2018-07-28 / Columnists

Nature Notes

Fireflies Light Up Night Sky During Summer on the Island
By Patricia Martin

Not long ago, I was biking home along the east shore of the Island when a light caught my eye. It flickered and disappeared, and then another appeared, and another. It was the lightning bug, or firefly, and I don’t mean the musical festival, a restaurant, or a television show, which all turned up when I Googled the word “firefly.” I remember the lightning bugs fondly when I was a child spending summers at Silver Birches where, usually in a muggy, warm August, they would light up the night.

Fireflies are winged beetles in the order Coleoptera. They are conspicuous because of the bioluminescence which appears during twilight. The light, which they produce owing to a chemical reaction, occurs in the lower abdomen and may be yellow, green, or pale red with wavelengths between 510 to 670 nanometers. The light would be referred to as “cold light” with no infrared or ultraviolet frequencies present. The light that they emit has a purpose, and that is to attract a mate or prey or as a defense. There are, in fact, about 2,100 species of fireflies found in temperate and tropical climates, many of them in marshes or wet woodlands where the larvae have lots of food. The form or stage of the insect that emits light varies from species to species. Sometimes the larvae glows, sometimes female larvae, sometimes adults, sometimes the eggs actually glow.

The bioluminescence occurs in specialized light emitting organs, usually on the firefly’s lower abdomen. An enzyme, luciferase, acts on the luciferin, in the presence of magnesium ions, ATP, and oxygen to produce light. It seems that all fireflies glow as larvae and it seems that it is a warning signal to many predators, since most firefly larvae contain chemicals that are toxic or taste bad.

The chemical that causes them to be distasteful is a group of steroid pyrones, which are similar to a chemical found in some poisonous toads.

In adults, the light was thought to be used for a similar function as a defense, however, it seems that its primary purpose is in mate selection. Apparently, the bioluminescence is used to communicate. They may produce a steady glow or flashing signal indicating species or quality of mates. There are some fireflies that do not produce light, and these are diurnal, that is active during the daylight. These communicate by pheromones to signal mates, rather than light.

After selecting a mate, a few days later the female lays her fertilized eggs on or just below the surface of the ground. The eggs hatch three to four weeks later and the larvae feed until the end of the summer. Fireflies hibernate over the winter in the larval stage. Some species may do so for several years. Some burrow underground, while others find places on or under the bark of trees. When spring arrives, they emerge again. After a few weeks of indulging on other insects, snails, and worms, they pupate, a process that takes between one to twoand a-half weeks. At the end of that period, they emerge as adults. The larvae of most species are specialized predators and feed on other larvae, terrestrial snails, and slugs. Some inject, through grooved mandibles, digestive fluids directly into their prey. As adults, fireflies may be predators, while others feed on plant pollen and nectar.

Fireflies do not flash at the same time every year. Unlike many of us, they love warm, humid weather. It actually helps them to survive. If there is a warm spring and it feels summer-like, it fools the fireflies into appearing early. A mild winter means a larger firefly population, as more of the larvae survive the winter. Wet springs also lead to earlier firefly flashes, usually thought to be owing to the fact that snails, slugs, and pill bugs that the rains bring out are more food for the larvae of the fireflies. Damp weather provides the ideal environment for the fireflies, and dry conditions provide the worst. Droughts and heat waves stress firefly eggs and larvae, causing them to die off before they have time to emerge. Heat stress will also delay the date when fireflies appear.

As I mentioned before, they are also prone to be located near wetlands, and it was true the other night that I saw the most flashes near Lone Lake (the fen on the east shore road) and the ditches along the road that are filled with water from springs or seeps.

Weather not only affects the dates when the fireflies appear, it determines how many flashes. On warm, humid evenings, they can light up a field, as if they were Christmas lights. On cooler nights, when the temperatures are below 50 degrees Fahrenheit, it may take much longer for you to see a single flash of the fireflies. The reason for this is that fireflies, like most insects, are cold-blooded, which means that they take on the ambient temperature, so as the temperatures rise, their activity rises, and the opposite also holds true. When the temperature drops, their activity slows.

Human behavior has been having an effect on our flickering beetle friends. The changes in land use, disruption of habitat, have caused adverse effects on the firefly population. In addition, these insects are sensitive to light pollution. Since they depend on their own light to reproduce, the production of artificial light disrupts their mating.

A study done in 2015 showed for the first time the negative effect of direct illumination on populations, and other studies have shown the detrimental effects of artificial night lighting. Light pollution is a real thing, and it does impact the natural world around us, including to disrupt the romancing of the fireflies. We are fortunate that here at Mackinac, and in much of the Straits area, that we have lots of dark sky places where there is no artificial light.

Trish Martin is a year-around resident of Mackinac Island, has earned a master’s degree in botany from Central Michigan University, and owns Bogan Lane Inn.

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