Fig. 1.—

Visual organs in representatives of Panarthropoda. Although most onychophorans and tardigrades have a pair of simple, ocellus-like eyes (arrows in A and B), arthropods usually show two types of visual organs: The compound eyes (arrowheads in C) and the median ocelli (arrow in C). (A) Scanning electron micrograph of the head of the onychophoran Principapillatus hitoyensis in dorsal view. Scale bar: 1 mm. (B) Light micrograph of the head of the tardigrade Hypsibius dujardini in dorsal view. Scale bar: 10 µm. (C) Stereomicrograph of the head of the hymenopteran Ectemnius cavifrons in dorsal view. Scale bar: 1 mm.