Luna moth tails have only one purpose – evading bats

In a groundbreaking investigation into the world of Luna moths, Actias luna, researchers have conducted two complementary studies to explore the role of the insects’ tails in predation and sexual selection. 

Previous studies have shown that Luna moths, along with other silkmoths, utilize their lengthy, trailing tails to evade bat attacks. These studies aimed to delve deeper into the potential benefits or hidden costs of these tails.

Juliette Rubin, a doctoral student at the Florida Museum of Natural History and lead author of both studies, described the structure of Luna moth tails: “They have projections off the back of the hindwing that end in twisted, cupped paddles. From experimental work with bats and moths in a flight room, we’ve found that these structures seem to reflect bat sonar in such a way that bats often aim their attacks at the tails instead of the main body.”

Rubin’s curiosity led her to question whether the twisted tails of Luna moths could serve additional purposes or come with hidden costs. This phenomenon, where traits evolve for one specific function and are then co-opted by natural selection for another, is not uncommon in nature. 

Silkmoth tails, for instance, have evolved independently across three continents and vary significantly in length. The longer the tail, the more likely the moth is to successfully evade a hunting bat.

Yet, silkmoth tails are not simply utilitarian decoys for bats; they are often visually stunning, akin to decorative streamers trailing behind a kite. Many structures in the animal and plant kingdoms have dual functions, such as the vivid colors of strawberry poison dart frogs (Oophaga pumilio), which both deter predators and help males attract mates. 

Similar dual-function traits can be found in male deer and other ungulates, which use their antlers for fighting off rivals and signaling their vigor to females. Even some moths that use clicks or chirruping sounds to disrupt bat echolocation use the same sounds during courtship.

Male Luna moths don’t need their tails to charm females

Luna moths, however, lack mouths to produce sound and ears to hear it. Instead, they rely on their sensitive eyes and powerful scent-detecting antennae. When female Luna moths are ready to mate, they emit a pheromone that triggers a response in the male antenna even at the molecular level. Males of the closely related Indian moon moths (Actias selene) can find females from over six miles away by following the pheromone plume to its source.

Rubin shared her thoughts on this process: “We don’t know how many males are traveling to a female each night. It’s entirely possible she’s able to call in multiple suitors and potentially have her pick.”

To test this hypothesis, Rubin conducted mating experiments involving a female Luna moth enclosed in a flight box with two males: one with normal hind wings and the other with its tails removed. Initial data suggested that females preferred males with intact wings. 

However, further controlled experiments showed that this preference was likely an incidental effect of tail removal. When both males had their wings clipped, and one had its tails glued back on, no difference in mating success was observed.

This fascinating research into the dual roles of Luna moth tails sheds light on the intricate world of these insects, revealing the interplay of predation and sexual selection in their evolution.

Do Luna Moth tails cause same confusion for birds?

The long tails of these moths have previously been shown to effectively deflect bat attacks, but Luna moths also need to avoid visually oriented predators such as birds that hunt during the day. Their electric green tails with bright pink borders could potentially make them more noticeable to such predators.

Similar trade-offs are observed in other organisms, like fireflies, whose bioluminescent displays help males locate potential mates but also make them more visible to nocturnal predators such as frogs and geckos.

Juliette Rubin, a doctoral student at the Florida Museum of Natural History and lead author of both studies, noted the brief life span of Luna moths: “This creates a very intense period of adulthood, where surviving the night is of the utmost importance.” These moths have about a week to find a mate and reproduce after emerging from their cocoons.

During the day, Luna moths are mostly inactive, reducing their chances of being caught midair. However, if they fail to adequately conceal themselves, they risk not surviving until nightfall. Rubin sought to determine if the visually elaborate tails of Luna moths put them at a disadvantage in this high-stakes game of hide-and-seek.

To investigate this, Rubin and her colleagues created moth replicas by wrapping mealworms in pastry dough shaped like Luna moth bodies and attaching real wings, half of which had tails. They partially hid these replicas among branches and leaves in an aviary and introduced Carolina wrens (Thryothorus ludovicianus), recording how many of the snacks the birds located and ate.

According to Rubin, the results showed that the tails had no effect on the birds’ ability to locate the fake moths. This may seem counterintuitive to humans as visually oriented animals, but there is evidence suggesting that birds might rely on search images when trying to distinguish food items from patterns in the background.

Rubin compared this process to humans searching for Waldo in a Where’s Waldo puzzle, explaining that people often look for the characteristic red, horizontal lines of Waldo’s shirt while scanning the page. It is possible that Luna moth tails don’t match the typical moth and butterfly patterns that birds expect to see while foraging, akin to Waldo wearing a solid red shirt instead of his signature stripes.

The studies, which were published in the journals Biology Letters and Behavioral Ecology, suggest that the complex structures of Luna moth tails evolved for a single function. 

Rubin emphasized the importance of testing assumptions about conspicuous traits in animals: “A trait that’s obvious to us, as visual creatures, might not stand out to the predators that hunt them, and the traits that we think are dynamic and alluring might not seem that way to a potential mate.”

Co-authors of the studies include Akito Kawahara of the Florida Museum of Natural History, and Nich Martin and Kathryn Sieving of the University of Florida.

More about Luna moths

Luna moths (Actias luna) are a striking and fascinating species of moth native to North America. They belong to the Saturniidae family, which is known for its large and beautifully colored moths. Here are some interesting facts about Luna moths:

1. Appearance: Luna moths are known for their stunning appearance. They have a wingspan of 3 to 4.5 inches (7.6 to 11.4 cm) and display a unique lime-green color. Their hindwings have long, elegant tails, and both the forewings and hindwings exhibit eye-shaped spots, which are thought to deter predators by mimicking the eyes of larger animals.
2. Life cycle: Luna moths go through four life stages—egg, larva (caterpillar), pupa (cocoon), and adult (moth). The entire life cycle lasts for approximately two months. Female Luna moths lay their eggs on the leaves of host trees, and the emerging caterpillars feed on these leaves. Caterpillars of Luna moths mainly feed on the leaves of walnut, hickory, and sweet gum trees. After a few weeks, the caterpillars spin silk cocoons around themselves, where they transform into adult moths.
3. Short adult life: Adult Luna moths have a remarkably short life span of about one week. They don’t have functional mouthparts, which means they don’t eat during their adult stage. Their sole purpose in this stage is to find a mate and reproduce.
4. Nocturnal behavior: Luna moths are nocturnal creatures, meaning they are active during the night. Their primary predators are bats, and their long tails have been shown to help them evade these predators by deflecting bat attacks.
5. Mating and pheromones: Female Luna moths release a pheromone to attract males when they are ready to mate. Males have large, feathery antennae that can detect these pheromones from miles away. Once a male locates a female, they mate, and the female lays her eggs shortly afterward.
6. Conservation status: Luna moths are not considered endangered or threatened, but they are becoming less common in some areas due to habitat loss and light pollution. As nocturnal creatures, they rely on the darkness of night to navigate and find mates, so increased artificial light can disrupt their natural behaviors.

Overall, Luna moths are remarkable insects, admired for their beauty and unique adaptations. Their fascinating life cycle, nocturnal behavior, and captivating appearance make them a truly intriguing species to study and observe.

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