Thieves in the Night: Kleptoparasitism by Fireflies in the Genus Photuris

Kleptoparasitism: stealing resources, usually food, from another animal of the same or a different species.
Most fireflies do not feed as adults, but there are exceptions. Certain species of Photuris actively hunt and devour other species of fireflies to acquire defensive compounds that they cannot produce themselves. Researchers have already identified two methods of hunting exhibited by Photuris: mimicking the courtship signals of other fireflies to lure the males within reach, and aerial "hawking"---catching male Photinus fireflies in flight. This paper describes a third method of hunting: kleptoparasitism, or stealing trapped fireflies from the webs of orb web spiders.

Over the course of nine nights, Photuris were observed perched or flying near the webs of orb web spiders, perhaps awaiting an opportunity should a Photinus firefly be trapped in one. During this observation period, twelve occurrences of kleptoparasitism were noted. In each case, a female Photuris was seen feeding on a web-wrapped Photinus firefly.
In cases where the feeding Photuris was confronted by the spider, the outcomes varied. The Photuris was sometimes able to fend off the spider and continue feeding. At other times, the spider managed to thwart the firefly by (a) driving off the Photuris, (b) cutting strands to release the thieving Photuris from the web (sometimes with the wrapped Photinus), or (c) subduing and ensnaring the Photuris, wrapping her in silk, sometimes with her prey.
The relative size of predator to prey seemed to be a determining factor in the outcome of these events. A spider smaller than the Photuris in its web often cut the threads to release the firefly, sometimes along with its prey. However, when the spider was the same size or larger than the feeding Photuris, the spider was more likely to attack.
In an earlier study, another researcher recorded seeing a female Photuris "quietly feeding" on silk-wrapped Photinus males "in a web from which the spider had departed." This study demonstrates that Photuris fireflies actively hunt spiders' prey, "dive-bombing" into webs to feed on the wrapped victims---often risking a confrontation with the spider that may be fatal.
Even though the spider poses a danger to the Photuris, kleptoparasitism appears to offer Photuris a reliable source of lucibufagins, the defensive compounds produced by some fireflies such as Photinus. Photinus fireflies become trapped in spider webs often, particularly during their peak mating season when several may be found in a single web---offering a tempting reward for the daring Photuris.
When attacked, Photinus fireflies will "reflex bleed," emitting a white sticky mass along with their lucibufagins. This sticky mass can gum up the mouthparts of the Photuris aggressor, sometimes allowing the Photinus to escape. However, Photuris avoids this problem altogether when they steal a web-ensnared Photinus, because the Photinus reflex-bled when initially caught in the spider's web.
Female Photuris fireflies turn predatory after mating, searching for Photinus fireflies to feed upon. If a male Photuris approaches one of these rapacious females, he must convince her to mate or he too may end up becoming her dinner. The researcher observed a female Photuris capturing and feeding upon a Photinus. Within minutes, a male Photuris landed and began mating with the feeding female. Further studies need to be done to determine if male Photuris purposely position themselves near webs to mate with females that are feeding on the bound fireflies in order to minimize the danger that that the female would rather eat than mate with them.
Generally, Photuris fireflies have been assumed to locate ensnared Photinus by the rhythmic flashes fireflies often emit when trapped. But there is also a possibility that Photuris perceives chemical clues that are being emitted. As noted earlier, trapped Photinus fireflies reflex bleed and discharge their lucibufagins; possibly Photuris can locate this prey by detecting their released chemicals.
Several questions remain to be answered:
What causes a Photuris female to shift from predatory to kleptoparasitic feeding?
Do male Photuris also steal wrapped prey from spiders' webs?
Do male Photuris perch near webs to find mates?
Are Photuris only attracted to webs that have already caught fireflies, or do they wait by a web until a prey is caught?
Does one Photuris steal from another Photuris? Do Photuris feed on dead prey a well as live?
These questions need further study to be resolved.