We’ve all heard the song, they lit up the world as I fell asleep, and so on. Soft bodied beetles of the family Lampyridae, frequently known as fireflies are often cited as a prime example of bioluminescence. But how do they do it? The answer lies in their anatomies, some impressive optical physics, and partially, in urine!
The source of a firefly’s light comes from its light organ. This light organ is composed of three main segments, being the ventral photogenic layer, the dorsal reflector layer, and the cuticle. The photogenic layer is where light is actually produced in the firefly’s light organ, and the cuticle helps to transmit this light to the outside world. The dorsal reflector layer is composed of specialized tissue which amplifies light emitted by the photogenic layer, by reflecting light towards the outside world, hence its name (Chen et al. 2015).
Uric acid is one of the most ubiquitous byproducts of metabolism in the animal kingdom, and it is an important chemical in the process of waste removal in a majority of animals. A firefly’s reflector layer is made up of many spherical granules made of uric acid crystals. These crystals are the mechanism that allows the reflector layer to reflect light that would not have reached the outside world otherwise (Goh et al. 2013).
Figure 1. The ventral layer of a firefly’s light organ. Pictured on the left is the ventral photogenic layer. On the right is the dorsal reflector layer. The dorsal layer still emits light, despite not naturally producing any on its own, due to the reflective uric acid crystals (Goh et al. 2013).
These crystals are extremely effective at reflecting light of wavelengths between 400nm and 800nm, with the percentage of light reflected off of them reaching 82% at a wavelength of 550nm. When scientists recreated the uric acid crystals using hollow silica microparticles, the intensity of the light produced by chemiluminescent reactions increased by a factor 55.3 (Chen et al. 2015).
The cuticle of the firefly is also quite important to the optimization of light emission from its light organ. The cuticle is made of longitudinal nanostructures, organized in a highly ordered fashion. These nanostructures help the cuticle to act as a single layer which has a refractive index of 1.24, contributing to the reduction of refractive index mismatch between the firefly body and the air. This allows for a brighter and more efficient light organ (Kim et al. 2012).
Firefly cuticles have been the subject of studies pertaining to light, with LED lenses inspired by the cuticle of the firefly light organ being considered as a candidate for more efficient and powerful displays (Kim et al. 2012).
Fireflies use their light organs for multiple reasons. Notably, the light is used as a defense mechanism, communicating to predators that they are unpalatable if eaten. The light is also used as a mating ritual, with brighter lights and specific light patterns in males signalling to females that they are more fit than their competitors (Scientific American, 2005).
Though they may seem simple at first glance, fireflies have complex and impressive mechanisms that allow them to make the night glow in the way that humanity (and Owl City especially) knows and loves!
References
Chen, Linfeng, Xiaodi Shi, Mingzhu Li, et al. 2015. “Bioinspired Photonic Structures by the Reflector Layer of Firefly Lantern for Highly Efficient Chemiluminescence.” Scientific Reports 5 (1): 12965. https://doi.org/10.1038/srep12965.
Goh, King-Siang, Hwo-Shuenn Sheu, Tzu-En Hua, Mei-Hua Kang, and Chia-Wei Li. 2013. “Uric Acid Spherulites in the Reflector Layer of Firefly Light Organ.” PLoS ONE 8 (2): e56406. https://doi.org/10.1371/journal.pone.0056406.
“How and Why Do Fireflies Light Up?” Scientific American, September 5, 2005. https://www.scientificamerican.com/article/how-and-why-do-fireflies/.
Kim, Jae-Jun, Youngseop Lee, Ha Gon Kim, et al. 2012. “Biologically Inspired LED Lens from Cuticular Nanostructures of Firefly Lantern.” Proceedings of the National Academy of Sciences 109 (46): 18674–78. https://doi.org/10.1073/pnas.1213331109.
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