Insects have evolved physiological adaptations and behavioral strategies that allow them to cope with a broad spectrum of environmental challenges and contribute to their evolutionary success. Visual performance plays a key role in this success. Correlates between life style and eye organization have been reported in various insect species. Yet, if and how visual ecology translates effectively into different visual discrimination and learning capabilities has been less explored. Here we report results from optical and behavioral analyses performed in two sympatric ant species, Formica cunicularia and Camponotus aethiops. We show that the former are diurnal while the latter are cathemeral. Accordingly, F. cunicularia workers present compound eyes with higher resolution, while C. aethiops workers exhibit eyes with lower resolution but higher sensitivity. The discrimination and learning of visual stimuli differs significantly between these species in controlled dual-choice experiments: discrimination learning of small-field visual stimuli is achieved by F. cunicularia but not by C. aethiops, while both species master the discrimination of large-field visual stimuli. Our work thus provides a paradigmatic example about how timing of foraging activities and visual environment match the organization of compound eyes and visually-driven behavior. This correspondence underlines the relevance of an ecological/evolutionary framework for analyses in behavioral neuroscience.

Responses of Formica cunicularia foragers to monochromatic light stimuli of 370, 440, 540, 590 and 640 nm were evaluated in different experimental conditions using a Y-maze apparatus and a circular orientation platform. The results showed that foragers responded significantly to all test wavelengths at certain intensities but could only discriminate 370 and 540 nm from alternatives irrespective of intensity changes. Furthermore, they were also capable of discriminating two long wavelengths, 590 and 640 nm, using a photon catch mechanism by their green photoreceptors. Foragers also discriminated stimuli pairs of same wavelengths based only on intensity differences they provide. The overall results show that F. cunicularia foragers have a dichromatic colour vision system based on inputs of two possible photoreceptor types sensitive to UV and green. The results also yielded evidence showing that their visual systems provided foragers a sensitivity also for wavelengths corresponding to blue and red ranges of the spectrum.

SUMMARYIn the present study, we report the first evidence that ants discriminate and learn perceptually close colour stimuli. Foragers of the ant species Cataglyphis aenescens and Formica cunicularia were trained in a Y-maze choice apparatus to monochromatic light stimuli of a constant intensity associated with a food reward. Two stimuli, with a mean wavelength of 40nm perceptual distance, were chosen from the UV (340nm vs 380nm) and the green (510nm vs 550nm) range because these species are UV-green dichromats. Foragers were trained with two conditioning paradigms [absolute conditioning (AC) and differential conditioning (DC)]. In the UV range, C. aenescens foragers failed to discriminate when presented with a small colour difference in both training procedures. Foragers also failed in the green range when trained with AC but showed significant bias towards the rewarded stimulus when trained with DC. Formica cunicularia foragers achieved the task in the UV range when trained with DC only. In the green range, F. cunicularia foragers showed clear preference for the rewarded stimulus in both training conditioning procedures. Foragers never failed in choosing the rewarded stimulus in DC even when the intensity of the rewarded stimulus was reduced by one log unit. This clearly indicates that DC is of paramount importance to discriminate perceptually close colour stimuli.

Ant responses were tested under both the natural geomagnetic and artificially induced Earth-strength electromagnetic field. Foragers were trained for a month to visit a food source at the north arm accessed through an orientation platform assembly. Under the natural geomagnetic field, when all other orientational cues were eliminated, results indicated significant heterogeneity of ant distribution with the majority seeking geomagnetic north in darkness. However, in light, foragers failed to discriminate geomagnetic north. Under shifted artificial electromagnetic field, orientation was predominantly on the artificial magnetic N/S axis with a significant preference for the artificial north in both light and dark conditions.

Ants constitute one of the most intriguing animal groups with their advanced social lifes, different life histories and sensory modalities, one of which is vision. Chemosensation dominates all other modalities in the accomplishment of different vital tasks, but vision, varying from total blindness in some species to a relatively well-developed vision providing ants the basis for visually-guided behaviors, is also of importance. Although studies on ant vision mainly focused on recognition of and guidance by landmark cues in artificial and/or natural conditions, spectral sensitivities of their compound eyes and ocelli were also disclosed, but to a lesser extent. In this review, we have tried to present current data on the spectral sensitivities of the different ant species tested so far and the different methodological approaches. The results, as well as the similarities and/or discrepancies of the methodologies applied, were compared. General tendencies in ants’ spectral sensitivities are presented in a comparative manner and the role of opsins and ant ocelli in their spectral sensitivity is discussed in addition to the sensitivity of ants to long wavelengths. Extraocular sensitivity was also shown in some ant species. The advantages and/or disadvantages of a dichromatic and trichromatic color vision system are discussed from an ecological perspective.