During our brief British summers, glow worms have only a few weeks to mate in a blaze of glory before the season ends and their time is up. Which means the pressure's on to find a mate. Fortunately, the females make it easy for the males to track them down, guiding their suitors with a glowing light. David Booth explains that the relationship between the male's visual sensitivity and his glowing amore's spectrum is finely tuned, so the male's retina is most sensitive to the female's glow. While male American fireflies fine tune their vision with filtering pigments to home in on females at dusk, Booth wondered whether British glow worms, which mate after dark, use the same visual mechanism? Or are they able to distinguish between attractive glows and other colours with colour vision? After all, male glow worms are sensitive to blue wavelengths, even though they fail to respond to them. Could they distinguish between colours to fine tune their vision to the female's signal? Testing male glow worm's colour preferences, Booth was amazed to discover that British glow worms do have colour sensitive vision, to home in on a mate after dark (p. 2373).
But before Booth could begin testing the insect's spectral sensitivity, he had to overcome the problem of their short seasonal lifespan. Fortunately, after months of painstaking work, Booth discovered how to rear the Lampyris noctiluca larvae to adulthood, in a fraction of the natural time, at any time of year. He was even able to reset their circadian rhythms, so he could test their visual preferences during the day.
Curious to know whether male glow worms use optical filters or colour vision to tune their spectral sensitivity, Booth scrutinised the male's eyes with a high powered microscope and a found a layer of yellow pigment; ideal for filtering out blue light and tuning the male's vision to his mate's greenish glow. But Booth needed to know whether the glow worms failed to respond to blue wavelengths because the yellow pigment filtered the wavelengths, or distinguish between colours with true colour vision and reject glowing spectra that didn't match their amore's greenish glow.
Working with Daniel Osorio, he set glow worm males the equivalent of a blind date test; he offered them a choice between a female glow worm's green spectrum, and another which had been tweaked slightly with blue wavelengths, and waited to see which mate-mimic they chose.
Booth admits that the yellow pigment in the male's eyes had convinced him that the males would head for the blue enhanced glow; but they didn't! Out of 23 males, 20 headed straight for the green glow worm decoy. The males were using colour discrimination and true colour vision.
Having discovered how male glow worms improve their chances of catching a gal, Booth and his colleague Alan Stewart are keen to find out how the insects cope with light pollution from our 24 hour culture. After all, British glow worms have tuned their visual systems to track down females after dark.
- © The Company of Biologists Limited 2004