Tuesday, September 1, 2009

Dragonflies in autumn

Dragonflies are insects that love warmth, so the adults fly through the spring, summer, and fall and disappear in winter, overwintering in the larval stage in the water or as eggs in the case of a few species. The best month in which to see dragonflies in the Pacific Northwest is July, when the spring species are still around, summer species are in full swing, and the first of the fall species are appearing. By the end of August, when these photos were taken, the spring species are gone, the summer species are waning, and fall species dominate.

Certain groups are especially common in fall. The big blue darners (Aeshna) peak in August and September, when we see them in numbers at just about any wetland. The males are looking for females, and they alternate flying along the shore and hovering in one spot, occasionally dipping down into the aquatic vegetation to see if they can spot an ovipositing (egg-laying) female. When two males intersect, there may be a fierce chase. When a female is spotted, the male grabs her and mating takes place, usually up in the trees and often lasting an hour or more. The female will soon make her way back to the water and lay her eggs in plant stems or rotten wood.

Another common group in the fall are the meadowhawks (Sympetrum), members of the skimmer family (Libellulidae). These little dragonflies, most of them red, fly in numbers around just about all lentic wetlands. (Lentic = still, such as ponds and lakes; lotic = flowing, such as streams and rivers.) One species, the Cardinal Meadowhawk (Sympetrum illotum), flies throughout the summer and can be recognized by its very bright red color and orange-suffused wings with a dark streak at the base. This species breeds in permanent ponds and lakes, and a larva, after hatching from the egg, develops over the summer, goes into dormancy in the winter, then emerges as an adult the following spring. If it develops especially rapidly, it will emerge in the fall, and it thus has a bimodal flight season, common in spring and fall and less common in midsummer.

But most meadowhawks don't emerge as adults from the water until July or August, then fly well into October. Another very common species is the Striped Meadowhawk (Sympetrum pallipes), recognizable by its striped thorax. The individual shown here is a bright red female, colored just like a male, but other females are brown. This is another example of polymorphism, with red and brown morphs, and we can only speculate on the factors that cause the evolution of such color variation. This species breeds in temporary wetlands that often dry up during our relatively dry summers. The females drop their eggs while in flight around the edge of these wetlands, usually on dry ground. Then the wetlands flood over the next winter, inundating the eggs, and they hatch in early summer when temperatures are high enough. They develop quickly and emerge in midsummer, and the adults fly during the fall, repeating the cycle. Two related species can have quite different life histories.

Dennis Paulson


Anonymous said...

Hello Dennis and Museum crew! Fantastic that the museum is doing a blog! I would love to contribute, and will announce any new posts on facebook, twitter and my blog to help get it out there.

Fantastic first two posts! It would be really cool to have sort of a bi-weekly, or monthly featured museum specimen/organism, and a post on the prepping process would be fantastic too!


Eileen said...

Great blog, Dennis and Museum people! It's great that I can keep up with you from all the way over here in PA!
Sounds like you had a great birding trip to the coast. I miss you guys!

Aleksei Molodtcov said...

Great blog and great nature creature, like a jellyfish that eat small fish