stemma

A stemma (plural: stemmata) is a simple photoreceptive eye-spot found in many invertebrates, particularly insect larvae. When set into a pit, the stemma limits the angle of light that can reach the photoreceptors — only light entering at specific angles activates the receptor. By comparing activity across multiple stemmata arranged at different orientations, the animal can infer the direction of a light source.

Stemmata are found primarily in holometabolous insect larvae — for example, caterpillars (Lepidoptera larvae), beetle larvae (Coleoptera), and sawfly larvae. They typically appear as small dark dots arranged in rows or clusters on the head. Adult insects generally replace stemmata with compound eyes during metamorphosis, though some primitive insects retain simple eyes (ocelli) in adults.

Stemmata are considered intermediate in complexity between simple pigment spots and true compound eyes. Unlike compound eyes, which use many independent optical units (ommatidia) for panoramic vision, each stemma is a single optical unit. Compared to adult ocelli (simple light-sensing organs), stemmata are more structurally complex and can form crude images in some species — particularly tiger beetle larvae.