Lots of things in the natural world emit light. Bioluminescence is a chemical process through which living organisms produce “cold” light. The mechanism can be different for different species but basically, living things make light the same way a glow stick does.
The oceans especially are full of bioluminescent creatures: cephalopods like the vampire squid, bacteria (including those that live under the eyes of some fish) and crustaceans like ostracods (also known as seed shrimp). One of the most numerous vertebrates on earth, the lantern fish, is (as their name implies) glow-in-the dark.
Here in Chicagoland, we also have bioluminescent organisms. The most popular are the fireflies or lightning bugs in the family Lampyridae. There are more than two dozen species that occur in our region; my kids identified at least two species in our yard last night. Species can be differentiated by size and anatomy as well as by how they flash. That said, when girls flash their message, the boys respond with a different pattern of flashes. In addition, females of some species can flash messages that attract the males of other firefly species. Since they are different species, the female is not trying to attract a mate, rather she’s ordering dinner. By eating the male that she lured in, she not only gets more protein to make strong eggs, she helps ensure that there will be less competition for her babies once they hatch.
Lightning Bug / firefly / glowbug
Maybe I should point out that whether you call them fireflies or lightning bugs, they are not flies (order diptera), nor are they bugs (order hemiptera), they are order coleptera—the beetles. This means they lay eggs that hatch into caterpillars that look a lot like little mealworms. (We have such a limited vocabulary for insect common names; these are, of course, also coleopteran--not worms, which are not even arthropods). You might not know it but you appreciate these little “worms” because they are voracious predators and eat a lot of garden and flower pests. However, because they have to live in your yard for a year before reproducing, a heavy application of pesticide will kill off most of your predatory “good” insects, including lightning bugs, for the rest of the year (or more) but will only kill of the herbivorous “pest” insects for a short time. Pests reproduce more quickly and disperse more widely and will re-colonize your yard long before the predator populations recover.
One of the neat things about fireflies is that in many species both their eggs and their larvae glow. And (given our limited vocabulary for common names) what do we call the glowing larvae of a beetle? Glowworms, of course! I have never been lucky enough to find glowing beetle eggs but I have regularly seen glowworms. Unlike the adult form, glowworms are relatively dim. In fact, lightning bugs may be the brightest bioluminescent organisms in the Midwest. It might seem that glowing at all life stages would just attract predators but, there are actually relatively few things that can tolerate the toxins of a lightning bug--don't feed them to your pets.
Despite their biological brilliance, light pollution can make even adults hard to see well and their terrestrial larvae are even more obscure. So, to find glowworms and most other luminescent life, you must acclimate yourself to the dark.
Seeing in the dark is not a ninja mystery but we are so reliant on bright-as-day electric lights we have forgotten how our ancestors functioned for half their lives. First, unless you’re doing something technical like fixing a car’s fuel line on a deserted road at one in the morning, I don’t think you need a flashlight. General hiking certainly does not require a flashlight, even on a moonless night. That said, when you first step from the light of a campfire (or recall my advice and put your flashlight back in your pocket) you will not be able to see. There are many neurological reasons for this involving rods and cones but they don’t matter practically; all you need to know is: be patient. Get away from artificial light sources, pause, even close your eyes, and count to 30 or more. Don’t squeeze your eyes shut, just rest them for a moment. When you open them, you’ll be able to see. Don’t expect to be able to see in color (read about those rods and cones if you you'd like) and don’t expect to see lots of detail. Do expect to see a new world of shapes, impressions, movement and sound.
Once you have learned to see in the dark, start walking around in the forest. Look to the ground and you’ll likely see glowworms and maybe more.
In the Midwest, especially in forests that are old and moist, you might see foxfire. This is a fungus that grows on decaying wood. To modern eyes it is a very dim blue green glow. Apparently though, it was bright enough to American colonist’s eyes for Ben Franklin to suggest its use to illuminate boats and scientific instruments. Once you can appreciate the brilliance of foxfire you’ll be well on your way to appreciating the fireworks that nature puts on for us every evening through a variety of amazing biological reactions (and maybe you’ll have a second thought about spending money on outdoor lighting around your home.) Keep looking for neat nature near your home and you'll discover a whole new world.