The Function of Eyespot Patterns in the Lepidoptera

Abstract

1. Lepidoptera belonging to several different families bear eyespot patterns on their wings, often with a very strong resemblance to the vertebrate eye. These have been derived from different morphological structures in different groups, and their evolution is therefore convergent within the order. 2. A study of two individual hand-reared Yellow Buntings has shown that the pursuit of flying insect prey is fully released at the bird's first encounter with a flying butterfly, but that the orientation of the attack is modified by experience. 3. Eyespot patterns are associated with displays which exhibit them. In the case of a Nymphalid butterfly, Nymphalis io, an experiment has shown that the display releases escape responses from Yellow Buntings, and that the major part of this effect is due to the four ocelli on the upper surface of the fore- and hind wings. Experiments with other species of insects suggest that other factors contributing include the bright field of colour surrounding the eyespots, the rhythmic movements of the display, and the rustling noise which accompanies it. The bird's responses to this display waned rapidly in the majority of individuals, but some birds became conditioned to avoid the butterflies altogether. 4. Experiments with models showed that circular patterns presented suddenly to feeding birds (Chaffinches, Yellow Buntings and Great Tits) release escape responses more effectively than do non-circular patterns of the same area and perimeter. An increase in the perimeter of the models, achieved by the use of three concentric circles of the same area as the single circle, increased its releasing value. A flat, eye-like pattern which was so shaded and distorted as to appear three-dimensional was more effective than a similar flat pattern which was not shaded, and hence was more bright. Large models were more effective than similar small ones. Waning was again rapid. The differential responsiveness to circular and non-circular patterns is inborn in the case of Great Tits and Chaffinches. 5. An experiment with mealworms artifically ornamented with small eye-like spots at their extremities has shown that such markings, placed on a prey-object, tend to direct to themselves the attack of Yellow Buntings. The waning of the element of disturbed orientation in the responses is slow, and may not be significant. 6. The deflection and intimidation effects are discussed. The origin of the predator's responses to deflection marks is not known, but it is suggested that intimidating eyespots act by mimicking the eyes of the large avian predators preying on the small insectivorous passerines which are among their natural enemies. It has been shown that many small passerines possess inborn responses to their predators, and it is probable that these are parasitised by the eyespot patterns of insects. Eyespot patterns must be regarded as wholly distinct from pseudaposematic colouration. The effect of experience on small passerines, either of the predator-model or of an ocellated imitator, is most usually the reverse of that sustained by a predator encountering the model or mimic in a true case of Batesian or Mullerian mimicry, and the restrictions on the population size of a pseudaposematic insect relative to its model do not apply to insects utilising the inborn responses of their predators.