Jewels of the Night

        Moths are not at all butterflies ugly cousins. Often when mentioned, the first thing many people think of are dusty little creatures flapping around a dim light or camouflaging against dull colored branches. But moths are by far from boring. And they are far from ugly.

        To give just one example of these exceptional Lepidoptera we will look at North American’s native Actias Luna, more commonly known as the Luna Moth or the American moon moth. Like other moths in the family Saturniidae, A. Luna was named after Greek and Roman mythology, specifically the goddess of the moon. These silk moths are characterized physically by their long “tails” extending from their hindwings wings and their various shades of green to yellow. Additionally, these moths are far from little, on average their wings span about 4.5 inches.

            Luna moth compared to a human hand

If you were to observe A. Luna across North America for a few years, you would find that they are univoltine in Canada, meaning they have one generation per year, but in more Southern places like the United States they can have two or three generations per year. Additionally, the more generations per year the more yellow their wings get per generation. This means the Northern Canadian Luna moths have more blue-green tints, while the overwintering Southern Luna moths who may be the second or third generation of the year, are more yellow-green.

As with any moth, they begin as eggs, which are about 1.5 mm and can be found on the undersides of leaves. Once they hatch, they progress through 5 instar stages with each instar getting longer and longer. By the fourth or fifth instar these caterpillars often begin to display a line of colorful dots down their sides. A. Luna caterpillars do have some defenses. As a warning, they can make clicks with their mandibles, then as the defense they can expel intestinal contents from their mouths, which has been seen to deter some of their predators.

   

                                                              Fourth or fifth instar with circle pattern

        As herbivores, these larvae feed on several species of broadleaf trees, primarily white birch (Betula papyrifera), persimmons (Diospyros virginiana), sweet gums (Liquidambar styraciflua), hickories (Carya), walnuts (Juglans), and sumacs (Rhus). They are able to eat plants with a lot of juglone, like walnuts, because their digestive systems have high concentrations of enzymes that neutralize the juglone.

Though usually green, when the larvae are preparing to pupate, they may become a reddish-brown color. Interestingly, once the fifth-instar goes to the ground and makes its silk cocoon, it uses more silk to attach dead leads to the exterior of its cocoon to aid in camouflage. Generally, they emerge from their pupa in the evening, since they must allow several hours for their wings to gain their full size and since they are nocturnal, then they will be ready to fly by night. Adults do not eat because they have a vestigial proboscis, instead they rely on fat stores for energy that they built up as caterpillars for the 4-7 days they live as adults (inaturalist, Chesapeake Bay Program).

There is very little sexual dimorphism between adults at first glance, but females tend to have larger abdomens because they contain hundreds of eggs and males tend to have longer and wider antennae, which help them detect sex pheromones that females release even from miles away (inaturalist).

Male vs Female A. Luna antennae

One danger to many insects, are other insects, namely, parasites. Interestingly, the parasitoid most damaging to A. Luna isn't native, nor is A. Luna it's only host. It is a tachinid fly, Compsilura concinnata, from Europe which was introduced to North America in order to try and manage the gypsy moth (Lymantria dispar) population, but over time C. concinnata expanded to other moth hosts, including the beloved Luna moth. Though this parasite kills them, it isn't to a degree that would drive A. Luna to extinction. In fact, A. Luna is a fairly stable species. Though they are somewhat affected by humans because of light pollution and urbanization, but they are still secure in status (NatureServe Explorer).

                                                                                Hindwing Tails

Another danger to A. Luna, are predators, specifically owls, birds, and bats. However, these moths have developed adaptations to give them an advantage over their hunters. To counteract echolocation, they have developed a false target of sorts. The “false target” hypothesis explains how the tail on the hindwing evolved as a way to reduce predation by bats. Since their hindwing tails are long and spin, then this causes excess movement that the bats echolocation picks up on more easily. This makes the bats more prone to attacking the moths tail, which doesn't serve any other purpose than to be a distraction. Experiments have been done supporting this hypothesis where some A. Luna kept their full wings, while the tail portion of other A. Luna were removed. And it was seen that bats did attack the hindwing tail more frequently than the body, indicating that this tail is advantageous in this way. Additionally, the circles on their wings, often called eyespots or moon spots, are also thought to confuse or scare off potential predators, like owls or other birds (inaturalist, Chesapeake Bay Program, TrekOhio).

Not only are these moths beautiful, but their beauty is their defense. Their elegant tail is a sacrifice, their colorful patterns a threat. And to humans, they pose no threat, instead they ought to be treated like jewels of the night.

Resources

https://www.inaturalist.org/taxa/47916-Actias-luna (inaturalist)

https://explorer.natureserve.org/Taxon/ELEMENT_GLOBAL.2.109121/Actias_luna (NatureServe Explorer)

https://www.chesapeakebay.net/discover/field-guide/entry/luna-moth (Chesapeake Bay Program)

Images

//www.inaturalist.org/taxa/47916-Actias-luna/browse_photos (inaturalist photos)

https://trekohio.com/2015/05/06/luna-moth-male-female/ (TrekOhio and male vs female photo)