Natural History and Flash Repertoire of the Synchronous Firefly Photinus carolinus

A spectacular firefly display, called the "Light Show," occurs on June nights in Elkmont, Tennessee. Thousands of male Photinus carolinus fireflies flash in unison in a flash pattern of 4-8 flashes followed by 6-9 seconds of darkness. Meanwhile, females remain hidden on the ground or on low vegetation, responding to the males with a double flash about 3 seconds after the males' final flash.
This paper describes certain aspects of the natural history of the southern Appalachian population of P. carolinus.

Field marks for identifying P. carolinus

P. carolinus is mid-sized between the larger P. pyralis and the smaller P. macdermotti and P. marginellus. Both males and females have a black margin on the front of their pronotum (thorax covering) and, unlike similar species, have a completely dark underside except for the lanterns. The female is slightly wider than the male, with a half-moon shaped lantern on segment 6 of the abdomen, while the male has full lanterns on segments 6 and 7.
When flashing, males fly horizontally 3 to 9 feet off the ground, rising into the treetops with their final mating flash.

Life Cycle Observations

Eggs. Females in captivity laid an average of 24 eggs in up to four clutches. Eggs were laid individually on moss when available.
Larvae. The larvae live underground and were seldom seen in the wild. Larvae that were hatched from eggs laid in captivity 18 days earlier were fed earthworms. It is unknown if larvae go dormant in winter or burrow deeper in the ground. In May, during the last larval stage, the larvae appear under leaf litter, often near damp logs, where they pupate.
Pupae. Occasionally, an adult male can be seen guarding the pupa of a female in preparation for mating when she emerges as an adult.

Male and Female Flash Repertoire

P. carolinus exhibit five additional flash behaviors beyond those of their courtship flash pattern:
Female walking/flashing behavior. As well as the double flash response to the males, females displayed walking/flashing behavior, a double flash given every 1.2-2.5 seconds while walking briskly, even hopping or flying short distances, in the open. This behavior did not seem to attract males.
Male and female single flash courtship dialog. When approaching the female, the male switches from a multi-flash to a single flash as he begins to circle above the female, who at this point also often switches to a single flash. The male and female continue to alternate single flashes as the male lands and walks toward her. Once mating begins, the pair will generally cease flashing and seek shelter. Mating can last for 11 hours or more.
Chaos flashing behavior. Occasionally many males will cluster around a female and emit rapid single flashes. This chaos flashing lasts less than 10 seconds before ending abruptly. Within minutes of this flashing, the males are seen tightly clustered around the female, or stacked as high as 6 males on top of her, aggressively wrestling for dominance. Eventually one male succeeds. Often, females were seen rejecting these males, whereupon the male cluster reforms, but usually without the chaos flashing.
Male pseudo-female flash behavior. The male in the cluster who was rejected by the female was often seen crawling away emitting the double flash of the female. Other males were often attracted to this double flash and dropped from the air or abandoned the female they were courting to follow this male. After the rejected male had crawled some distance from the cluster, he would resume the typical male flash behavior once again.
Distress Flashing. P. carolinus caught in a spider web or by harvestmen emitted flashes repeated every 1.5-3 seconds. Distress flashes often attracted other males, who then became caught themselves.

Variations in Male Flash Signals

The male flash pattern had significantly more flashes later in the season after the females had emerged as compared to before female emergence.

Female Flash Behavior and Mating/Re-mating

When placed in captivity with males, females that had exhibited female walking/flashing behavior were just as likely to mate and lay eggs as females that had exhibited the normal courtship behavior.

Natural Enemies

Orb weaving spiders prey on P. carolinus. Late at night, after courtship had ceased, the distress flashes of fireflies caught in webs were the only light seen. Harvestmen were seen carrying both adults and pupae. Photuris fireflies in captivity ate P. carolinus but predation has not been observed in the wild. Phorid flies parasitize P. carolinus by laying their eggs within the firefly's body.