Simple eye in invertebrates

A simple eye or ocellus (sometimes called a pigment pit) is a type of eye found in many small animals. Unlike the complex eyes of humans and many other animals, a simple eye has just one lens. This makes it different from compound eyes, which have many small lenses working together.

Head of Polistes with two compound eyes and three ocelli (circled)

The way an animal's eyes develop depends on where it lives and what it needs to see to survive. Animals such as arthropods live in many different places and need to see many different things, like finding food, staying safe, or finding friends. Because of this, they have developed many different kinds of eyes to help them see better.

The word simple eye can mean different things depending on how we look at it. For example, the eyes of larger animals are often called camera eyes because they work like a camera, using one lens to focus an image. But even these eyes can be called simple in some ways. Some small animals, like jumping spiders, have eyes that work in ways that are more like the complex eyes of larger animals. Many insects have many lenses in their compound eyes, but each small part of the eye works together to create a clear picture.

Ocelli or eye spots

Some animals like jellyfish, sea stars, flatworms, and ribbonworms have very simple eyes called pigment spot ocelli. These eyes have colored spots that look red or black but lack parts like a clear covering or a focusing lens.

Other animals, such as box jellyfish, have more advanced eyes with special layers to help see better. Many snails and slugs also have these simple eyes at the ends or bases of their tentacles. Even giant clams use ocelli to let light reach their soft bodies, known as mantles.

Simple eyes in arthropods

Spider eyes

Main article: Spider vision

This jumping spider's main ocelli (center pair) are very acute. The outer pair are "secondary eyes" and other pairs of secondary eyes are on the sides and top of its head.

Spiders do not have compound eyes. Instead, they have several pairs of simple eyes, each pair adapted for specific tasks. The principal and secondary eyes are arranged in four, or sometimes fewer, pairs. Only the principal eyes have moveable parts that help them see. The secondary eyes have a special layer at the back that helps them see better. In hunting spiders, one pair of eyes helps them see prey from far away. Night spiders have large eyes that help them see in very dim light.

Dorsal ocelli

Moth larva about to moult; the new stemmata are visible behind the old head capsule

The term "ocellus" (plural ocelli) comes from the Latin word for "eye," meaning "little eye." In insects, there are two types: dorsal (top-most) ocelli and lateral ocelli. Many insects have dorsal ocelli, while stemmata are found in the larvae of some insects. These eyes are different from each other in structure and function.

Dorsal ocelli are light-sensitive organs on the top or front of the head in many insects, like bees, ants, wasps, flies, dragonflies, grasshoppers, and mantises. These ocelli work alongside compound eyes, giving insects two different ways to see. The number, shape, and function of these ocelli vary among insect groups. In flying insects, they are usually larger and often found as a group of three, with two facing the sides and one facing forward. Some land insects, like certain ants and cockroaches, lack the central ocellus.

These ocelli consist of a lens and a layer of light-sensitive cells. The lens can be curved or flat, and the light-sensitive layer may be separated from the lens by a clear substance. The number of light-sensitive cells can vary widely. In bees, locusts, and dragonflies, the lens is strongly curved, while in cockroaches, it is flat.

An example of a sawfly larva. It has just a single pair of stemmata, and they are set higher on its head than the position of stemmata on the heads of lepidopteran larvae.

Research shows that some insects, like dragonflies, can use their ocelli to see shapes and movement, which helps them with quick and skilled flying. These eyes are also important for scientists designing small flying machines, as they can learn from how insects see and stay stable in the air.

Stemmata

The larva of one of the Acherontia species shown here, is typical of the order Lepidoptera. The head of the larva bears more than one pair of stemmata, all of which are set low down and are far more widely placed than the mouthparts.

Stemmata are another type of simple eyes. Many larvae of insects only have stemmata until they grow into adults. Some adult insects, like fleas and springtails, also only have stemmata and never develop compound eyes. Behind each lens of a stemma is a group of light-sensitive cells. The lens is curved, and the stemma has a clear core.

Although stemmata are simple eyes, some, like those of sawfly larvae, can be very sensitive and can detect polarized light. In the pupal stage, such stemmata develop into fully formed compound eyes. Their position on the head—usually to the side—helps distinguish them from ocelli, which are usually in the middle.

Genetic controls

Some genes help decide where and if simple eyes, also called ocelli, develop. For example, the orthodenticle gene can stop simple eyes from forming if it changes. In fruit flies called Drosophila, a special protein named rhodopsin Rh2 is only found in simple eyes.

Certain genes like eyeless and dachshund work in the complex eyes but not in simple eyes. Another gene called epidermal growth factor receptor (Egfr) helps simple eyes to form by supporting the work of other genes.