Dorsal ocelli are light-sensitive organs found on the dorsal surface or frontal surface of the head of many insects,

The term "ocellus" (plural ocelli) is derived from the Latin oculus (eye), and literally means "little eye". In insects, two distinct ocellus types exist: dorsal (top-most) ocelli, and lateral ocelli (often referred to as ocelli and stemmata, respectively), most insects have dorsal ocelli while stemmata are found in the larvae of some insect orders. Despite the shared name, they are structurally and functionally very different. Simple eyes of other animals may also be referred to as ocelli, but again the structure and anatomy of these eyes is quite distinct from those of insect dorsal ocelli.

Dorsal ocelli are light-sensitive organs found on the dorsal surface or frontal surface of the head of many insects, including Hymenoptera (bees, ants, wasps, sawflies), Diptera (flies), Odonata (dragonflies, damselflies), Orthoptera (grasshoppers, locusts), Mantodea (mantises), and various groups within Heteroptera (true bugs). These ocelli coexist with compound eyes; thus, most insects possess two anatomically separate and functionally different visual pathways.

The number, forms, and functions of the dorsal ocelli vary markedly throughout insect orders. They tend to be larger and more strongly expressed in flying insects (particularly bees, wasps, dragonflies and locusts) where they are typically found as a triplet. Two ocelli are directed to either side of the head, while a central (median) ocellus is directed forwards. In some terrestrial insects (e.g. some ants and cockroaches), the median ocellus is absent. The sideways-facing ocelli can be called "lateral ocelli", referring to their direction and position in the triplet, however this is not to be confused with the stemmata of some insect larvae, which are also known as lateral ocelli.

In many members of the suborder Heteroptera (true bugs), such as stink bugs Pentatomidae, assassin bugs Reduviidae, and seed bugs Lygaeidae, only two dorsal ocelli are present. These are typically positioned symmetrically on the vertex, between or slightly behind the compound eyes. This two-ocelli arrangement is a distinguishing trait in several Heteropteran families and differs from the more common three-ocelli configuration found in other insect orders.

A dorsal ocellus consists of a lens element (cornea) and a layer of photoreceptors (rod cells). The ocellar lens may be strongly curved or flat. The photoreceptor layer may also be separated from the lens by a clear vitreous humour. The number of photoreceptors also varies widely, but may number in the hundreds or thousands for well-developed ocelli. In bees, locusts, and dragonflies, the lens is strongly curved; while in cockroaches it is flat. Locusts possess vitreous humour while blowflies and dragonflies do not.