Clearwing moths are mimics of bees and wasps. (Photo by Jeff Mitton)
By Jeff Mitton
Flowers appeal to the human sense of aesthetics -- they are ornate structures with beautiful colors and delightful fragrances.
But naturalists have more reasons to stop, smell the flowers and inspect them. Nutritive awards in pollen and nectar attract butterflies, beetles, bees and flies.
The host of pollinators, in turn, attracts predators such as crab spiders and assassin bugs that lie in wait, using the flowers as lures.
I visit flowers not only for their aesthetics, but also to see which pollinators are attending and who is hunting. Over the years, my curiosity has been most richly rewarded by showy milkweed, Asclepias speciosa.
At the mouth of Gregory Canyon, a strand of milkweeds in full flower caught my attention. I found one of them was littered with dead honeybees; an assassin bug sat motionless near the uppermost flower. On another milkweed I saw an insect that I mistook for a wasp. After closer inspection I knew it was not a wasp, but I had no idea what it was.
The antennae were iridescent blue, of medium width and gently curved at the ends. Segments of the wings were clear, while the margins had iridescent coral scales and the trailing edges of the wings were fringed with hair. The legs were yellow with conspicuous tufts of hair. The body was predominantly black with several bands of yellow hairs. The most perplexing aspect of the morphology was a fan-shaped tail of hair, mostly black but with a yellow fringe.
After consulting with my colleagues, I learned that the mysterious insect was a clearwing moth. The family Sesiidae contains 1,100 described species of clearwing moths, some of them well known because they are horticultural or crop pests. The larvae burrow into the stems or roots of trees, shrubs and some crops, damaging or killing them. Many have common names referring to their primary hosts, such as peach tree borer, currant borer and viburnum borer.
I have only seen two clearwing moths. The second was also on showy milkweed, in the meadow west of the National Center for Atmospheric Research buildings. I am still not certain which species these were, for I did not collect them and therefore we have only photographs to work with.
Identification is further complicated by the fact that some of the species have considerable geographic variation, changing pattern and color from place to place. The individual in the photo is probably a strawberry crown moth,Synanthedon bibionipennis, but might also be a dogwood borer, Synanthedon scitula.
Clearwing moths are convincing mimics of bees and wasps; at my first encounter with a clearwing moth, I thought it was an unusual wasp. In some species, the legs have conspicuous tufts of hairs that are tipped with yellow scales. These structures look very much like the pollen baskets on the hind legs of honeybees. The clear wings and black and yellow banding lead naturalists and predators to classify clearwing moths with bees and wasps.
Poisonous stings punish naive predators that attack bees or wasps; experienced predators have learned to avoid the nasty insects with clear wings and yellow and black bands around their bodies. By mimicking stinging insects, clearwing moths avoid many predators and are thus able to fly during the day with impunity. Remember that most moths fly at night.
The fantail still perplexes me. Butterflies and moths do not have fan-shaped tails made of hairs. So perhaps the fantail developed during the evolution of mimicry, in the same way that clearwings evolved tufts of hairs and scales on their legs to mimic pollen baskets. But neither bees nor wasps have tails extending from their abdomens. So why do clearwing moths have fantails?
Easter daisies are already blooming, and I saw a leafhopper on one of them. The first pasque flowers will appear soon.
Jeff Mitton (mitton@colorado.edu) is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.
January 2012