About a week ago while sitting on the back porch watching night fall, I saw the first golden flash of summer. Soon a dozen fireflies, or lightning bugs as they are often called, patrolled the backyard. Another handful flashed from perches in the tall grass on the edge of the yard.
Flashing fireflies mean summer has arrived. When my daughters were little girls, we’d catch a few fireflies and put them in a jar to make a natural lantern. It’s hard not to marvel at a creature that carries a built-in flashlight.
Purpose
But the purpose of firefly bioluminescence is not to light the way. Just as birds communicate by singing and silk moths use pheromones, male and female fireflies communicate with unique Morse code-like flash patterns.
These flash patterns vary in color, number of flashes, duration of flashes, time between flashes and time of night. Males flash in flight; females respond from stationary perches in shrubs and tall grass.
It’s one of those gee-whiz phenomena that always fascinates.
There are many species of fireflies, and each uses a specific flash pattern. Entomologists use flash patterns to identify fireflies in flight.
Unique flash patterns prevent males of one species from wasting time and energy trying to mate with females of the wrong species.
Flash system
For a particular species, for example, the flash system works something like this. At sunset males crawl out of the grass and begin to fly around the yard. Every seven seconds each male emits a bright yellow, half-second flash. The flashes are not synchronized, so a casual glance suggests these insects are flashing at random. Follow one individual, however, and a pattern emerges.
Sooner or later a female recognizes the male flash pattern for her species. Precisely three seconds after each male flash, the female replies with a half-second flash. When the male sees the appropriate flash coming at the appropriate time interval, he flies closer to investigate.
All the while the male flashes at seven second intervals, and the female responds in kind every three seconds. They may repeat this exchange as many as 10 times.
Ultimately the male flies down to the female, who is hidden in the grass, and the pair mates.
Easy to test
This system is easy to test. Study the flash patterns of the fireflies in your backyard. After identifying the length and rate of the female flash (the one in the grass), mimic it with a flashlight. If your impression is good, males will approach in search of a receptive female.
But males must be careful and not respond too quickly because if they respond to an impostor, they pay the ultimate price. Females of some species mimic the flash pattern of other species, especially species in which males are small.
If a male of the smaller species is fooled by an impostor, he will approach with only one thing on his mind. But when he locates and approaches the larger female impostor, she grabs him and eats him.
Fewer flashes
In some places, though, people are seeing fewer flashes, and entomologists don’t know why. Some speculate that firefly numbers are down because of our lawn care habits.
During the day fireflies rest in the grass, and they may succumb to the blades of lawn mowers. Or perhaps our extensive use of fertilizers, weed killers and pesticides are taking a toll.
Another possible problem, especially in urban and suburban areas, may be light pollution. Street lights and backyard security lights may disrupt fireflies’ ability to detect and interpret flash pattern of the opposite sex.
Whatever the problem, scientists at the Boston Museum of Science, Tufts University and Fitchburg State College have launched a citizen science project to assess firefly population numbers nationwide.
Firefly Watch
Firefly Watch is recruiting volunteers to devote just 10 minutes each week to monitor firefly numbers in their backyard. Complete details and instructions are available at www.mos.org/fireflywatch/.
Like Projects MonarchWatch, FeederWatch and Bud Burst, Firefly Watch is a great way for any citizen to collect valuable data that is used in real research. And it’s a great way to get kids outdoors and spark an interest in nature and science.
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