Thursday, September 12, 2013

Glow Worms

Lightening bug- Shelly Cox
I don't have to describe fireflies for anyone who as a child ran around on a July evening in the Midwest.  If you didn't collect enough to make a lantern in a jar, you probably at least wore a "firefly ring," created at the expense of a recent captive.

Fireflies at night- Click to enlarge
There are over 2,000 species of fireflies world wide, which aren't flies and don't produce fire.  Heck, they aren't even lightening "bugs."  They are actually beetles which lack the typical hard shell we are accustomed to seeing.  Instead their elytra (front wings) are leathery covers for the flying hind wings.  Some species lack the lights and others have flightless females.


Ventral view-

Our common fireflies rest during the day, then come out at night, the female sticking to the low lying grass and shrubs, sending out her Siren call.  Meanwhile we are more likely to focus on the male who patrols, looking for the special flash of the female of his species.  When he finds the signal he drops down to do his thing.

Actually, the first theory concerning their light was that it was an aposematic warning, a bright color saying "Don't eat me, I taste bad."  In fact they do have distasteful chemicals which may not help the victim but will help its classmates in the future.  Only later did the sexual signaling become apparent.

Females of the genus Photuris are the Mata Hari's of the family Lampyridae, mimicking the mating flashes of other firefly species to lure them for nefarious purposes.  The excited male lands for a tete-a-tete, only to be eaten by this femme fatale of fireflies.  As it is also with many spiders, sex is a risky proposition.

A few days after mating, a female will lay her eggs on the ground and a few weeks later the larvae emerge and start to feed.  This will continue through the late summer and as the "glow worms" grow larger through successive instars, their light becomes more obvious on moonless nights. 

September is the time to see the product of all those flickering summer nights.  The gravel along the edge of Bull Creek starts to light up with faint luminescent spots of light, not the flash of a firefly but the subtle slow wink of its larvae, the glow worms.  Collecting them is tricky as they are moving along leaf-litter, frequently disappearing from view.  Their light is weak and brief, and I lose my night vision when I turn on a flashlight to capture the critter.

Firefly larva
Ventral view of larva- note jaws- Click to enlarge
When disturbed by picking them up or touching them too much, they roll up in a protective ball similar to a pill bug.  They can't make the perfect ball, leaving their sides and legs somewhat exposed.  On the other hand, they no longer look edible.

Larva rolled up in defensive position
The light of a firefly is produced by a chemical called luciferin which is heat resistant and can be triggered to glow by the enzyme luciferase.  All of the energy of this reaction is released as light with no heat produced unlike our lightbulbs.  Now that is a reaction to bottle!

Larval green light-
The larvae light is only a third as bright as the adult and is shifted toward the green spectrum.  Even the light mechanics and location differs from the adult firefly.  I don't know why it uses energy to produce the light as it seems to be of no biological advantage.  One theory is that it is aposematic color, warning off predators that it will taste bad.  My theory is that they are practicing for adulthood, like a group of junior high boys and girls. has lots of practical information on fireflies and their diminishing populations.
Detailed scientific information on fireflies is at this MDC site.
More than you may want to know about raising fireflies is at this link.

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