A Firefly Symbiont of Ants

Firefly larvae, as well as some female adult fireflies with minimal or no wings, often live underground or under debris, feeding on snails, earthworms, and other invertebrates. Finding larval and adult fireflies with other organisms would presumably be coincidental, resulting from overlapping habitats. However, one firefly species, Pleotomodes needhami, has been found living in the nests of two (possibly three) types of ants—a fungus-growing ant (T. septentrionalis) and a carnivorous ant (O. clarus).
This paper describes observations of ant/firefly interactions, as well as firefly activity, larval foraging behavior, bioluminescence, and mating behavior.

Methods
All specimens for this experiment were collected at the Archbold Biological Station in Highlands County, Florida—the only place P. needhami has been found. Field observations took place between 1988 and 1993. Ant nests were excavated and placed in specially constructed ant farms, along with the attendant fireflies.

Results

Location of Fireflies Relative to Ants
Twenty larvae, two pupae, three adult females, and four male P. needhami were found in the queen, brood and fungal chambers of eleven fungus-growing ant colonies. Four P. needhami females were found in three carnivorous ant nests, but whether favoring specific locations in the colony was indeterminable due to the loose structure of these nests.
Adult P. needhami are glowworms, using their light to find mates. Glowing females were seen near the entrances of both T. septentrionalis and O. clarus nests. In one instance, no ant nest was found at the signaling site, but T. septentrionalis ants and evidence of excavation were found nearby, suggesting a close association between the fireflies and ants.
Behavior of Fireflies inside Ant Colonies
In fungus-growing ant colonies in the wild and in captivity, P. needhami larvae were found in the fungal chambers, in nest tunnels, and on the soil surface above. When firefly larvae encountered ants, they pressed themselves to the substrate, and the ants ignored them.
Interactions between Fireflies and Ants
No obvious interactions occurred between firefly larvae and ants. Larvae were generally motionless when ants were present, and ants often squeezed by them in partially blocked tunnels. No evidence was found of ants feeding the larvae. Firefly larvae in the nests exhibited a singular behavior: they retracted their heads deeply into their prothorax, possibly a defensive adaptation for living among the ants.
Larval Luminescence
Three larvae in the ant farms were observed emitting a steady glow, both on the surface and underground.
Adult Luminescence
Females emerged to attract a mate, but remained near the nest entrance. The female signals by holding her abdomen up over her back, directing her light organs skyward. This activity lasted about an hour; if she did not attract any males during this time, she dimmed her light and retreated into the nest.
Males produced a steady glow in the presence of females, but were also observed emitting light near nest entrances when females were absent. Possibly these were reacting to pheromones a female had released earlier.

Discussion

It seems remarkable that P. needhami coexists with ants of such different temperaments as the gentle fungus-farming ant T. septentrionalis and the predaceous O. clarus. Especially remarkable is that P. needhami is found in the brood, queen, and fungal-mass chambers-centers of reproduction that the ants guard well.
Why the ants ignore the fireflies is unclear. By comparison, T. septentrionalis ants instantly attack and dismember milichiid flies found in their nest. Although carnivorous, there is no evidence that the firefly larvae feed on the ants. Larvae observed in captivity fed only on snails, and did so on the soil surface, not in the nest.
If not to feed on ants, what does P. needhami gain by living in ant colonies? Perhaps the nest interior provides greater protection from the harsh, sandy surface than what they could dig for themselves. In addition, the ants own defense of their nests indirectly defends the fireflies from predation as well.
How do the fireflies colonize new nests, as female P. needhami are soft-bodied, slow-moving, and wingless? One hypothesis is that the larvae possibly move from nest to nest by following ant trails.
Why firefly larvae glow inside the nests is unknown.