Glow-worms shine in order to reproduce

A small green light that can be seen in a summer night is a female glow-worm with reproduction as the sole purpose of its approximately two-week adult life. Anna-Maria Borshagovski is studying these beetles which lure their mates with light. She travels all around the south of Finland measuring the intensity of the light that they emit.

With glow-worms it is the female that seeks to dazzle a mate, contrary to how things generally are in the animal kingdom. The rear end of the female has a light-emitting organ whose pallid green light is produced enzymatically: the luciferase enzyme oxidises a luciferin compound to produce oxyluciferin. The female glow-worm lights her lamp on summer nights.

"The topic of my doctoral thesis is to study the impact of illumination conditions on the shine of glow-worms and the sense of sight of the males", Anna-Maria Borshagovski explains.

"I have raised glow-worms native to both Finland and England because I want to compare the impact of illumination conditions with the characteristics of the glow-worms. If individuals of the two populations, which are raised in the same environmental conditions show differences, then we might be able to conclude that the differences are genetic. However, if the differences are the result of environmental conditions alone, the individuals that are raised would not show differences. I will soon start to analyse this."

Doctoral Candidate Anna-Maria Borshagovski is studying how the light emitted by a female glow-worm and the ability of the male to see have developed according to light conditions. Photo: Antti Yrjölä.

Glow-worms are useful predators

"The larvae eat snails. Small larvae in groups are able to kill a copse snail, and larger larvae will kill a copse snail with a single bite. The larva secretes a poison that paralyses its prey and then bites pieces from the snail with its pincer-like jaws.  Strong digestive enzymes dissolve the snail into very liquid nourishment", Borshagovski says.

Copse snails are an annoyance in gardens, as they multiply rapidly and eat fairly large amounts of plants in a short time. Consequently, glow-worms are a gardener's useful helpers.

The larval stage of the glow-worms last long - from two to four years. Adult glow-worms do not eat anything.

"The task of the larvae is to eat and grow big. The only task of an adult glow-worm is to find a mate and reproduce."

Those wanting to see the glow-worms shine should head to the shore.

The range of distribution of the glow-worms covers parts of Finland south of a line drawn from Kokkola to Kuhmo.  Glow-worms are most common along the south coast.

"Glow-worms thrive best near waterways in damp locations, or wherever there is food for their offspring."

Does the amount of light, temperature, or some other factor limit the northern spread of glow-worms?

"The likely limiting factors probably include the temperature and the length of the growing season, and consequently nutrition as well. There are fewer snails and they are smaller in the north, and copse snails are found only in the southernmost parts of Finland."

"We have received news of sightings of glow-worms from many people at their cabins and in the yards of their homes. Many have stories about glow-worms that are connected with childhood. Boys would collect glow-worms and hang them on their caps in order to impress girls in the village", Borshagovski laughs.

Bitter-tasting offspring

"We have been raising glow-worms for research purposes in the laboratory. The female is fertilised and it lays its eggs very quickly - within just a couple of days after mating. The female places its eggs in soil for protection. The soil needs to be suitably damp, and it is important that it doe snot dry or grow mould. After about a month tiny larvae about a couple of millimetres in length", Borshagovski says.

"We give the larvae snails to eat.  The larvae keep busy eating and shed their skins as they grow. The shedding of the skin is often difficult for the larvae and some of them die."

Glow-worm larvae have been raised in a laboratory for research purposes. Photo: Anna-Maria Borshagovski.

A glow-worm larva crawls and wriggles in a jar where Borshagovski keeps a few of her specimens. A black larva a couple of centimetres long has about ten yellow or orange spots on both sides. 

"The larvae groom themselves with their chaeta and we are not sure if they wash themselves or if they are spreading a substance that makes them smell and taste bad."

Glow-worms do not have natural enemies, as birds and other predators avoid them because of their bad taste.  "Actually, I have not had the nerve to test the bad taste by tasting one", Borshagovski laughs.

Choice of mates studied using tests

"We have equipment at the Tvärminne research station in Hanko that we use to measure the intensity and colour of the light emitted by the females. I move around between the Hanko Peninsula and Konnevesi with this gadget to measure females, so I get good readings for making comparisons on the north-south axis."

Anna-Maria Borshagovski wants to find out what factors affect the intensity of the glow, how the males sense the light, and how they choose from among the females.

"At the latitude of Konnevesi the summer nights have more light than in Hanko, so the living environment is quite different in the north-south direction. I am interested in how the glow of the females and the male's ability to see have developed under the influence of ambient light."

"It would also be interesting to study variations between east and west. My colleagues have compared the genetic variation between the populations and it seems that differences between the countries are quite considerable. Another interesting question is the route by which glow-worms originally came to Finland."

Light pollution makes it difficult for glow-worms to mate. A male will not notice the glow of a female under a street lamp, and will select a female that can be seen outside the beam of light.

LED lamps, especially green ones, closely resemble a female glow-worm.

"We use green LEDs to catch males", Borshagovski says. "A glow-worm trap is simple to make: the top part off of a large soft drink bottle is cut off and turned upside down as a funnel into the lower part. A green LED is placed above the trap, attracting lovesick males that fall into the trap."

"The males are used in experiments in selecting partners, among other things. We had a hypothesis that the male chooses a large female with a conspicuous light and which produces many eggs. It was surprising that size turned not to matter, as the males chose both large and small females. Perhaps the male did not need to be all that choosy as there were several females within reach. The males can actually inseminate at least two females at a time."

However, in nature the male seeks out the largest female with the brightest glow.


Text: Satu Räsänen

Main photo: A female glow-worm shines in the summer night. Gautier Baudry.

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Last updated: 12.7.2018