• by Dr. Sarah Oktay, Director, University of Massachusetts Nantucket Field Station •
The gentle glowing of fireflies that surround me at the UMass Boston Nantucket Field station is a welcome summer visitor. I was surprised when a group of students from Worcester was shocked and delighted to see these creatures. Light pollution in the larger cities makes the subtle glow of fireflies hard to distinguish and our dark skies allow us to view these beauties easily each summer. I wrote about fireflies back in 2011 and 2009 (http://yesterdaysisland.com/2011/science/9.php/) and have learned a bit more about them since then. I am especially curious as to when exactly they first light up each summer; their pyrotechnics seem to coordinate with humidity and temperature and inquiring minds want to know.
Fireflies, also known as lightning bugs, are neither flies nor bugs but are beetles (Order Coleoptera) from the family Lampyridae, which is a pretty straightforward name. There are a lot of differences between true bugs and beetles and we will save that for another column this summer, just trust me on this……fireflies are not bugs. The larval form and larvae-like females (larviform) of some firefly species are sometimes called glowworms to distinguish them from the winged forms. There are about 2,000 firefly species! These insects live in a variety of warm environments, as well as in more temperate regions, and are a familiar sight on summer evenings.
Fireflies love moisture and are native to the more humid regions of Asia and the Americas. In drier areas, they are found around wet or damp areas that retain moisture. Some fireflies are strictly crepuscular (love this word; sounds both creepy and muscular, like an athletic zombie) creatures which means they are primarily active during twilight as opposed to nocturnal (true night-time loving) and diurnal (daytime) animals. The life cycle of most firefly species takes two years. A few days after mating, which occurs in the spring, a female lays her fertilized eggs on or just below the surface of the ground. The eggs hatch 3-4 weeks later and the larva feed on worms and slugs by injecting them with poison and slurping them up (yum!). In the United States, fireflies may persist in the larvae stage for 1-3 years. Fireflies overwinter (some species for several years) during the larval stage. Some do this by burrowing underground, while others find places on or under the bark of trees. After several weeks of feeding, they pupate for one to 2.5 weeks and emerge as adults in late spring early summer. The adult firefly’s life span lasts only a few weeks, during which reproduction is their main goal. Fireflies can grow to a size of one inch (2.5 cm). Not all adult firefly species glow, but most of the larval forms do. The ones on Nantucket appear to be the most active at twilight.
Fireflies get their name from the bioluminescence coming from a light-producing organ in the beetle’s abdomen that contains a substance called luciferin (a substrate) which when combined with luciferase (an enzyme), ATP (adenosine triphosphate), and oxygen reacts chemically to release energy in the form of light. This chemically-produced light, emitted from the lower abdomen, may be yellow, green, or pale red in color, and has a wavelength from 510 to 670 nanometers. The eggs, larvae, and pupae of members of this family are often luminous as a way of signaling to predators that they are not tasty, and can be mildly toxic. Firefly larvae even flash like the adults in response to gentle stimulus such as tapping them. A firefly’s glow is an extremely efficient process, converting 90-100% of the energy to light instead of heat. The average incandescent light bulb only converts 10% of its energy to light, wasting the rest as heat. Their bioluminescence produces a cool light that does not burn your hand, or the creature creating it.
It makes sense for the larvae to glow as a protective measure. So why do the adults glow? Well, just as one would think, to pick up a date (or a meal as we’ll see below). Around dusk, the male takes to the air and flies over and around vegetation, flashing its species-specific code. The female, usually perched on vegetation, responds by mimicking the signal a few seconds later. After five to ten signal exchanges, the male will have homed in, and mating takes place. Aspects of male flash patterns are also thought to be affected by sexual selection. Female fireflies have been shown to prefer certain characteristics of a male’s photic signal (such as increased flash rate) and respond preferentially to males that possess these “sexy” signal components (Branham, M.A. and M.D. Greenfield 1996. Nature 381:745-746). Where this light dance goes horribly awry for one of the pair is when it involves the female of the genus Photuris, a large eastern U.S. Species, who is the “femme fatale” of the firefly world. She is capable of mimicking the flash code of other species and thus lures in many different males – not for mating but for eating!
Males from a few species, such as Photinus carolinus in the US found in the Smokey Mountains synchronize their flashing. Scientists are not sure why they do this but they think this helps them attract mates in masse and then leave the sorting up to the females (http://www.nps.gov/grsm/naturescience/fireflies.htm).
We know the larvae eat slugs and worms using their B Movie skills to track other insects via slime trails and inject poison in them in order to subdue and eat them. Adults typically feed on nectar or pollen, though scientists believe that some adults might not eat at all. This Tufts University web site shows you everything you might want to know about how fireflies make their flash and how to induce that flashing behavior using nitrous oxide (http://ase.tufts.edu/biology/Firefly/). I found a lot of firefly research and videos as well as advice on how to photograph them at http://www.firefly.org/firefly-resources.html. Fireflies are a relatively unexplored topic for entomologists and in many areas of the world where fireflies were extremely abundant, like the Mae Klong River in Ban Lomtuan, Thailand there is some worry that firefly populations may be crashing. Pesticide use, habitat destruction, and light pollution from nearby cities which can confuse and distract fireflies trying to mate have been listed as some of the possible causes for the reduction in firefly numbers observed worldwide. How big of a problem is this? How many bugs have we lost? No one seems to know. Researchers have had difficulty finding enough scientists to study the short-lived (as adults), delicate, and sometime elusive creatures. Citizens to the rescue! This is an excellent example of a subject that citizen scientists can help illuminate (sorry, easy pun). The Museum of Science in Boston has started the Firefly Watch in conjunction with scientists from Tufts and Fitchburg State (info at https://legacy.mos.org/fireflywatch/). By logging in, you can become an observer and add data to the national database. You can even determine the species of Photinus fireflies by the timing of the flashes, which vary between males and females.
In recent news, it appears that a glowworm species which is part of the beetle Family Phengodidae is responsible for beheading and killing the millipedes on Tuckernuck! Here is a site showing one actually inside a millipede: http://www.whatsthatbug.com/2010/09/24/glowworm-10/! The burnt out remains I found with my junior rangers on Tuckernuck indicate that a glowworm species is likely one of the millipede assassins. We will be checking again this summer.
The chemical magic that goes into a firefly’s abdomen may help scientists detect abnormal cells! From http://www.firefly.org/facts-about-fireflies.html: “Fireflies are medically and scientifically useful: The two chemicals found in a firefly’s tail, luciferase and luciferin, light up in the presence of ATP. Every animal has ATP in its cells in amounts that are more or less constant—or should be. In diseased cells, the amount of ATP may be abnormal. If the chemicals from fireflies are injected into diseased cells, they can detect changes in cells that can be used to study many diseases, from cancer to muscular dystrophy. But that’s not all they’re used for. Electronic detectors built with these chemicals have been fitted into spacecraft to detect life in outer space, as well as food spoilage and bacterial contamination on earth.”
If glowing were not bad enough, when attacked, fireflies shed drops of blood in a process known as “reflex bleeding.” The blood contains chemicals that taste bitter and can be poisonous to some animals. Because of this, many animals learn to avoid eating fireflies. And it is important to know that you should never feed fireflies to lizards, snakes and other reptilian pets.
The relatively limited amount of light pollution and pesticide use on island means Nantucket should be an ideal place to study them. To attract fireflies (and wildlife and other beneficial bugs) to your yard, cut back on pesticides and outside light. One of the reasons why summer, although chilly, can be enchanting on Nantucket is our limited light pollution which makes it easy to pick out these glowing beauties which have been around for a couple of weeks. But scientists still are not exactly clear on what triggers these guys to start flashing every summer. About a week and a half ago, when the nighttime temperature was at least 68 degrees, we had a light show of displaying fireflies. And last night, July 6th, the twilight temperature was 19 degrees Celsius (66.0 degrees Fahrenheit) and I could see at least a dozen fireflies flashing away. Keep your eyes open and enjoy these summertime wonders.
Other cool websites about fireflies:
- National Park Service: http://www.nps.gov/grsm/naturescience/fireflies.htm
- How fireflies control their light: http://www.scientificamerican.com/article/how-and-why-do-fireflies/
Featured Photo by Jenn Forman Orth