I mentioned on the main page that species of Photuris are behaviorally rather than biologically distinct. The concept of behavioral species is not actually all that weird! For example: green lacewings (Neuroptera: Chrysopidae, Chrysoperla carnea group) have been used as biological control for greenhouse pests for a long time. People started noticing that, from time to time, the population of lacewings they'd purchased would just crash for no apparent reason. Turns out... C. carnea is a group of cryptic species with identical morphologies but different mating songs. The insects were physically compatible (lock and key hypothesis) but behaviorally distinct, and thus finding a mate was luck-of-the-draw -- even moreso than usual, due to the random selection of lacewings in each greenhouse.
Some people actually don't even believe in a "species" as a concept -- in the past several years (maybe decade or so?) something called Phylocode has arisen and articles have been written to argue that Linnaen taxonomy is outdated and no longer useful. Instead of the fixed taxonomic ranks that the ICZN recognizes, Phylocode uses the concept of a Least Inclusive Taxonomic Unit, or LITU. Its classifications are based entirely on cladistics -- mammals, for example, would be defined as "the smallest possible group that includes all of the mammalian synapomorphies" rather than "class Mammalia". (A synapomorphy is a derived trait that separates that group of organisms from others. Clear synapomorphies include the ability to produce milk (mammals), a hard shell formed of modified forewings (beetles), or an ovipositor modified to deliver venom in addition to eggs (stinging bees, ants, and wasps), to name some.) One of the arguments is that these fixed ranks imply equivalency, but in reality there is no standard of what's a family versus a genus versus a subfamily versus... a system of LITUs solves that problem by simply removing the rankings. I'm not certain that I can get behind that, but I do think that that's a solid point.
And, to further complicate things... taxonomy is a very slow, contentious science of consensus and weight of evidence. It doesn't function on a system of "the last person to publish has the say"; nothing holds the weight of gospel. A hypothetical Dr. Jones may have published a paper in 2022 elevating the wasp genus Polistes to family level, but Dr. Smith may completely disagree with the reasoning in Jones 2022, and continue to treat Polistes as a vespid. As a real-world example, let's talk briefly about mayflies. The family Isonychiidae contains one genus: Isonychia. This genus has been floating around between various families for decades, and it isn't difficult to find papers placing it in Oligoneuriidae or Baetidae, among some others. Many mayfly workers now accept Isonychiidae as a family, but some do not. That's part of why, when you read taxonomic papers, they have sections in them that are devoted to cataloguing past names of the taxon in question!
Okay, now let's look at how you can tell if you've got a firefly in front of you.
Identifying adults of the family (and the genus), per Arnett: head hypognathous, pronotum shield-like and covering head (in Photuris, the head is partially exposed and the pronotum does not conform to the shape of the eyes), tarsal formula 5-5-5, male abdomen with 7 visible sternites (5 and 6 completely occupied by the light-producing organs in Photuris; female Photuris also have light-producing organs on sternites 5 and 6, but they are bordered by non-illuminating regions (note: Barber describes sternites 6 and 7 as bearing the light-producing organs in this genus)).
(That's a lot of words that you don't actually need to know. The long and short of it is: adult fireflies will have their heads covered by a broad shield, and the last few segments on the underside of their abdomen will be light in color.)
Identifying adults of the genus, per Arnett, Barber with notes by McDermott: pronotum with lateral borders flared out (explanate) and opaque over the eyes, antennae simple and long with 11 segments (first segment typically the longest), labial palpi strongly asymmetrical and kind of a "long, narrow mitten shape, with the 'thumb' projecting at a right angle", anterior foretarsal claw with two sub-symmetrical apical points and all posterior tarsal claws simple (i.e. not bifid -- the anterior claws of both fore- and midtarsi are bifid), male genitalia with lateral filament on basal piece, posterior margins of sternites nearly straight on sternites 1-4 and generally emarginated on sternites 6 and 7 (which tend to be longer than any of the first four), proportionally very long legs, spurs prominent on meso- and metatibiae.
(To simplify that one: adult Photuris will have that head-covering shield flared at the edges and opaque over the eyes, they'll have four thin claws on each hind leg, and their legs will be lanky.)
Identifying larvae of the family, per Arnett: sclerotized, head elongate and prognathous, mandibles curved and channeled, antennae 3-segmented and retractable with a cluster of stemmata near the base, prothorax elongate (as long as meso- and metathorax combined) and narrowed anteriorly, legs 5-segmented, abdomen with paired light-producing organs on the epipleura of abd seg 8 (visible ventrally). Larvae of Photuris are apparently not well known (Arnett and also an email I received from Oliver Keller), but are the most commonly found.
(And let's put that one in clearer language, too. Larval fireflies will look like predators and not at all like the adults. Their mandibles will be out in front of them and ready to pierce into their prey, and they'll have light-colored regions on the underside of the end of their abdomen.)
It seems that Barber 1951 is really the "definitive" work that everyone else links back to. I put "definitive" in quotes because, according to Arnett 2002 (American Beetles, vol. 2) "keys to spp., Barber 1951 and McDermott 1967, should not be relied upon" (p. 193). Part of this is undoubtedly due to the difficulty of separating previously collected specimens by morphology (although Barber clearly thinks poorly of this practice -- "Taxonomy from old mummies which fill collections is a misguided concept. It leads to the misidentification of rotten old samples in collections. How these poor fireflies would resent being placed in such diverse company... what contempt they would feel for the 'damned taxonomist'.")
Here's something really cool: on the main page, did you see that firefly larvae are generally predatory? Well, in the genus Photuris, some adults are predatory as well! The females are referred to as femmes fatales, because they'll mimic response flashes of other species to lure in the males, who then are consumed. Part of this is so that they can sequester the defensive chemicals that the male produces; fireflies are very well-defended, chemically speaking, and they are quite distasteful to predators. Young bats have been observed catching fireflies and immediately spitting them out, and older bats won't even start to attack an offered firefly.
So why do the females need those chemicals? In many cases, they need them for their kids! A lot of these femmes fatales, when they lay eggs, will coat them in defensive compounds to protect against predation and parasitism. Eggs are easy targets, because they just sit there and they're full of protein. Life's tough when you're a small bug out in the big world!
While this aggressive mimicry is pretty awesome, it does make the genus even more difficult to study. As I mentioned, Photuris spp. are distinguished by their flash patterns rather than external morphological characters. It's rare to find a place with fireflies that hosts only one species, and Photuris spp. are very common; since many females mimic patterns of other species, it's very difficult (sometimes impossible) to know what you have. There are some that are only active during very specific periods, so you can narrow the choices down in that way! For the most part, though, unless you know for a fact that only one species is around at that time of year, identifications generally stay at the genus level.