Common grass blue butterflies (Zizina otis) are toxic throughout their geographic range.

Aposematism consists of promptly identifiable signals paired with a secondary defence (e.g. toxins) and is an effective anti-predatory strategy. Visually aposematic signals are typically described as shades of red, yellow or white paired with black that generate conspicuous signals. Blue animals, however, are often overlooked and seldom tested for toxins even when conspicuous. Using Daphnia mortality assays, I show that the common grass blue butterfly, Zizina otis, is toxic (∼40% fatality, while controls killed near 0%). Female Z. otis were more toxic than males, and tropical populations were more toxic than subtropical ones, with temperate populations being the least toxic. Population colour saturation and luminance, however, did not predict toxicity. My results demonstrate a literature bias in aposematic research and encourage the exploration of toxins in previously neglected taxa. Discovering more colour patterns associated with toxicity will help clarify how aversive learning works, and how signals that communicate unprofitability evolve.