Monday, July 31, 2017

Wandering Glider

If I told you there was a species that flies thousands of miles each year, stopping at Midway each spring to breed at Midway Atoll, you’d probably think I was talking about something covered in feathers. And you’d be wrong.

Among the insects are some species known for their long migrations. The famous Monarch butterfly (Danaus plexippus), for instance, travels back and forth each year from breeding areas in northern Great Plains to their overwintering sites in southern Mexico. The Monarch seems to have never made its way to Midway and, in fact, just a single species of butterfly can be found on the atoll, the Painted Lady (Vanessa cardui). The amazing insect voyager that I want to tell you about, though, isn’t even a butterfly.

The Wandering Glider, known also as the Globe Skimmer (Pantala flavescens), is a large dragonfly species that occurs across a wide swath of tropical and temperate areas of the Americas, Asia, Australasia, and Africa as well as oceanic islands. The wide distribution of this species was not fully appreciated until recently when it was discovered that they make incredible, long-distance migrations. The “population” studied, migrated each year from India to Africa, stopping in the Maldive Islands along the way, following the monsoon rains and then returning via the approximately same route. It is part of an elaborate, multi-generational, movement that may exceed 18,000 km (11,185 mile) in total distance and which requires individual insects to journey over 6,000 km (3,728 mi), including a 3,500 km (2,175 mi) non-stop leg over the open ocean. 

How do they do this? By following the winds. This is not a simple thing to do though as often winds are blowing in the opposite direction that they need to travel. So they exploit weather fronts, often flying at altitudes exceeding 1,000 m (3,281 ft). Their large, long wings make them very strong fliers; they've even been seen in the high mountains of the Himalaya at elevations exceeding 6,000 m (19,685 ft). The globe skimmer, it turns out, has the longest known migration of any insect! 

A Globe Skimmer in Oahu, Hawaii (Photo credit: Forest and Kim Starr)

I remember how surprised I was when I first saw dragonflies on Midway last summer hovering and chasing each other just outside my house. It led me to do some research on them which turned out to be extremely enriching.

Decades of inadvertent introductions of exotic insect species – arriving in food, plants, soil, or other items brought to the atoll – have had significant effects on the arthropod fauna of Midway. Over a dozen surveys of insects and other arthropods have been conducted at Midway, beginning with a study done by Henry Palmer and  George C. Munroe in 1890, creating a long-term record of the extensive introductions (and even some extinctions) that have occurred.

No dragonflies were encountered until Gordon Nishida noticed the Globe Skimmers while conducting an extensive arthropod study in the late 1990s. Because of this I had thought, at first, that the skimmers probably weren’t a native species but Nishida listed them in his report as being “indigenous”, which is basically to say that they occurring here at Midway naturally.

All dragonflies need fresh water in order to reproduce. Globe Skimmers traveling between India and Africa sometimes stopover in the Maldives and other islands that lack freshwater, perhaps to rest before moving on. Are the Midway skimmers breeding here or are we just a stopover site? Two pieces of evidence suggest the former. That the Globe Skimmer was not known at Midway until 1998, combined with the fact that this date corresponds well to the introduction of Laysan Ducks to Midway, an effort that required the construction of artificial freshwater seeps and ponds, argues pretty well that they are breeding on Midway. This shouldn’t be too hard to verify – just some aquatic invertebrate surveys. I will admit to not having put enough study into the situation. An hour long sit-down down by one of the ponds might be enough to yield an answer as Globe Skimmers, like many other dragonflies remain connected to each other during their mating flights.

And if these dragonflies are breeding on Midway in manmade ponds, does that really mean they are native as was suggested by Nishida? I guess it depends on your criteria, but in either case it is a good illustration of some of the existential challenges faced by biologists working in these kinds of island ecosystems.

The Globe Skimmer has been featured on postage stamps from several nations, including Botswana and the remote Pitcairn Islands, a British Territory in the South Pacific.

A recent genetics study by Daniel Troast and others showed that the peregrinations of Globe Skimmers are so extensive that it may represent a single worldwide population among which genes flow freely. A truly global insect!

Two additional dragonfly species have been recently observed at Midway by Forest and Kim Starr, the Green Darner (Anax junius) and Black Saddlebags (Tramea lacerata). Both of these species are also known to migrate long distances but probably never stopped for very long at Midway prior to recent times when surface freshwater ponds and seeps were constructed. Similar to plants, the insects of Midway tell a story of continual change and illustrate how effectively species can discover and adapt to changing conditions, abilities that will be as important as ever in a time of rapid environmental change.

Postscript: If you are interested in learning more about the Globe Skimmers I recommend you check out the TED Talk by Charles Anderson. Also, this is the first time I've posted a story without a single photo that I've taken. Turns out my photographic skills and patient were not sufficient to capture a decent image of one.


  1. Wow, this is a fascinating blog post! I had no idea that Globe Skimmers made such an enormous migration--what a unique species to have on Midway. I'm definitely going to check out the seeps and wetlands and take a closer look at the skimmers. Thanks!

  2. Very interesting! Such hardy creatures!