Saturday, October 29, 2011

Citrine Forktail

I photographed this beautiful albeit miniscule citrine forktail, Ischnura hastata, a few weeks ago in southern Ohio. These tiny damselflies are so small that it is quite easy to overlook them. This is the smallest odonate found in North America. A big citrine forktail is barely over an inch in length, and they're prone to lurking in dense vegetation where they fade and appear like will-o'-the-wisps. This one - and a number of others - was hunting in a cedar glade prairie, where it engaged in typical damselfly hunting strategy. Unlike the incredible aerial acrobatics of their brethren, the dragonflies, damselflies patrol low amongst the vegetation, employing a rather sluggish and jerky flight style. They grab tiny bugs from the foliage in a pounce and pick hunting style.

Proportionately huge many-faceted eyes mean that the citrine forktail overlooks very little that enters its sphere. This is a successful damselfly: citrine forktails are common throughout all but the northwest quarter of the United States, and range throughout the Caribbean, Central America, and even dip into South America.

There is a bizarre and noteworthy blip in the citrine forktail's distribution and life history. This map (courtesy Wikipedia) shows the Azores archipelago outlined in red. The Azores are a chain of nine volcanic islands positioned near the middle of the North Atlantic, nearly 2,500 miles east of North America's Atlantic coast. Colonized by sea-going Portuguese explorers in the 15th century, the Azores are home to some 240,000 people.

And, oddly enough, citrine forktails. Some authorities think that these forktails arrived recently, perhaps at the tail end of the 19th century. How did such a seemingly fragile, weak flyer manage to cross over two thousand miles of rough North Atlantic waters? Probably impossible to say with certainty, but several other species of tiny odonates - including other Ischnura forktails - are well known island colonizers. It seems likely that these insects are capable of getting swept up in storm cells and can survive lengthy if unintended aerial journeys to new lands.

But the tale of the Azores' citrine forktails gets much stranger. Researchers began intensive studies of this disjunct forktail population about twenty years ago, and quickly realized that all of the animals were female! It turns out that Azorean citrine forktails reproduce entirely by parthenogenesis, which is a type of asexual reproduction. Many species of animals are known to reproduce by parthenogenesis, and the various mechanisms by which this works are complicated. Thelytokous parthenogenesis is the term for the forktail's reproductive strategy. Suffice to say that embryonic growth and development is possible without direct contribution of sperm from males.

I hope that thelytokous parthenogenesis does not find its way into Homo sapiens.

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1 comment:

Anonymous said...

Very interesting! Do the citrine forktails in the west still get it on or are they asexual across the world?