Tuesday, September 8, 2009

green darner, Washington state insect

Most states don’t have state insects, and most people in Washington state don’t know about our very own state insect! Do you know what it is? You probably do by now, having seen all the photos, but you may not know its name. It’s the Common Green Darner, Anax junius. This species is a member of the family Aeshnidae, suborder Anisoptera, order Odonata. This insect order contains the dragonflies and damselflies.

In 1997, a group of students at Crestwood Elementary School in Kent brought the idea of a state insect to the legislature. About 25,000 students from over 100 school districts statewide were asked to pick a state insect from among several contenders, and the dragonfly won in a landslide.

Male Common Green Darners are easily recognized by their green thorax and mostly blue abdomen. Females are similar, but the abdomen is usually greenish (rarely blue). Common Green Darners are the only North American darner in which the pair stays connected after mating and flies around in tandem while looking for places to oviposit.

Common Green Darners are among the best-studied of North American dragonflies. They occur all across North America, breeding in ponds and lakes from southern Canada to southern Mexico, as well as in Hawaii. They are known to be highly migratory throughout their North American range. Mature adults arrive rather early in spring at the north end of their range, often the first dragonflies to be seen in flight each year. They breed and lay eggs as long as they live, then die after a few weeks. Their eggs hatch and their larvae develop in the water through the summer.

In Washington, as elsewhere in the North, the larvae have grown rapidly in the warm-water environment and are ready to emerge from the water in August. There is usually a hiatus in summer where not many are seen, but then immature individuals, colored like adults but with a reddish-violet abdomen, become common. After their emergence, you can see their exuviae (cast skins) along the edges of wetlands everywhere.

The immature Anax very soon begin to head south. Obvious migratory movements are rarely seen in Washington, but farther south in California, Common Green Darners can be everywhere in fall. They move through the landscape in numbers, steadily heading south.

If you wander through Florida or southern Texas in September and October, Common Green Darners are ubiquitous. These are end points in the long migration, and by the time the migrants reach these lower latitudes, they have attained sexual maturity and are ready to breed. They do so, and – to make a long story short – their larvae mature through the tropical winter and are ready to emerge in spring. These spring emergers then head north to repeat the annual cycle.

So in this dragonfly, there are two alternating generations, one that emerges in fall in the north and flies south, another that emerges in spring in the south and flies north. Thus this species shows an amazing and unique migration pattern, unlike that of any other animal. At first glance, it seems something like the migration of Monarch butterflies, but it is actually somewhat different.

In Monarchs, northern adults that emerge from their chrysalis in the fall head south and winter in southern Mexico. These same individuals begin a northward migration the next spring, so in that way their migration is more like those of birds. But they stop and breed somewhere in southern US, then their offspring continue northward and do it again. There may be three generations of northbound migrants, then, as they emerge from their pupae late in the summer, the offspring of the northernmost breeders head south instead. There are indeed similarities to the darners in that more than one generation is involved in the annual cycle, and the northernmost breeding populations have a northbound and a southbound generation.

But it turns out that the darners have been varied in the evolution of their annual cycles. Larvae growing rapidly through the summer in Washington emerge in fall to fly south, seemingly the most common mode, while those growing more slowly are forced to overwinter as large larvae and emerge the next spring. Thus there is also a “resident” population of individuals with a life cycle more like other dragonflies, that is they fly during the summer and their larvae develop during the winter. It was hypothesized that these two populations might even show genetic differences because they didn't interbreed. However, recent studies have shown that the migratory and resident populations are genetically identical.

With global warming, the summer growing season should lengthen, possibly causing further variation in the life cycle of this interesting species.

Dennis Paulson

No comments:

Nature Blog Network