Does a Glowworm See? Sigmund Exner’s Study of the Compound Eye

by Christoph Hoffmann

The essay begins:

In 1891, the Viennese physiologist Sigmund Exner published the first comprehensive study of complex, multifaceted eyes. Today, his book The Physiology of the Compound Eyes of Insects and Crustaceans is a classic in its field. Scholars still refer to Exner’s findings, even if “it took the scientific community 80 years to catch up with his state of knowledge.” The most popular outcome of Exner’s studies was a picture proudly printed opposite the title page. According to the caption, the figure shows “the erect retinal image in the eye of the firefly (Lampyris spldl.).” The original photograph was made with the help of Josef Maria Eder, author of the Handbuch der Photographie and renowned director of the Viennese Graphische Lehr- und Versuchsanstalt (Academy of graphic arts). Eder placed the eye—fully stripped of all tissue behind the lens structure—under a microphotographic apparatus, focused on the plane almost directly behind the inner endings of the ommatidia, and then recorded the image that would have formed at this point on the removed retinal layer. The resulting photograph shows the window of the laboratory room; the letter R, slightly distorted, mounted on one of its panes; and finally, beyond the pane, the “Schottenfeld Church and church tower, approximately several hundred paces distant” from the laboratory. This, at least, is what a human observer notices when studying the picture. Far more questionable is what the glowworm might see. Even if the photograph exactly reproduces the image “which the eye of the glowworm beholds,” as Eder claimed, it is not at all clear whether the resulting impression is comparable to the one perceived by a human being, and, even less clear, whether we can suppose that seeing always takes place in the same way.

From today’s perspective, Exner’s monograph indeed marks a moment in which the then still uncontested equating of the visual process with the perception of images took shape as a widely valid but nevertheless limited understanding of vision. The phrase “from today’s perspective” deserves special emphasis because Exner himself never drew this conclusion in his writings. My suggestion is motivated, rather, by a tension between Exner’s focus in research on the mechanisms of image formation and his ideas about the purpose of the compound eye in animal life. This tension is most noticeable in the photograph of the retinal image of the glowworm (the folk name for the “firefly”). The impression captured is in some respects misleading because the depiction of objects, according to Exner, plays only a minor role in seeing with a compound eye. Instead, for him, the main purpose of such an eye was to detect perceptible changes in the environment.

In the following pages I argue that the unspoken conflict marking Exner’s discussion of the compound eye may be considered a window into the way Western scientific thought has constructed a framework for vision since at least the beginning of the seventeenth century. In part 1 I focus on the fact that Exner, with the photograph of the retinal image, visually cited a crucial experiment. For scholars in physiological optics, the retinal image did not constitute one phenomenon among others, but represented the very moment when seeing began: the formation of a picture of the outside world in the back of the eye. Originally based on observations made with the single-chambered-lens eye, image formation also guided scholars who, since the end of the seventeenth century, had begun to explore the capacities of the compound eye. As part 2 shows, the very different structure of such an eye only underscored the notion that the formation of the retinal image must take place slightly differently. Exner’s studies followed this line of research; for the two basic types of compound eyes, he successfully documented how the complex lens apparatus is able to form a retinal image of the outside world. Nevertheless, as already noted, Exner also subverted this line of research by emphasizing that such an eye must be particularly apt for perceiving changes in the environment. In part 3, I frame the resulting inconsistency, to quote Ludwik Fleck, as due to the “tenacity of systems of opinion.” I argue that the concept of vision based on the perception of images was so dominant that Exner overlooked the potential consequences of his own ideas. Continue reading …

What does seeing mean with respect to different living beings? Is seeing with a compound eye similar to seeing with a single-lens eye? In what way is vision conceptually coupled with the formation of a retinal image? These and other questions arise from consideration of a famous photograph of the retinal image formed by the eye of a glowworm in Sigmund Exner’s authoritative treatise on compound eyes of 1892.

CHRISTOPH HOFFMANN is Professor of Science Studies at the University of Lucerne. His current research focuses on experiments in animal communication and data work in the sciences.