We’ve already heard (previous post) about the weird hunting habits of Pyractomena larvae – they’re amphibious. So let’s continue our Pyractomena tour, moving along to consider another strange behavior: they pupate en plein air!
Once they’re ready to metamorphose into adult fireflies, the larvae of most North American lightningbugs seek safety underground. Immobile, they huddle for weeks in soil chambers, finally emerging to light up our skies as adult fireflies. But no cold dark cellars for Pyractomena fireflies! Instead, these strange fireflies climb up onto vegetation to undertake a feat known as aerial pupation.
Pyractomena borealis, widespread across eastern North America, is one of our earliest spring lightningbugs. They can be seen flashing in the still-leafless treetops during March & April in southern locations, late May in New England and eastern Canada. In late winter, their fully grown larvae start climbing up the trunks of particularly large trees with furrowed bark. Seeking out a sunny spot, the larva nestles into a crevice and glues its tail onto the bark. Securely fastened, its head hanging down, it will remain attached and immobile for 2-3 weeks as it undergoes an astonishing transformation into an adult firefly.
So what’s up with aerial pupation? It seems quite risky compared to doing it underground. A typical firefly pupae inside its underground chamber gains more protection from dessication and from temperature extremes. Why did this unusual behavior evolve in just this particular kind of firefly? Firefly expert Jim Lloyd suggests that frequent springtime flooding of their marshy adult habitats has selected these larvae to seek higher ground.
As they climb up the trees, these mottled brown larvae can be pretty cryptic. Yet if you look carefully, you may spy one – once you see one, keep searching the same tree and you’ll likely find several more. In fact you’ll be able to find them year after year, as they’ll keep returning to the same colony tree year after year.