Luna moth (Photo credit: John Obermeyer/Purdue Entomology)
Luna moth (Photo credit: John Obermeyer/Purdue Entomology)

As an entomologist living in the central part of Indiana, there is always one thing I can count on in late May or early June. That would be a phone call or an e-mail, almost always beginning with something like, "There's a humongous moth hanging on the wall by the porch light."

Humongous is not a word I use on a regular basis. But I do hear the term mentioned on occasion. Apparently, humongous entered the general lexicon in the late 1960s and has been described as a fanciful combination of huge and monstrous. For sure, anyone who uses the term is thinking about something larger than ordinary and maybe a little on the scary side.

To be honest, when I hear the word humongous used to describe a moth, the first thing about is the science fiction classic Mothra. This 1961 Japanese film was inspired by radiation from an atomic blast. In the film, a giant egg - an insect egg as it turns out - is discovered on a remote, tropical island. After being transported to Japan, the egg hatched into a very large caterpillar. In spite of the best efforts of the Japanese military, that caterpillar almost destroyed Tokyo before it metamorphosed into a gigantic moth.

Of course, real moths do not approach the size of the radiation-induced dimensions of the sci-fi Mothra. But a few species of moths would be what some people might consider humongous. At least, they are when compared to most moths that humans encounter.

These unusually large moths are classified in the family Saturniidae and are called giant silkworm moths. There are several species native to North America, and four are some of our largest moths. In addition to large size, these moths also appear imposing because they have wings marked with transparent eye spots.

The largest of our native giant silkworm moths is the cecropia. It is reddish-brown in color with a wingspan of 5 to 6 inches. The middle of each wing has a crescent-shaped white spot. In addition, the tip of the forewing’s shape and eye spot gives the illusion of the head of a snake. Such a moth is not as imposing as Mothra, but definitely a creature that would attract attention.

Another of the giant silkworm moths is the promethea. This moth is named after Prometheus, one of the Greek Titan gods. The promethea is smaller than the cecropia but the female is similar in color and marking pattern. The male, however, is much darker colored and exhibits the somewhat unusual moth behavior of flying around during daylight hours.

Also in this group of moths is the polyphemus. The polyphemus is a brown-colored moth that has a large purple-colored eyespot on each of the back wings. This moth is also named after a giant in Greek mythology. In this case, Polyphemus was one of the Clycopes described in the Odyssey.

Probably one of the most recognizable of the giant silkworm moths is the luna. This is a lime-green moth that has long tails on the hind wings. It gets its name from the crescent-shaped marks on each of the forewings that resemble a quarter moon configuration. This moth has very feathery antennae and is often attracted to lights at night. The luna moth has become the symbol for advertisements of a common sleeping medication. There aren’t many insects used to hawk modern products, but I have to concede that the choice of the luna moth to promote a sleeping aid makes sense.

As you might imagine, the caterpillars of full-grown, giant silkworm moths are large. These caterpillars are also armed with dangerous-looking tubercles and spines. While these caterpillar protrusions are not harmful, they appear that way. For that reason, these caterpillars will attract attention, especially when they leave their food plants and crawl around looking for a place to spin a cocoon - using silk, of course.

One interesting aspect of the biology of the giant silkworm moths is that the adults do not have functional mouthparts. That means that these moths do not feed. These moths are also well-known for the fact that the females produce mating-attractants called pheromones. Male giant silkworm moths will sometimes fly a mile or more toward the females, following the pheromone trail carried by the wind. And on an empty stomach to boot!

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