Morphology of the compound eyes as an indicator of life-style in carabid beetles

Twelve diurnal, 9 nocturnal, and 6 intermediate species of carabid beetles common in central Europe were investigated with respect to hunting behaviour and the morphology of the head and eyes. Diurnal visual hunters are characterized by typical visually guided predatory behaviour, which consists of a turn toward the prey, followed by a jerky approach and a lunge. Nocturnal species do not react to visual stimuli but use exclusively chemical or tactile cues for orientation. Visual hunters have broader heads than the other two groups (≈ 26% of the body length as opposed to ≈ 20%), shorter antennae (≈ 45% of the body length compared with ≈ 52% in the other two groups), ≈ 50% larger eye surfaces, and, correspondingly, 50% more ommatidia per eye. The binocular overlap of the frontal visual fields of both eyes is more than 50° in visual hunters; in nocturnal and intermediate species it is usually less than 40°. Nearly all species have acute zones with small interommatidial angles in the frontolateral parts of the eye, but in visual hunters these are much more distinct. As a readily measurable indicator of the hunting behaviour and life-style of a species we describe an eye–antenna angle that is more than 60° in typical visual hunters and, in most cases, much less than 55° in nocturnal species.