August 2016 Featured Paper

“The Dark Side of the Light Show: Predators of Fireflies in the Great Smoky Mountains,” Sara M. Lewis, Lynn Faust, and Raphael De Cock.
Psyche, Volume 2012, Article 634027
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Many animals, including fireflies, have conspicuous courtship displays and often breed in dense colonies. One would expect that these animals would attract many predators. Fireflies, however, are known to have toxic chemicals, lucibufagins (LBGs) that provide protection against predators. Studies have shown that these chemicals deter birds, spiders, and ants.

There are some animals that feed on fireflies despite these toxins. One of the main predators of fireflies is Photuris. Fireflies in the genus Photuris do not manufacture their own LBGs. They must acquire it by eating other fireflies.

The Great Smoky Mountains in East Tennessee host many species of fireflies, both nocturnal and diurnal. Among these are Photinus carolinus and Phausis reticulata, two species that occur in large numbers.

This study first surveys the invertebrate predators of P. carolinus and P. reticulata in the Great Smoky Mountains. Secondly, the Photuris fireflies are singled out for study to determine if they exhibit a feeding preference for certain species of fireflies.

Field Studies of Nocturnal Firefly Predators

Many orb-weaving spiders construct their webs at night and dismantle them before the next morning. In areas of high firefly concentration, 72% of the insects trapped in orb webs were fireflies, mainly P. carolinus males. Two P. reticulata males and one P. carolinus female were also caught. Both of these fireflies are active after dark and are much more likely to encounter an orb web than fireflies that are active during the day or at dusk, before the spiders have constructed their webs.

Many of the trapped males continued to flash after they were wrapped in silk by the spider. Some researchers have suggested that this acts as a lure to attract other fireflies. If so, further study is needed to determine if the spiders are somehow able to induce the firefly to continue to glow once it is trapped.

Harvestmen (otherwise known as daddy longlegs) were also seen feeding on both live fireflies and fireflies that had already been caught in a web and wrapped in silk.

Other predators observed feeding on fireflies were cobweb spiders, an assassin bug, and a hanging fly.

It is surprising that these invertebrates prey on fireflies, as it is known that they are protected by their LBGs. However, not all species of fireflies have been tested for LBGs. The chart below indicates the fireflies in this study that had been tested. The general prevailing assumption that all fireflies other than Photuris produce LBGs has never been confirmed. Two fireflies* in this part of the study had not been tested, so their predation could be due to a lack of LBGs.

Fireflies tested for LBGs   Fireflies not tested  
Photinus ignitus dark-active    Photinus carolinus* dark-active
Photinus marginellus dusk-active     Photinus brimleyi dusk-active
Photinus pyralis dusk-active     Photinus macdermotti dusk-active
Lucidota atra daytime-active     Phausis reticulata* dark-active

Lab Tests of Photuris Feeding Preferences

A number of different species of fireflies were placed in containers with Photuris to determine if Photuris exhibits a preference for specific fireflies.

The Photuris readily fed on the fireflies offered, but did not feed on other insects, indicating that Photuris are specialist predators of fireflies.

When attacked, fireflies release a sticky white substance from their thorax and leg joints—a process known as reflex bleeding. This substance can glue together the mouthparts of an attacker, allowing the firefly to escape. Both P. pyralis and P. carolinus release copious amounts of the white fluid. P. reticulata did not reflex bleed, but often feigned death when attacked.

In three Photinus species, P. carolinus, P. marginellus and P. macdermotti, 60–76% of the offered males were devoured within 24 hours. The two species of the daytime firefly Lucidota were also consumed.

Of the P. pyralis males, only 12.5% were eaten. Most P. reticulata also remained uneaten. Survivors of both species, however, exhibited bite marks and dried blood, indicating that they had been attacked, but then rejected.

One would expect that dark-active fireflies, i.e., those that are most likely to encounter Photuris at night, would have evolved methods to deter Photuris predation and be spared predation over those that do not encounter them regularly. The results of this survey do not support this. As the chart above shows, Photuris fed on dark-active, dusk-active, and daytime fireflies.

Could Photuris fireflies be choosing prey with high LBG content? No determination could be made here because not all the fireflies could be tested for their LBG content levels. The possibility also exists that the fireflies rejected by Photuris contain some other chemical deterrent to Photuris predation.

This study of fireflies in the Great Smoky Mountains demonstrates that a diverse assortment of generalist predators feed on fireflies despite their toxic defense. In addition, the specialist Photuris predators prey more readily on some species of fireflies than others. This study suggests that the evolution of firefly defenses is driven by a combination of generalist predators and specialist Photuris predators.

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