Hairstreaks are a group of butterflies in the Family Lycaenidae. In the US, hairstreaks are widespread but frequently go unnoticed because of their small size and often drab appearance. When they are noticed, they’re commonly mistaken for moths. Their fast, erratic flight lends to this confusion. If you get the opportunity to observe a hairstreak closely, though, it’s worth the effort.
Like all butterflies, hairstreaks are susceptible to predators unless they develop some type of strategy to reduce the likelihood of being noticed, caught, and eaten. Birds, lizards, and other invertebrates like mantids and spiders are just some of the predators that butterflies have to be able to evade if they are going to survive long enough to pass on their genes. Many hairstreaks show some degree of crypsis, or coloration that helps them to blend in with their backgrounds. A few, like the Colorado Hairstreak Hypaurotis crysalus are cyrptic on the ventral surface (underside), but vibrantly colored on the dorsal surface (upperside). This bright purple hue is most likely an adaptation to distract predators, although colorful signals like this can often be used to signal to other butterflies- potential mates, competitors for mates, or individuals of a different species.
Many hairstreaks, however, have gone beyond the standard cryptic coloration mechanisms to evade predators. The trailing (back) edge of their hind wings exhibits a “false head“. Typically this false head consists of some combination of eyespots and “hairs” or tails resembling antennae and/or legs. The idea is that if a predator notices the hairstreak, its attention will be drawn to the false head and it will aim its attack there, because a successful attack on the head is more likely to incapacitate the butterfly than an attack on another part of the body. Instead, the predator comes away with a mouthful of flimsy wing, which breaks off while the butterfly makes its escape. Experimental evidence (link above, reference below) suggests that this adaptive strategy is successful in increasing survival in butterflies that have this trait.
As if that isn’t cool enough, though, many hairstreaks have taken this deception even further. Hairstreaks have a habit of perching with their wings closed, upside down, and rubbing the trailing edges of their hind wings together. This subtle movement is enough to further direct the predator’s attention to the false head, and it strongly resembles the movements made by the butterfly’s head while she sips on flower nectar.
Keep your eyes open for hairstreaks the next time you’re outside. If you’re fortunate enough to see one, take the time to observe it while it perches and see if you get tricked by the false head! Want to learn more about the research on false heads? There are several great studies out there. Here’s just one (linked above):
R. K. Robbins 1981. The “False Head” Hypothesis: Predation and Wing Pattern Variation of Lycaenid Butterflies. American Naturalist Vol. 118, No. 5 pp770-775