont.