How to Identify and "Call" Lightning Bugs or Fireflies
An "Enlightening" Conversation
As summer rolls around, it's time again for the emergence of fireflies or lightning bugs. They are the bug all kids love to chase and catch. These benign insect ambassadors have been many children's introduction to the world of beetles and friendly insects. For today's post I want to introduce you to the three most common groups of fireflies in the Eastern US, and how you can use LED flash patterns to call the fireflies to you. It makes a great lesson plan for evening programs, or just something fun to do with the kids on a summer night.
Fireflies, or lightening bugs, are beetles in the order Coleoptera, and there are about 170 species in North America. Unlike their cousins, that have hard-bodied elytra (or wing coverings), their bodies and wings are relatively soft, with leathery wing coverings. Their bodies are usually about 2 cm long, and blackish, with reddish or yellow spots on their head covering (also called the pronotum). Around the world, there are over 2,000 species of fireflies, and most live in tropical, moist and damp areas, in part because of their soft bodies. Lightning bugs are called this because their abdomens glow or light up, using a chemical process called bioluminescence. We'll get more into this in a bit, but first, let's look at their life cycle.
In the Eastern US there are three common families of fireflies that we see, Photinus, Pyractomena, and Photuris. You can distinguish them apart by looking at their pronotum (big segment with dot behind their head) and their wings.
Photinus are the most common, they are about 1/2" long and have a big black spot on their pronotum along with yellowish edges around the pronotum. Their wing edges are also yellowish.
Futures are the largest of the three species, coming in at about 1" long. They have a yellow arc across their pronotum, and below it is a red arch that is split with a black stripe. Their wings have a brownish line down the middle and brownish color around the outside. Their legs are also very long.
Pyractomena are a bit harder to distinguish, because they are the same size as the Photinus, about 1/2" long, but they have a much darker pronotum, with black edges and very little red. There is a also a big black dot in the middle of the pronotum. Their wing edges are yellowish.
Both females and males can fly, which is unusual for creatures that use bioluminescence like jellyfish, scorpions, fish, and some fungus. Most are also crepuscular, meaning that they can be seen mostly at dusk and dawn (this term was on the TV show Jeopardy, it's a great one to pull out! Everyone loves a Jeopardy champion!). However, there are a few purely nocturnal and diurnal species. The diurnal species typically don't flash. Most fireflies can glow throughout their lives. However, the glowing of the larvae is more of a warning to predators that they are distasteful, while the glowing of the adults is to advertise for a mate, but we'll talk about this in just a bit.
After the females mate, they then lay their eggs in moist leaf litter. When the eggs hatch, the larvae then burrow down into the soil and begin to eat other firefly larvae, slugs, snails, and soft bodied invertebrates that live underground. The larvae of some species have a special set of mandibles with chewing mouth parts. They can inject a type of anesthetic substance through hollow fang-like ducts in their mandibles into their prey. This immobilizes the prey and helps digest them. As the larvae get larger they become more terrestrial, and move above ground. These larvae are sometimes called "glow worms" because they glow with bioluminescence. In several species of firefly the females are larviform, which means that they retain the characteristics of the juvenile larvae (they look like soft, squishy grubs) but their rear lights up, and they have large compound eyes so they can search for males.
What do adult fireflies eat? Good question! Most adults prefer a vegetarian diet of pollen (protein) and nectar (carbohydrates), but there are a few species that eat insects. In one or two cases the adults don't even have mouth parts. In the Eastern US, most of our species, as adults, prefer nectar and pollen. Fireflies are not top on the dinner menu of nocturnal predators. The adults and larvae are foul-tasting because of the chemicals they produce in their bodies. If you want to really geek-out you can read a paper all about the steroidal pyrones, or chemicals found in fireflies which are similar to those found in poisonous toads, in the paper by Eisner et. al. in the Proceedings of the National Academy of Sciences.
There are several ways to distinguish males from females, too. As mentioned earlier, some females remain larviform. As a general rule, you can also look for behavior. Males usually fly and flash, while females usually remain on vegetation (sitting still) and flashing. Height preference, as to where they sit, varies by species.
If you remember, in the Eastern US there are three common families of fireflies, Phytinus, Pyractomena, and Photuris. Telling males and females apart for these species is a bit trickier, but looking at their abdomen can be reliable. This is because it is final segments of the firefly's abdomen that light up, so they are lightly colored and glowing at night. In the common species of Photinus and Pyractomena the females have their light organs on the next-to-last segment, while the males have theirs on the last two parts. Typically the males have larger abdominal light segments too. In the Pyractomena (another typical family of fireflies) the females have light organs on the last two segments, on the sides, with armor like plating in the middle. Photuris is a bit harder because both males and females have their light organs on the final two body segments, but if you look close at the females, their light organs don't go all the way to the edge of their abdomen. This leaves a clear space around the borders of her light segments.
So, how exactly do the fireflies make their abdomens glow? If you've ever caught them, then you know they are not flaming balls of fire and they're not hot to the touch. The type of bioluminescence they use is chemical, and it's a cold process, which means that it doesn't produce heat from the chemical reaction. The enzyme that they produce is called luciferase, and it acts on luciferin. This happens with a cocktail of the luciferase, luciferin, ions and ATP for energy, along with two parts oxygen (O2). The luciferin is produced by specialized cells, that then convert it into the enzyme luciferase. The luciferase reacts with oxygen to form an inert (non-heat producing) molecule called oxyluciferin. Want to learn more about this process? You can geek-out even more and read the publication, "The Colors of Firefly Bioluminesence: Enzyme Configuration and Species Specificity" in the Proceedings of the National Academy of Sciences (warning, the article is very technical). It's fascinating how each species has its own unique cocktail of butt-glowing-enzymes.
Now to the flashy-part. Along with their own unique butt-glowing cocktail of enzymes they also have their own unique flash pattern. These flash patterns can be broken down by looking at the following features;
- Time of night they are active
- Temperatures (flash patterns also vary by outside temperature)
- Length of flash
- Number of flashes
- Time between flashes
- Flight pattern (sitting or flying, and the patterns in which they fly)
Of the three most common species I mentioned (for the Eastern US), here are their colors:
- Photinus: Yellowish-green flash
- Photuris: Dark-green flash
- Pyractomena: yellowish to orangish flash.
By changing all of the variables listed above, several species of firefly can co-exist in one area, and eliminates confusion over who belongs to what species. However, the Photouris species are the femme fatals of the firefly world, because the females have figured out how to mimic the flash signal of the males of other species (in particular Photinus and Pyractomena). The females will "flash" the males and call them closer, whereupon they will pounce upon the unsuspecting male and eat them! This almost sounds like something out of Grimm's Fairy Tales, or lessons learned from the praying mantis mating manual!
A great resource for learning flash patterns is from the Museum of Science in Boston, MA. They have a citizen science program called "Firefly Watch" which allows you to record and report your firefly sightings. Their website has a nice variety of information, including the "Firefly Flash Chart" which give you information about the flash pattern of various species. The chart is somewhat hard to read, and it's really more for the sciency types who can interpret graphs easily, but it's useful. Their information page gives you more background on how to use the chart.
Dr. Firefly's page also has a great graphic to help you visualize the flight patterns in one nice presentation. There's also more from Terry Lynch (Dr. Firefly) here: http://www.firefly.byteland.org/firefly_faq.html.
As they fly, male flash patterns can range from a quick "blip-count-one-two-blip-count-one-two" to something resembling a "J stroke blip-count one-two-J stroke blip-count one-two" and so forth. Firefly patterns can even resemble Morse code, with three quick dashes of light and then three seconds of nothing, followed by three more quick dashes of light. It all depends on the species. Females also vary their patterns in concert with the males, at very specific time intervals. You can observe this at night if you focus on one single male or one single female. It makes for a great science project for kids.
HOW TO CALL THE EASTERN FIREFLY OR BIG DIPPER FIRELY
If you know the flash pattern of a firefly, there's no reason you can't try "calling" them. My own favorite is to try to call in the Eastern Firely or Photinus pyralis (one of the most common in this area).
You can use almost any common LED flashlight, but if you need to buy them in bulk you can purchase them online. (Here's one that I like from Amazon, the Mini-LED flashlight.) They run about $4 for 10. As a rule of thumb I ask participants in this activity to put red cellophane over their flashlights, so as not to "night-blind" everyone, and to limit disruption during the activity. Remember, insects and animals can be very sensitive to bright white light, interfering with many important behavior patterns.
I start by handing out the LED lights, and going over a few precautions. If you're working with children or younger students, then you should remind them that chasing down a firefly doesn't work. You have to be patient and wait for it to come to you (in the case of calling males), or if you're calling females you need to move carefully and slowly so you don't step on them! I find the biggest hurdle in this process is over-anxious "flashers." I will allow them to hold fireflies, but I'll go over how to be gentle if they do catch them to look at them. And we're always "catching and releasing."
Next, remember that timing is crucial. You have to practice your flash pattern and counting. If you're too fast or slow, then it's like you're "saying" something foreign to the firefly. I usually have the entire group stand in a circle, with the LED lights, and then practice "syncing" up counting and flashing.
Here are the patterns I use:
If you want to pretend to be a male, calling a female use this flash pattern:
- Hold your light about 1 1/2 feet over the ground (you can use meter sticks if it helps)
- With the LED light create the shape of a J, by starting at the small hook end of the J and sweeping up about 6-8".
- Restart the next flash approximately 2 seconds later (one Mississippi, two Mississippi) from the "bottom" of the J again (1 1/2 feet off the ground). These fireflies usually hover around the same area so stay close to where you started.
- You can "hover" and move forward in small increments if you start to see a female respond. She should be flashing from near the ground or in the leaves, with the pattern listed below.
If you want to pretend to be a female, calling a male use this flash pattern:
- Sitting on the ground, hold your light in the grass or about 4-5" off the ground.
- After seeing a male flight pattern (the one that looks like a "J") wait two seconds till after he finishes his flash, then give one short flash.
- Repeat this, "calling back" procedure until the male comes in to you.
FLASHING PATTERN GAME: You can also modify the "syncing" activity by writing flashing patterns, in pair on note cards. For example, you could write 0X0X or 00X0 as a flash pattern, with X meaning one rest and 0 meaning a flash. You can even change this up and add patterns like J's or longer pauses. Randomly give each person an index card with a pattern, and then have them spread out and flash their pattern and see if they can find their partner. If you don't have LED lights you can do this with hands, in the light. With open hands being a flash and a fist being a pause.
When you conduct a firefly calling activity, it's best to have everyone spread out around the yard before they begin calling. Be sure that you can keep track of everyone, so that there are no accidents or lost visitors. As a general rule, it's always good to capture-and-release your fireflies. Be sure that if you are "capturing" them, that you use a jar with holes and that they are not overly hot or housed for too long. Fireflies are fun and fascinating creatures, with much to teach us about patience, and new ways of communicating!
Here are a few books that you might be able to use to supplement a firefly program:
- Fireflies in the Night (Let's Read and Find Out Science 1)
- Next Time You See a Firefly (Morgan)
- Incredible Fireflies (Incredible World of Insects)
- The Very Lonely Firefly (Penguin, Young Readers L2)