Firefly 'Femmes Fatales' Acquire Defensive Steroids (Lucibufagins) from their Firefly Prey

Fireflies are known to be unacceptable food to a number of vertebrates. Earlier research has shown that fireflies of the genus Photinus are unpalatable to thrushes due to steroidal compounds called lucibufagins (LBG), which had been found in all species of Photinus under study. Another genus of firefly, the Photuris, was also found to contain LBG, but these fireflies do not produce them on their own. Instead, the female Photuris acquires LBG in its blood by preying on the male Photinus firefly.

Firefly courtship involves species-specific flash patterns and responses. Female Photuris fireflies lure male Photinus to their demise by mimicking that species' female flash response, effectively masquerading as the female of that species. This paper, focused on the female Photuris only, presents data that demonstrates: 1) female Photuris fireflies in the field contain LBG in their blood; 2) when first emerged as adults, female Photuris fireflies do not contain LBG, but only acquire LBG from feeding on Photinus males; 3) jumping spiders are less likely to prey on female Photuris fireflies fed on male Photinus than on unfed Photuris fireflies; and 4) the degree of protection acquired depends on LBG content in the blood of the Photuris.

Firefly Reflex Bleeding

When disturbed, fireflies commonly emit drops of blood. The amount of blood emitted by this "reflex bleeding" can be considerable, up to 2% of body weight. Both Photinus and Photuris are known to reflex bleed. In the lab, effecting reflex bleeding in these fireflies yielded the blood samples necessary for LBG analysis.
LBG content of field-collected Photuris and Photinus: Photinus fireflies were found to contain substantially more LBG than Photuris. While female Photuris contain much more of this compound than their male counterparts, the LBG content of both male and female Photinus is, on average, about three times higher than in Photuris.
LBG content of Photuris females fed Photinis males: Photuris females reared in captivity were fed two Photinus fireflies. A day after eating, they contained high measures of LBG, allocated primarily to the blood. Unfed controls were essentially LBG-free.
Both Photinus and Photuris fireflies were offered to jumping spiders, one of the natural predators of these fireflies. All Photinus fireflies offered to the jumping spiders were initially attacked, but quickly rejected. Photuris fireflies fed sufficiently on Photinus to have a high LBG content were also rejected by the spiders, while Photuris with lesser amounts of LBG were eaten.

Discussion

By luring and consuming male Photinus fireflies, the female Photuris acquires the chemical defense against predators that she otherwise is unable to produce on her own, and stores this at high levels in her blood. When disturbed, she emits a substantial amount of blood through reflex bleeding, and with it, the defensive chemical LBG is also emitted.
The amount of protection acquired directly correlates to the amount of LBG in the blood. Females that contained an amount of LBG equal to eating even one Photinus male were invariably rejected by the spiders, while those with a lesser amount of LBG in the blood were not.
We do not yet know how male Photuris acquire LBG in their blood, but given that they lack them upon emergence from the pupa, they too must acquire LBG from an outside source. In the lab, Photuris males will also eat Photinus fireflies, sequestering LBG in their blood as a result, but whether they do this in the wild as well has yet to be determined.
While the strategy of acquiring chemical defenses from an outside source is common in nature, the Photuris female firefly is exceptional in that it aggressively mimics the female of its genetic relative, the Photinus, to attract the Photinus male, only to devour him. In so doing, these firefly "femmes fatales" effectively gain the chemical defense they lack to ward off predators themselves.