The absolute sensitivity of lens and compound eyes.

The numbers of light quanta available to photoreceptors of lensand different types of com­ pound eyes are calculated on the basis of photometric considerations. It is shown that the results depend upon the situation in the optical environment: For point-like lightsources such as stars receptors in compound eyes generally receive considerably less numbers of light quanta compared n. g. with the human eye. This is due to the small sizes of the ommatidial facets. For extended optical surroundings, however, the numbers of quanta reaching the receptors in typical insect compound eyes of the apposition type are comparable to those in the human eye. In this respect the optical superposition eye of nocturnal insects like Ephestia is an exceptional case, where there is an improvement in the numbers of quanta reaching the receptors by a factor 100 to 1000 com­ pared to the eyes of bee or man.