The night is clear and calm. Temperatures in the 50s makes for a mild September nocturnal walk. The moon, in its waning quarter phase, has not yet risen when I’m here at 9 p.m.
Overhead, the celestial show demands attention. I quickly see constellations of Big Dipper, Little Dipper and Cassiopeia. I also note views of our galaxy, the Milky Way. With the help of binoculars, I’m able to peer into deep space and find another galaxy: the Andromeda. I’m glad to be walking where all of this star gazing is visible.
A few sights and sounds tell me that I am not alone. Some late-season moths fly about. A toad slowly hops across the road in front of me as does a scurrying deer mouse. In the nearby woods, the local barred owls demonstrate their various calls to each other.
Off in the surrounding fields, a coyote family, now with partially grown young are celebrating the season with calls of their own. And there is more.
Not a sound, but a dim light is emanating from the roadside grasses. I pause for a closer look and reveal a small dark segmented critter, less than a half-inch long: a glowworm.
Living up to its name, it is producing a light from its body on this early autumn night. Glowworms are the larval form of the well-known fireflies (lightningbugs). Like the adults, which are neither a fly or a bug, the glowworm is not a worm. Instead the glowworm is larva (young) of the firefly.
They are a type of beetle. The life cycle of this beetle, like many insects, goes through different stages in what is known as metamorphosis. The eggs hatch in summer. Larvae that crawl from these eggs are out here now in the soils of autumn and will survive the winter under the snow as larvae. Next spring, they form the pupa stage and then emerge as adults.
We don’t usually see the larvae because they are dark, small and on the ground, but the adults are often seen. During nights of June and early July (some years, I have seen them in May and one species can be seen flying in late summer), they shine their living lights as they fly over fields, swamps, meadows and roadsides.
Typically, the males take wing and advertise their presence to females with a series of flashes, varying with the different species. If she, on the ground, likes what she sees, she turns on her light to invite him to come closer.
Animals producing light, bioluminescence, are fairly common in the ocean depths, but unusual on land. (It seems like the only other bioluminescence that I see on these dark walks is from some glowing fungi.) For the adult firefly, the light is used as a mating ritual.
Like other insects, fireflies can only mate when they reach maturity and develop the proper light-making organs and wings. Since larvae do not mate, the explanation of why these glowworms that I now see are producing light is a little harder to understand.
One believable theory is that a glow from the ground at this time is unusual and may be enough to confuse and deter a potential predator. Besides the glowworm imitating the adult by having a light, it also feeds like the adult.
Glowworms are predators on minute insects and snails. And so, if you see a dim light on the ground during a night walk now, you may be seeing a glowworm as it discourages predators while preying on others.