Structure and function of the larval eye of the sawfly, Perga

The visual properties of the stemmata of fourth and fifth instar sawfly larvae have been investigated by means of intracellular electrophysiological recordings, interference-, light-, scanning, and transmission electron microscopy, and ray tracing techniques. Each larva possesses one pair of lateral ocelli consisting of biconvex lens, a clear-zone of corneagenous cells devoid of pigment, and a retina made up of groups of 8 retinula cells with a central and fused rhabdom. Migration of pigment during the night is predominantly radial, i.e. directed towards the periphery of the retinula cell. Vertical pigment movement occurs after exposure to bright sunlight for several hours. A circadian rhythm controls the sensitivity of single units, which at night show an increased sensitivity by 2 to 3 log units. The difference between the mean values of acceptance angles for light-adapted units (11·46 ± 5·1° S.D.) and dark-adapted ones (13·83 ± 6·8°) is not statistically significant. The wide range of acceptance angles, with 4·5° being the narrowest and 31° the widest, is explained by the optics of the single lens: there is a small region of highest resolution where light is well focused to a spot, but towards the outer edge of the cup-shaped eye the focusing becomes less accurate. Sawfly larvae turn their heads towards an approaching object if it subtends at least 4° of arc. This limit does not change over a range of 3 log units. A polarization sensitivity of up to 10 : 1 was determined electrophysiologically. Electron microscope studies of the rhabdom show a system of highly oriented microvilli which is thought to be responsible for the polarization sensitivity. Two different waveforms, both occurring after resting potentials of 40 to 70 mV, were found in electrophysiological recordings: (a) hyperpolarizations and (b) depolarizations. Throughout the study only depolarizing units were taken into consideration. These showed characteristics of both the compound eye and the ocellus.