There are only two glowing insect species that I know of in Latvia and those are both glow-worms, i.e., the European common glow-worm (Lampyris noctiluca) and the lesser (or short-winged) glow-worm (Phosphaenus hemipterus).
The latter species is less prevalent and I believe that any observations of glow-worms in our area are those of the common glow-worm.
As far as the species’ life cycle has been described, females lay eggs in summer and the eggs hatch within 2 – 3 weeks (or longer in colder climates).
The larvae then proceed to hibernate during winter and the entire development can extend over 1 – 2 (or even more) years (1 – 2 winters) until the final moult after which they turn into adult insects.
It has mostly been assumed that glow-worm larvae do not produce the glow by themselves but that the glowing substance has been deposited in the eggs by the female whence it is ‘inherited’ by the larva which can then use it for a period of time (probably as a defense mechanism aka aposematic signal suggesting its unpalatability to predators (see, e.g., De Cock, R. & Matthysen, E., 2003).
However, my observation of the larva actually emitting light occurred in early May (after at least its first hibernation).
Females can keep laying eggs quite long into autumn if the weather is permissible (on occasion they do not become fertilized).
Even last year I still observed glowing females in September.
I wonder if there is a possibility that some of the eggs could not hatch due to cold weather and they only hatched this spring.
It seems unlikely that eggs would survive winter.
However, there is also a possibility that the larva hatched but then almost immediately underwent winter diapause and, as a result, did not use up its bioluminescence reserves which were still accessible in spring (it would seem that glow-worm larvae resume activity in April – May).
Of course, the current science might be wrong and the larvae might have evolved a self-produced light mechanism rather than relying on residues left by their mother.
I wonder which of these scenarios would best explain the observation of a glowing larva on May 3.
There is, nevertheless, another bewildering aspect to this puzzle.
Namely, the glow-worm larva resemble the adult female in their form but this larva did not quite have the appearance of an adult female firefly and it rather looked like, e.g., larvae produced by Diptera species.
Of course, it was probably not any type of fly but rather some forest ground cover/woody substrate/leaf litter-associated species. At the moment, I am not nearly educated enough in forest invertebrates to give a respectable example of which species larva it could have been (but it was ca. 1 – 1.5 cm long and perhaps 3 – 4 mm wide).
However, I do not really trust my observation because I only caught a brief glimpse (about 30 seconds) and then I was forced to move on.
Perhaps the glow pattern was different from the larval form (much like the adult female does not glow all over) and, therefore, judging by the glow alone, the luminescence did not perfectly represent the actual shape of the larva.
On the other hand, it has been (somewhat vaguely) stated that the glow-worm larvae emit signal pulses much like the adult females do while this larva was glowing non-intermittently.
It had a faint, persistent glow which did not cease during the 30 seconds of observation.
The bioluminescence, on the larva, should also be emitted from two small spots on its body but this larva was glowing all over.
Also, the glow was nothing like the adult’s glow – it did not have a cue of colour to it. The glow simply seemed pale grey.
This consideration has made me wonder whether it might be another insect species entirely that also has glowing larvae.
Last year (unfortunately, I do not recall which month it was but it might have been August), I observed similar glowing ‘worms’ (the glow-worm larvae do not truly remind of ‘worms’ while these did) and I also observed very thin, thread-like worms (< 1 mm of width and perhaps ca. 7 mm – 1 cm of length) which were glowing bright silvery and moving very actively.
In fact, it appeared as if they were swimming (they were circling one another in a swift, flowing motion) but they were on bare ground where there were no rain puddles.
I have been unable to find much photographic evidence of glow-worm larvae soon after hatching (prior to the first moult) but the one photo that I discovered (by John Tyler) did not suggest that glow-worms hatch as thread-like organisms although these were not photos of the lesser glow-worm which might have a different larval form.
The lack of glow pulse emissions and the shape of the glowing larva (as well as the colour of the luminescence) give me a pause – could there be another insect species in Latvia which has glowing larvae?
Or perhaps this is some other type of phenomenon whereby larvae of a non-glowing species have acquired bioluminescence, e.g., through having developed near tree roots infected with bioluminescent Mycena spp. or Armillaria spp.?
Or perhaps those are fungivorous larvae (or worms, or even large nematodes in the case of the thin thread-like organisms observed last year) that have acquired their glow via consumption of bioluminiscent fungi?
Armillaria spp. seem the likelier ‘donor’ because they produce glowing hyphae that infect the woody substrate while Mycena spp., typically, have bioluminescent above-ground macro-organs.
Regardless of the pathway (exposure to woody/soil substrate or fungivory, hyphae would be involved rather than above-ground macro-parts.
(It should be mentioned that I observed the thin silvery-glowing organisms approximately 30 metres (in a riparian forest habitat, on a trail between the river and the forest) from a site where a tree had been felled by a windstorm and that tree had bioluminescent Armillaria fungi which had colonized its wood.)
***
On May 11, 2024, at ca. 1.30 – 2 am, on a trail leading through a riparian forest (in the middle of the barren ground path) I, once more, spotted the silvery, thread-like invertebrates that I had seen last year in an entirely different place (but also on a trail through a riparian forest).
One of the invertebrates was approximately 1 mm wide and perhaps 2 cm long.
I did not have any flashlight or other means to illuminate them and, accordingly, I can only describe the shape that was seen glowing but the actual individual could have been of a different shape (much like a glow-worm adult bug is not constituted of a glowing tail as one might assume after having simply observed the bioluminescent organ).
The other invertebrate emanated the same silvery luminescence but it appeared that the glowy substance was not distributed uniformaly along its body in the same thread-like manner.
Instead, it was concentrated toward one end of the individual and it also seemed spotty (as if the individual had rubbed off some of the glow and now it was patchily covering only its front of the length of about 0.5 cm).
The longer individual (who might not have been longer as such but who had a longer glowing segment) was perfectly still but the other individual (with the patchy, short glow) was moving very rapidly.
It closely resembled the scenario I had described from last August (or September) where also 1 – 2 individuals were lying almost as if dead but one was circling them at great speed.
During the earlier incident, the circling individual was also glowing as a thin, long thread and its movements appeared very smooth and gliding.
This time, however, the motion was far more angular.
While it was too dark to clearly see, the movement suggested that the individual was either wriggling sort of like a snake (in a zig-zag) or perhaps it had legs which it was using to move in a searching pattern (not in a straight line).
If it had legs, it is possible that the individual was wider than the glowing thin line would suggest.
The surrounding habitat included plenty of coarse woody debris although none of these boles were fresh (at least half a year of decay).
There was also leaf litter, there were shrubs – a rather typical deciduous, wet riparian woodland.
References
De Cock, R. & Matthysen, E. (2003). Glow-worm larvae bioluminescence (Coleoptera: Lampyridae) operates as an aposematic signal upon toads (Bufo bufo). Behavioral Ecology. 14. 103-108. 10.1093/beheco/14.1.103.
Tyler, J. The Glow-worm (photographs on the left side), http://www.johntyler.co.uk/glowworm.htm
Notes by Ieva 
