The Lampyridae Raffinesque 1815 are the beetles commonly known as fireflies and lightning bugs. There are over 2000 species in over 100 genera, with this being a quarter of the expected diversity (Viviani, 2001). They’re found all over the world and in all sorts of habitats, including aquatic ones (Fu et al., 2005).
Their classification has classically not followed a phylogenetic system – as McDermott (1964) wrote, their classification “should not be construed as indicating phylogenetic relationship”. Of course, I don’t agree with such a view, being a paraphyly-hating systematist rather than a taxonomist. Lampyrid subfamilies are differentiated with easy-to-spot characters, such as the position of the light organs or the dorsal visibility of the head (how far under the pronotum it goes – it can be completely covered, as in cockroaches, or more exposed). At lower levels, male sexual characters are needed, most often the aedeagi (penises), but flashing patterns can also be used. Needless to say, this makes their identification very challenging for the non-specialist. In particular, cantharids (soldier beetles), elaterids (click beetles) and lycids (net-winged beetles) are often confused for lampyrids; it’s no surprise that at one point, these (besides the elaterids) were grouped together as the “Malacodermata”, a polyphyletic group.
Systematically, they belong to the Elateroidea. The monophyly of the Lampyridae is not conclusive though. For example, Bocakova et al.‘s 2007 molecular analysis of the Elateriformia recovered it as paraphyletic. Similarly, the thorough morphological analysis of Branham & Wenzel (2001) also recovered a paraphyletic Lampyridae, with several classically problematic genera found to fall outside the family and contradicting the classic conception of Lampyridae.
With these genera removed, 3 autapomorphies appear to define a monophyletic Lampyridae:
- head covered by the pronotum;
- trochanter attached obliquely to the femur;
- CuA1 intersect MP above the fork.
They are recognisable by any combination of characters. Their legs have less than five segments and a single claw (instead of two); the labrum and the clypeus are fused; the body is flattened; the ventral surface is membraneous; the head is retracted and usually not visible from above.
Less certain characters include the antennal sockets being pretty close to each other in comparison to other beetles, and elongated and slender mandibles with perforations.
Another character, not terribly reliable but evolutionarily interesting, is the soft body. If you ever touch a lampyrid, you’ll find that it’s relatively squishy, very different from other beetles with their hardened elytra. This is an example of neoteny, the continued expression of larval characters in adulthood, in this case of reduced sclerotisation and a flexible abdomen, both characters typical of lampyrid larvae. Around a quarter of all lampyrids are neotenic (Cicero, 1988).
Historically, they were first systematically studied by Oliver (1907), which remained the foundation on which all lampyrid taxonomy was based for decades, until the overhaul by McDermott (1964) and its slight update in McDermott (1966). Crowson (1972) did the last major revision, and the systems used nowadays are all hybrids of Crowson and McDermott’s classifications, but it is universally acknowledged that the taxonomy of the Lampyridae is unsuitable and in further need of revision.
The latest nomenclatoral revision of beetle families (Bouchard et al., 2011) classifies the lampyrids as containing 5 subfamilies: the Psilocladinae, Amydetinae, Lampyrinae, Luciolinae and Photurinae.
The picture above (Lawrence et al., 2011) shows the hindwing venation of Photinus, and can be taken as a generalised model for lampyrids; notice the development of the radial cell and the absence of the anal notch.
Lampyrids are most well-known for glowing; Darwin wrote about this in his Voyage of the Beagle (see Appendix 1). It must be kept in mind that this is not autapomorphic for the adults and is a relatively derived condition (see table above; Branham & Wenzel, 2003): basal lampyrid adults can’t glow, and this is the plesiomorphic condition (Branham & Wenzel, 2003). The main purpose of the glowing is for attracting mates. In the non-glowing lampyrids, pheromones are used instead; they’re active only during the day. It’s worth noting that flashing is extremely energy-efficient (Woods et al., 2003), even more efficient than walking, so it is easy to maintain evolutionarily.
The glowing is achieved by a family of proteins called luciferases; the same luciferases are found in lampyrids as in phengodids and elaterids, and other members of the luciferase family are found in bioluminescent bacteria and in deep-sea bioluminescent organisms. The way luciferase in fireflies works is summarised by Fraga (2008) in his excellent review paper. On reacting with oxygen in the presence of ATP, a pigment called D-luciferin gets oxidised to oxyluciferin; this reaction is catalysed by the luciferase. Oxyluciferin, upon relaxing, emits a photon, with the light produced peaking at ~560 nm (Hastings, 1983). Firefly luciferase and the gene coding for it are used in biomedicine and biochemistry as indicators (for what, I don’t know; just something I remember from an entomo-course).
In general, it’s the males that flash the most brightly. Both sexes have the photic organs that produce the light, and in both sexes they’re arranged in species-specific patterns (stripes, entire area, spots, etc.) on the ventral side of the 5-7th abdominal segments (see picture above, from Lewis & Cratsley (2008)). Associated with the organs is an increase in nerves and tracheoles (for better gas exchange) (Buck, 1948).
The flashes appear to us as varying in colour: twilight/dusk flashes seem to be orangeish, while ones at night are a lime green. But there’s no evidence that colour has any effect on the fireflies, they only respond to the flashing pattern. That said, Seliger et al. (1982) showed that the colour is most visible for the time of day – lime green is best for night, and orange is best for twilight, and that their photoreceptors are adapted to seeing those specific emission spectra. According to Seliger et al. (1982), this is achieved not by differences in photorecptor pigments, but by coloured pigments in the retina that filter the incoming light.
The general procedure is as follows: the males fly towards the females, flashing their coded messages; the motionless females reply, the males fly closer, ad nauseam until they get together and copulate. The flashing of the male acts as an honest signal of the male’s fitness, with average or faster-than-average flashes being preferred (Branham & Greenfield, 1996).
Copulation lasts ~20 minutes, and each female mates only once, and multiple times under exceptional circumstances (Wing, 1984). The system is apparently very successful, as lampyrids rank as being very successful at mating (Rhainds, 2010).
Some males produce a very nutritious spermatophore to give to the females as a nuptial gift after mating (van der Reijden et al., 1997). It is transferred at the same time as ejaculation, but is moved to a special compartment, where it they are digested, the protein used to supply the oocytes in vitellogenesis.
Competition does occur. While in general, large males get to mate much more, when many males are around, smaller males are simply more agile and faster than the larger ones, and so they arrive at the female before the large males (Vencl & Carlson, 1998).
Most flashes are nothing more than crude glows just to signal presence, and are used in combination with pheromones for long-distance communication. Another factor to consider is the “shape” produced by the movement of the firefly while flashing; this can also be species- and gender-specific.
In some species and genera, there is dynamic flashing, which can rightly be called a conversation, with the flashes consisting of comments, questions and replies – they truly are as sophisticated as a language, and the control over them is very precise (Trimmer et al., 2001). The differentiation is very fine, with different species having different patterns, and the patterns are all sexually dimorphic. In some cases, e.g. in the case of Pteroptyx of Southeast Asia, these conversations last for months (Case, 1980). They can take place between individuals or in aggregations, the latter of which is even more interesting, considering that there needs to be some sort of synchronisation and that lampyrids are not social insects.
This synchronisation is generally sorted according to a system devised by Buck (1988). The synchrony can be continuous or discontinuous, the former being a regular flashing pattern among individuals and the latter being a pattern with the entire group flashing at the same time. Additionally, both of these can be in unison, i.e. all at the same time, or in a wave (like a Mexican wave).
The flashing is controlled by the nervous system (hence the higher innervation of the photic organs), and the synchrony in aggregations by a pacemaker (Case & Buck, 1963). As further testament to the flexibility of this “language”, the control by the pacemaker is dynamic and occurs as a response to other visual clues; while the flashing patterns are built-in, the speed and fine control of the flashing depends on the environment.
As testament to just how much control they have over their flashing, consider Photuris females (Lloyd, 1965). They’re capable of mimicking the female flashes of other firefly females of the cohabitant Photinus and Pyractomena species, thus attracting the males of those species. They then eat them, as pictured above (Lewis & Cratsley, 2008). Hence their nickname as firefly femmes fatales. As a bonus, they also gain protection against jumping spiders from this behaviour, by acquiring lucibufagins (defensive steroids) from ingesting the males (Eisner et al., 2007).
And if that’s not amazing enough, get this: the Photuris males, in order to seduce the females, resort to also mimicking Photinus and Pyractomena, but only the males (Lloyd, 1980). This attracts the Photuris females and so they get to mate. In other words, this genus has lost its own method of communication and has to rely on mimicking others. Also, keep in mind that the femme fatale can recognise the mimicry and will respond differently to a male of its own species than to a male of the prey species; she also tends not to eat males of the same species.
A large worry nowadays (hopefully) is the gradual, but very noticeable, decline in firefly abundances. Morse (1914) described an event he saw in 1864, with “hundreds of fireflies flashing in unison”. It’s unheard of now to see such a sight in urban or suburban areas, and they’re becoming rarer in rural areas. The most likely culprits are, of course, insecticides and habitat destruction, but light pollution might also be a large factor. It’s a real shame, as such sights are sure to be pretty spectacular (and they have been the inspiration for artists over the centuries; cf. this very good movie).
Unlike the adults, the plesiomorphic condition in the larvae is for them to glow (Branham & Wenzel, 2003), a trait inherites from their cantharoid ancestry. They all produce light from the same photic organs on the ventral side of the 8th abdominal segment. Interestingly, the larval light of some lampyrids differs from the adult light in that it’s polarised (Seago et al., 2009) – this is also a pretty rare phenomenon in general. You can find them in any humid habitat at night, where they go hunting. They’re quite vicious predators, chemically sensing their prey (Schwalb, 1960) and injecting them with digestive juices with their mandibles. They then slurp up what remains and wipe their mouths using characteristic organs covered in small hooks on the 10th abdominal segment (see Appendix 1).
The purpose of the glowing in the larvae is most likely as a warning for predators (de Cock & Matthysen, 1999) – it’s an aposematic signal. This isn’t deceptive though: the larvae are toxic, or at least not nice to eat, since they accumulate steroids (lucibufagins) in their blood.
The glow persists even in pupation, which mostly takes place underground, although some construct so-called igloos, or do it in preexisting cavities in wood; a couple of species have even been known to pupate on trees, like butterflies and moths. In those non-luminescent adults, the glowing is lost after pupation.
An interesting factoid: Indian folklore says that a lampyrid potion can give its drinker night vision (Lockwood, 2009). How did I find this out? Back in 2002, the Indian military, apparently desperate for ideas, looked into ancient Indian texts to see if they can get some inspiration. And if I remember correctly, India has nuclear weapons. Oh dear.
Another, somewhat more chilling role of lampyrids in warfare comes from the Cold War and the work of a German scientist called Adolf-Henning Frucht (Lockwood, 2009). In the 1960s, he was hired to conduct research for Eastern Germany, to find a way to detect airborne toxins (nerve gases). The nerve gases of the time all contained organophosphates, since they were derived from insecticides. And he found out that even the slightest amount of organophosphate is enough to stop the glow of a lampyrid. That was a perfect detector. His life after this discovery took a turn for the worst, as he found out he was being manipulated – he was told this was for a protective measure, while it was actually going to be used offensively. He turned to espionage, was caught and spent a decade in prison doing manual labour, until he was released to the West as an old geezer in a prisoner exchange.
Anyway, a bit on other parts of lampyrid ecology. The larvae can be terrestrial, semi-aquatic or completely aquatic (the latter have ventral gills on the last abdominal segment), and are predators feeding on soft animals (snails, earthworms), insects, or on carcasses of dead vertebrates. Their digestion is extraoral (Borror et al., 1989). They can get parasitised by phorid flies (Brown, 1994).
Some larvae construct dwelling chambers out of smeared mud. They’re usually subspherical in shape.
The larvae of the genus Photuris have unique structures at the end of the abdomen called pygopodia, which they use to move. When resting, they’re inside the body. When the larva wants to move, muscles push them out and they act like small suckers. It wouldn’t surprise me if these are found in other genera – Photinus are by far the best studied lampyrids.
I already mentioned that larvae are poisonous to eat; the same is true for adults. In fact, it wouldn’t surprise me if the adult loss of a hardened elytra is related to this (maybe someone knows of a relevant paper?). In any case, lampyrid adults are notorious among predators, which may also be why some sympatric cantharids, elaterids and lycids resemble them so much – a form of Batesian mimicry (in which harmless organisms mimic harmful ones, hoping that the predator doesn’t notice).
Since they’re often active at night, lampyrids are some of the arthropods known to produce antifreeze proteins (Duman et al., 1982).
They have a fossil record, including some Baltic amber specimens caught while mating (e.g. Grimaldi & Engel (2005), fig. 10.46). They’re also found in rocks.
For the collectors among you, pitfalls traps are enough to capture decent amounts of them, although the best way, in my experience, is to use a light trap.
“I found that this insect emitted the most brilliant flashes when irritated: in the intervals, the abdominal rings were obscured. The flash was almost co-instantaneous in the two rings, but it was just perceptible first in the anterior one. The shining matter was fluid and very adhesive: little spots, where the skin had been torn, continued bright with a slight scintillation, whilst the uninjured parts were obscured. When the insect was decapitated the rings remained uninterruptedly bright, but not so brilliant as before: local irritation with a needle always increased the vividness of the light. The rings in one instance retained their luminous property nearly twenty-four hours after the death of the insect. From these facts it would appear probable, that the animal has only the power of concealing or extinguishing the light for short intervals, and that at other times the display is involuntary. On the muddy and wet gravel-walks I found the larvae of this lampyris in great numbers: they resembled in general form the female of the English glowworm. These larvae possessed but feeble luminous powers; very differently from their parents, on the slightest touch they feigned death and ceased to shine; nor did irritation excite any fresh display. I kept several of them alive for some time: their tails are very singular organs, for they act, by a well-fitted contrivance, as suckers or organs of attachment, and likewise as reservoirs for saliva, or some such fluid. I repeatedly fed them on raw meat; and I invariably observed, that every now and then the extremity of the tail was applied to the mouth, and a drop of fluid exuded on the meat, which was then in the act of being consumed. The tail, notwithstanding so much practice, does not seem to be able to find its way to the mouth; at least the neck was always touched first, and apparently as a guide.”
- Charles Darwin on lampyrids, from The Voyage of the Beagle.
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Research Blogging necessities :)
Branham, M., & Wenzel, J. (2003). The origin of photic behavior and the evolution of sexual communication in fireflies (Coleoptera: Lampyridae) Cladistics, 19 (1), 1-22 DOI: 10.1111/j.1096-0031.2003.tb00404.x
Lewis, S., & Cratsley, C. (2008). Flash Signal Evolution, Mate Choice, and Predation in Fireflies Annual Review of Entomology, 53 (1), 293-321 DOI: 10.1146/annurev.ento.53.103106.093346
Lloyd, J. (1965). Aggressive Mimicry in Photuris: Firefly Femmes Fatales Science, 149 (3684), 653-654 DOI: 10.1126/science.149.3684.653