A black firefly crawls along a green stalk on July 6. (Photo: Tom Murray)

Do you remember chasing flickering fireflies as a kid? In Massachusetts, there are at least 20 species of them, and you can see them here. People have uploaded to iNaturalist firefly photos taken at Fresh Pond, Harvard University and Mount Auburn Cemetery and in Cambridgeport and the Cambridge Highlands, just to name a few spots.

Fireflies are not flies, but nocturnal winged beetles. Nonwinged beetles of the same family are called glowworms. Fireflies have the remarkable ability to take in oxygen and combine it with a substance called luciferin to produce light with almost no heat. Each species of firefly blinks in a different pattern.

A firefly larva hides in tree bark at Fresh Pond on April 27, 2009. (Photo: Joe MacIndewar/EarthwiseAware.org)

All species of firefly larvae glow, but only about 75 percent of adult species; it is believed that firefly bioluminescence evolved as a warning signal to predators: Stay away! I’m toxic! And it’s true – when attacked, a firefly releases tiny droplets of blood that contain a toxic and distasteful chemical. Birds, toads and other predators that ignore the glow will be reminded quickly why they leave fireflies alone. 

Fireflies blink to attract mates. The male flies around, while the female, who has nonfunctioning wings, does not fly but rests on the edge of a plant. When she spots a blinking light pattern that she approves of and that matches the pattern of her species, she blinks back. The male flies to the female to mate. If light pollution makes an area too bright, the male still blinks (but not as much) and the female does not respond at all.

One of the earliest fireflies to flash each year, this sneaky elf firefly crawls along a leaf in Sherborn, Massachusetts on June 27, 2022. (Photo: Tom Murray)

One kind of predatory female firefly blinks to imitate the pattern of another species. When the male of that species flies to her, she bites and drinks its white blood. Then she chews up the rest of the male, spitting out the hard parts. These predatory fireflies do not make their own defensive chemicals to keep jumping spiders and birds away. So they devour fireflies that do contain these chemicals.

A few days after mating, a female firefly lays eggs on or in the ground. About a month later, the eggs hatch. The larvae feed throughout the summer and fall, displaying another use for firefly toxin. Larvae prey on worms, insects and slugs by injecting them with it in the form of a numbing fluid. That makes it easier to eat them. (Adult fireflies eat mostly pollen and nectar, although some do not eat at all.)

A black firefly in Cambridgeport on June 15. (Photo: Brian Chan)

The larvae live relatively unnoticed in the ground for up to two years, hibernating over the winter. Some burrow underground; others hide in tree bark. In spring, the larvae emerge. They feed for a few weeks, then form a chrysalis, emerging as adult fireflies a couple of weeks later. Adult fireflies cannot fly at temperatures below 50 degrees Fahrenheit, so they emerge when nightly temperatures are appropriate for flight.

This is why fireflies do not live west of the Rocky Mountains and why in the East, the number of fireflies in a season is tied closely to the weather. Wet springs help more larvae survive, and more larvae lead to more adults.

A common eastern firefly on a tree on July 8, 2021. (Photo: Tom Murray)

The kind of fireflies that live in Massachusetts fly in greatest numbers from late May to late July. Look for fireflies in warm, damp, secluded places, but be aware that these locations are also the perfect spots for mosquitoes to thrive!

Adult fireflies do not live long – their primary purpose is to mate and start the life cycle all over again – while larvae can be killed be lawn pesticides. If firefly populations seem to be declining, it may be because of humans: Habitat loss, light pollution and the use of pesticides all contribute to declines.

A winter firefly crawls on a leaf in Groton, June 2, 2022. (Photo: Tom Murray)

You can take steps to help fireflies survive. Leave some leaf litter or woody debris at the edges of your yard to create a good habitat for larvae. Plant native trees and shrubs to help the soil retain the moisture that fireflies need. Install motion-sensor lights outdoors and close your curtains or turn off indoor lights. Above all, don’t use pesticides.

Have you taken photos of our urban wild things? Send your images to Cambridge Day, and we may use them as part of a future feature. Include the photographer’s name and the general location where the photo was taken.

Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.