Have you ever found yourself in a situation where you can’t tell if those leftovers in your fridge have gone bad or not? If so, what was the first thing you did? If you’re like me, you probably held it up to your nose and took a sniff to determine if it was still edible. Well, imagine if all you had to do was touch the leftovers with your hand or even your leg in order to smell it.
It may sound absurd, but this unique ability is one of the many interesting characteristics of the bluebottle butterfly, Graphium sarpedon, which lives in southern Asia and Australia. Known as “sensilla basiconica,” protruding structures on the butterfly’s legs are one of nature’s amazing examples of the relationship between form and function [1]. The swollen base and blunt tip of these appendage projections are specifically grouped and equipped with highly sensitive receptors (known as chemoreceptors) designed for olfaction and detection of chemicals. This is starkly different from humans and other species of animals that typically only have these sensory systems localized on and around their face—it is as if the bluebottle butterfly’s whole body is its nose!
These chemoreceptors are how butterflies experience most of their world. These sensory receptors are connected to neurons, and many of them are highly specialized to detect certain signals. For example, some of those sensilla basiconica on their legs are specially designed to sense dissolved sugar, thus pointing the butterfly in the direction of its next nectar meal.
In addition to finding food, both male and female bluebottle butterflies have chemoreceptors on their antennae that are critical for sexual reproduction and survival. Females will use these chemoreceptors to detect males that are secreting pheromones—a sexy scent particular to males that are ready to mate. After mating, the females will use different chemoreceptors located along the base of the sensilla basiconica to detect which flowers are suitable for hosting their eggs [1].
Taste and smell aren’t the only whacky sensory adaptations that the bluebottle butterfly possesses. This species of butterfly – along with many other species of butterflies and moths – possess a form of extraocular (or “beyond the eye”) vision that is localized in their genitalia [2]! This is made possible by the two types of photoreceptors that reside within their genital region. Photoreceptors are the very special cells that allow you, me, and pretty much all other animals to see! The cells convert light waves into nerve signals that our brain then uses to create a picture of the world around us. While humans only have photoreceptors in our eyes, the bluebottle butterfly appears to have them in their eyes and genitals. Light from the surrounding environment emits wavelengths that pass through a transparent “window” that lies on the outer shell of their genitals. While the localization of these sites varies slightly between males and females, both of them appear highly sensitive to violet and blue light. Considering that these “extra eyes” are localized near and around the sexual reproductive organs of the butterfly, it makes sense that their function is to promote successful mating. Males use their genital photoreceptors to achieve correct coupling, whereas females use them to confirm whether their sexual organ (the ovipositor) is properly pushed out [3].
Aside from having vision in their genital region, the bluebottle butterfly is also known to have a complex array of photoreceptors within its actual eyes that endow it with a wide array of spectral sensitivities – much more than humans! For example, butterfly eyes contain photoreceptors for fifteen distinct spectral sensitivities [4]. This is the most recorded in any single insect eye – not only can butterflies see all of the colors of the rainbow that humans can, but they can also see ultraviolet (UV) light! It is theorized that the bluebottle butterfly has evolved to visualize the UV radiation of the sun so that it can navigate and orient itself.
Despite all of these incredible visual features, there is one last difference between butterflies and humans in which WE are actually superior – this butterfly can’t focus its vision! The receptors within the eye of the bluebottle butterfly can see part of the whole, but they do not contain a lens through which they can focus their vision [4]. Thus, the butterfly spends its entire life seeing objects as rough blurry shapes rather than the finer forms humans are used to. Oddly enough, this seems sufficient for the butterfly to live, reproduce, and gather food. It’s amazing to think that despite all of our differences, every species has tradeoffs that may best the other.
Michael Saturno is a senior at Dartmouth College, majoring in Neuroscience with the intention to pursue medical school. He was fortunate enough to learn from Dr. Kelly Finn in her Exotic Sensory Systems class and learned much about the umwelt variations that exist between humans and non-human animals.
References:
[1] Dey, S., Hooroo, R. N., & Wankhar, D. (1995). Scanning electron microscopic studies of the external morphology of sensilla on the legs of a butterfly, Graphium sarpedon (Lepidoptera—Papillionidae). Micron, 26(5), 367-376.
[2] Arikawa, K., & Aoki, K. (1982). Response characteristics and occurrence of extraocular photoreceptors on lepidopteran genitalia. Journal of Comparative Physiology, 148(4), 483-489.
[3] Arikawa, K. (2001). Hindsight of Butterflies: The Papilio butterfly has light sensitivity in the genitalia, which appears to be crucial for reproductive behavior, BioScience, 51(3), 219–225.
[4] Chen, P. J., Awata, H., Matsushita, A., Yang, E. C., & Arikawa, K. (2016). Extreme spectral richness in the eye of the common bluebottle butterfly, Graphium sarpedon. Frontiers in Ecology and Evolution, 4, 18.
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[Edited by Meredith Lutz]
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