Visual adaptation in nocturnal and diurnal ants

Abstract

1. The compound eyes of three species ofCamponotus ants, one exclusively nocturnal (Camponotus irritans), one crepuscular and nocturnal (Camponotus ligniperda), and the third diurnal (Camponotus detritus) are compared with respect to day/night light sensitivity changes. AsCamponotus detritus sometimes stays outside the nest during the night, the strictly diurnal speciesCataglyphis bicolor is included in the comparison. Even though all four species are of about the same body size, the eyes of the diurnal species are much larger, contain more ommatidia and exhibit smaller cross-sectional areas of the cone tips than those of the nocturnal species. 2. During the natural light/dark cycle distinct ultrastructural changes occur in the distal parts of the ommatidia in all three species ofCamponotus, whereas inCataglyphis the ommatidial structures remain unchanged throughout day and night. In the eyes of theCamponotus species the most characteristic feature of the light-adapted ommatidium is the elongated proximal part of the crystalline cone, the narrow cone tract. In addition the length of the rhabdom is shorter (by about 10 μm) than in the dark-adapted state. At night the conical cone tapers steadily from its distal end at the corneal lens to the tip of the rhabdom. These dynamic responses represent the first description of retinomotoric changes within a eucone dioptric apparatus of insect apposition eyes. 3. The ommatidial dynamics occurring inCamponotus eyes take the following daily course: At dawn and dusk the retinomotoric changes require almost 2 h of the natural daily light cycle. As demonstrated by artificial light regimes they are based on an endogenous circadian rhythm. 4. Results obtained from electrophysiological (ERG) recordings are in accord with the anatomical findings. InCataglyphis bicolor the ERG-responses do not show any daily sensitivity changes, whereasCamponotus ligniperda exhibits a distinct circadian modulation of compound eye sensitivity. Between the light- and dark-adapted state sensitivity varies by about 1 log unit.