Masters of deception
Clearwing moths survive by resembling other creatures
On Sunday night, I decided to stay up and watch the Perseid Meteor Shower. It had been many years since the weather cooperated enough to give a chance at seeing them and I was rewarded for my effort. The sky was clear, the Milky Way was clearly visible and thousands of stars decorated the nighttime sky. It was just after midnight that many of them started to suddenly streak into motion.
Long before the festivities began, I was out on my porch enjoying the sounds of a beautiful midsummer evening. There are only a few birds that stand out in the crowd at this time of the year and they all seemed to be active. Mourning doves were singing their owl-like songs, cardinals were belting out their clear-whistled notes and from down in the meadow the occasional towhee suggested that I should drink my tea. Do those guys have a deal with Bigelow, or what?
At this time of year, the insects dominate more and more of the sounds. It started with the field crickets a few weeks ago, but as time has gone by, and the Earth sped toward the debris field that would eventually produce the shooting stars that I stayed up to watch, many other “voices” have joined the fugue. At this point, there are probably four different types of katydids and another half-dozen different crickets making noise out there. Some of them even persist in their singing during the daytime hours, which makes the landscape feel especially drowsy.
But what really caught my attention on Sunday evening, just as the light was starting to fade, was not a sound at all. It was dusk and I was sitting in an extremely comfortable chair and listening with my eyes closed. From time to time, my eyes would open and stare, half-lidded, at nothing in particular as I concentrated on the sounds of the insects around me. It was during just such a moment that I saw it.
I have several flower boxes up on my porch railings and one of them is filled with the most beautiful petunias. Royal purple and as sumptuous as velvet, these flowers have been blooming for weeks and have caught the attention of the local hummingbirds. So I naturally thought it was a hummingbird that was making one last visit to the flowers before turning in for the night. But there was a problem.
This was no hummingbird. It was already too late for the hummingbirds to be active and then there was the fact that there was no sound of wings humming. After long hours of observation, I have learned that hummingbirds can make a lot of noise with their wings if they want, but they can also tone it down and go into their closest approximation of stealth mode. There is no way that they can eliminate the noise altogether, however. So, like I said, this was no hummingbird.
What I watched that night was a moth. To be a little more precise it was a sphinx moth, but you’ll have to settle for just a little more precision. There are about 40 species of sphinx moths that can be found in our area and without capturing and examining the individual that hovered around my petunias, I cannot tell you exactly what kind of sphinx moth it was.
With so many species in the family (which is known as Sphingidae, if I haven’t already mentioned it), there is plenty of variation among the species. Still, there are enough similarities to allow even a casual observer of moths to identify a sphinx moth. At rest, the wings are generally narrowly triangular and held out from the body. This means that the body itself can be seen almost in its entirety. The shape of the body is a very elongated and sharply tapered oval.
Rather than fluttering, like a butterfly, sphinx moths have adopted a much more high-energy flight of rapid wing beats that is best compared to a hummingbird. These moths can hover, fly backward and gently move into feeding position in front of a flower without necessarily having to land on it; valuable skills for moths that feed on the nectar in flowers. But wait, there’s more.
While most sphinx moths are nocturnal, there are a couple that come out during the daylight hours. This is a rather unusual habit for a moth and it does come with some risk. The flowers they feed on are generally out in the open and their hovering technique does leave them somewhat exposed to avian predators. So how does a moth go about protecting itself? The answer is simple: Deception.
There is one species of sphinx moth that has made its best attempt at looking like a hummingbird. The antennae sticking out of the head are somewhat problematic, but the moths do attempt to replicate some of the hummingbird’s green in the hairs on its back. They even have tail “feathers” that splay out from the end of the abdomen in a fashion that actually does resemble the tail feathers of hummingbirds that are changing directions quickly. This particular species is actually called the hummingbird moth (Hemaris thysbe).
Another species (Hemaris diffinus) appears to be making its best attempt at resembling a bumblebee. This species is the same basic shape and size as the hummingbird moth, but instead of green this moth has adopted the striking black-and-yellow markings of a bumblebee. What doesn’t make a lot of sense is the fact that this moth also has tail “feathers.” I suppose the coloration has been so effective that the moth hasn’t experienced enough evolutionary pressure to lose the feathers. But wait, there’s more.
Another adaptation that assists in their deceptive ways is the fact that both species have clear panels in their wings. Flies, wasps, dragonflies and beetles all have transparent wing membranes, so it seems that opaque wings are more of the exception rather than the rule in the insect world. Moths and butterflies use their wings for a variety of things, including communication, defense, and camouflage, so they have added scales to their wings that give them color. Thus, a secretive moth can lay its wings flat on the bark of a tree and hide itself under its own camouflage tent.
Daytime species, stuck out in the open, can’t use their wings to hide under. Instead, if they are going to try to imitate species like hummingbirds (which have wings that move with blurring speed) or bumblebees (which have transparent wings that may move even faster) then it should stand to reason that wings with the least visibility would be the most helpful. Enter the “clearwings.”
A completely different family of moths, the clearwings (family Sesiidae) are another group that specializes in imitation. The only difference is that all of the 14 species focus on imitating one type of wasp or another. They have the same coloration and the same transparent wings as wasps do, but there is always the exception to the rule. Some clearwings have wings that aren’t particularly clear.
So as the summer starts to wane, you should definitely keep your eyes glued to any flowers that might grow in your yard. Both of the daytime sphinx moths are active until September. The hummingbird mimic is sometimes called the hummingbird clearwing, while the bumblebee mimic is known as the snowberry clearwing. Just remember that while both have clear wings, neither is a clearwing.
Bill Danielson has worked as a naturalist for 16 years. In that time, he has been a national park ranger, a wildlife biologist and a field researcher. He currently works as a high school chemistry and biology teacher. To contact Bill, or to learn more about his writing, visit www.speakingofnature.com