A research team from the Bavarian Natural History Collections in Munich has uncovered a novel communication method in harvestmen, a type of arachnid. The study, published in the journal Scientific Reports on January 26, 2026, reveals that five closely related species exhibit unique fluorescent patterns on their backs, which are particularly pronounced under ultraviolet light. This discovery suggests that these patterns facilitate species recognition, especially during twilight and moonlit conditions.
The team conducted their research in the lowland rainforest of the Peruvian Amazon, where they observed the five species of harvestmen during nocturnal surveys utilizing ultraviolet flashlights. Despite their similar sizes and coloration, each species possesses distinct pale patterns on their backs. These patterns, consistent within species yet markedly different between them, glow a vivid blue-green when exposed to ultraviolet light. Such visibility likely aids these arachnids in identifying one another in low-light environments, a critical advantage for nocturnal and crepuscular species.
Microscopic examinations indicate that the fluorescence emanates from the outer layer of the harvestmen’s bodies. Beneath this layer lies a complex structure of crystalline mirrors that reflects incoming light and enhances the emitted fluorescent glow. Previous studies have established that harvestmen can detect both short-wave light and their own blue-green fluorescence. The current research confirms that their eyes, while simple, are sufficiently sensitive to recognize high-contrast patterns from short distances. This capability is particularly useful as these arachnids navigate three-dimensionally through foliage and across varied terrain.
The importance of these findings lies in the necessity for reliable species recognition among closely related species that inhabit the same area and are active at the same times. Such recognition is essential for mating and avoiding mispairing. Stefan Friedrich, the first author of the study, stated, “Our study provides strong evidence that fluorescence serves the specific purpose of highlighting species-specific traits.” He emphasized that the combination of a fluorescent surface and a reflective underlayer likely serves a significant biological role, rather than being a mere byproduct.
The study outlines several key criteria that support the role of fluorescence in communication: the dorsal patterns are clearly visible, distinctly defined, and specific to each species, aligning with the lighting conditions under which the harvestmen are active. Furthermore, the arachnids’ eyesight enables them to perceive these patterns effectively.
Despite these compelling indicators, researchers acknowledge that further behavioral experiments are necessary to definitively establish the function of the fluorescent patterns. The research involved collaboration among scientists from the Bavarian State Collections of Natural History, Ludwig Maximilian University of Munich (LMU), and Munich University of Applied Sciences. Fieldwork was conducted at the Panguana Biological Research Station in Peru, highlighting the international effort to deepen understanding of arachnid communication.
This research opens new avenues in the study of biofluorescence and its potential functions in the animal kingdom, shedding light on the complex interactions that occur in one of the planet’s most biodiverse ecosystems. The findings not only enhance the understanding of harvestmen but also contribute to broader discussions about communication mechanisms in various animal species.
