Summer #9: Fireflies

A couple of weeks ago Marilou and John McNavage sent me a newspaper article about the fireflies in the Allegheny National Forest. The firefly community in the Allegheny Forest is one of the richest and most diverse in the world! Fifteen species of firefly including the rare “synchronous firefly” (Photinus carolinus) can be found within the boundaries of the forest! One of these fifteen species, and a very abundant species of firefly around here, is the official state insect of Pennsylvania, the Pennsylvania firefly (Photuris pennsylvanica). (Firefly image by James Jordan, Flickr).

“Fireflies,” though, are not really flies at all. They are beetles in the family Lampyridae that along with several hundred other closely related “firefly” species, have the remarkable ability, in all of its life stages, to biologically generate light. The adult beetle, which is the form most familiar to people, is � to � inch in length, with a flattened body that is predominately black in color with yellow highlights and prominent red spots on the back of its thorax. It has large eyes and long antennae and flies in a gentle, hovering manner. The light generating parts of these adults are in the terminal segments of their abdomens. The adult firefly has long, curved mandibles that suggest a predaceous life style, but only a few species have been shown to actually consume anything other than flower nectar or pollen during the very short adult portion of their life cycle. The less well known larvae of the firefly, called “glowworms,” live in leaf litter and are voracious predators. They eat other insects, mites, earthworms, and even slugs and snails.

The lights of the fireflies are used for communication. The female fireflies, which are predominantly sessile, perch on the vegetation and generate a species specific sequence of light flashes that attract the much more mobile males. The males respond with an answering light sequence and zero in on the females in order to mate. A few species of firefly have been shown to mimic the light sequences of other species in order to draw unsuspecting males to waiting, predaceous females. These females not only gain energy from consuming the males of these other species but also can accumulate chemicals from their prey which help to protect them from their own predators. This behavior is called “aggressive mimicry.”

After mating in the late summer, the females lay their eggs one at a time on the surfaces of woody or leaf debris. The eggs hatch in a few weeks and the emerging larvae enter the soil/litter habitat where they actively feed on a wide range of invertebrates. In late fall, the larvae burrow into the soil or under the bark of woody stems where they overwinter. In the spring, they re-emerge and continue to actively feed on their diverse array of prey species. After a few weeks, they re-enter the soil and pupate. They then emerge from their pupal chambers in early to mid summer as adult fireflies.

The mechanism for the production of light in fireflies is mediated by the enzyme “luciferase.” High energy phosphates generated from food molecules are coupled via luciferase to the direct production of photons of light. This coupling is extremely efficient (90%+) and generates almost no waste energy (heat).  The genes that regulate this light generation have been used in cancer research to mark and track metastasizing cancerous cells.

There have been many reports of declining numbers of fireflies throughout North America. The widespread use of pesticides and herbicides, the loss of the leaf litter habitat required by larval life stages especially in suburban areas, and drought have all been proposed as factors in the decline of the firefly.
   
This summer, though, has been a very good season for fireflies in our woodlots and field edges. The magnitude and duration of their displays have been spectacular and more than a bit of a relief after a decade and a half of very low numbers. I am not sure if the abundance of these beetles this year is due to our very wet spring and summer or to some other local factor (like increased leaf litter for the beetle larvae) but their resurgence makes our summer nights truly special.