I spy with my insect eye

 

Text and photographs Marlies Craig or as credited

Many organisms manage fine, or better, without eyesight, but nevertheless eyes are extremely useful. Eyes have evolved on at least 40 separate occasions, in different branches of the animal family tree. On one of those occasions, around 500 million years ago, the early insect/crustacean branch gained their eyesight.

Today, please join me on a quick tour of insect eyes. Insects as a group have two types of eyes: simple and compound.

Simple eyes

Simple eyes, or ‘ocelli’, only detect light intensity, they do not form an image. Gradual changes in light helps insects tell the time – both hours and seasons. Springtails (close insect relatives) have up to six ocelli, but in modern insects, ocelli are usually found in triplets – unless of course they are absent.

This flower mantis (see further below for a picture of the whole animal), has in the middle of its forehead three small, glassy ocelli.

Those bulging fisheye lenses must be exceptionally sensitive. One simple eye (seemingly with a faint pink filter) points forward, two simple eyes (with yellow filter) point left and right. One can only guess what sort of visual cues this flower mantis is recording as it picks the perfect perch. It also has two large, grey-green compound eyes.

Compound eyes

Compound eyes resolve images. Unlike human eyes, which have one large lens and thousands of rods and cones on the retina, insect compound eyes are made up of tens, hundreds, thousands or even tens of thousands of tiny compartments, or ‘ommatidia’.

The compound eyes of some ants have less than ten ommatidia. House flies have around 4000 while dragonflies have up to 30 000.

Compound eye of a drone fly in beautiful tiger-colours, containing thousands of ommatidia.

Each tiny ommatidium has its own light receptors and a separate little lens. Each ommatidium sends a light spot signal to the brain, which then pieces together the information to form one large image – or rather, two large images. The compound eye therefore works much like a digital camera, producing pictures made up of more or less pixels, depending on the number of ommatidia.

A chameleon, as seen with 1000 light spots… and with 10000.

Robber flies hunt other flying insects. The huge head-hugging compound eyes give them accurate vision in every direction, and depth perception where the fields of view of the two eyes overlap.

Insects detect movement as things flash across the fields of view of neighbouring ommatidia, causing a chain of signals. They can tell how far things are, and how fast they are moving. Insects see and react far quicker than we do. Flies for instance can detect 240 flickers per second, whereas we perceive only 50-100. This means they see the world in slow motion. Such high ‘critical flicker fusion’ is what helps flies to dodge the fly swatter, and what allows other insects to catch those same flies for lunch.

A house fly, which we find so impossible to catch, has been snatched in mid-air by a robber fly.

The large compound eyes, placed far apart on a wide, triangular head, gives this mantis excellent 3D vision.

For a successful attack, a hunter needs to know exactly where to aim. A mantis may rock from side to side, sizing up its prey, and judging how far away it is. Snakes, spiders and cats also do this. Dragonflies swivel their heads to get a better perspective. Changing your viewpoint improves your depth perception.

Cluster eyes

One order of insects, the ‘strepsiptera’, have such unique eyes that one scientist joked: “One is tempted to conclude that these eyes, so unlike anything in any other insect, either came from outer space or were put here by God to confuse scientists”.

These unique eyes are made up of 50 or so complex ‘eyelets’, each fitted out with its own complex retina, each producing its own little multi-pixel image. Once could say that stylopids see the world as a jigsaw puzzle.

Unique eyes of a male stylopid.

By the way, the females have no wings, eyes or legs. They parasitise other insects (such as bees). Only the males are free-living, and they are very short-lived: within a few hours they must find a female to mate with. This strangest of insect orders has a bunch of other very weird characteristics worth reading up on.

Colour vision

Colour vision results when different kinds of pigments in the eye absorb light of different wavelengths, creating different nerve impulses. The brain combines and interprets these messages, so that we see the world in colour.

Humans have three types of pigments, that absorb predominantly blue, green and orange/red light. Most insects on the other hand have only two pigments, one detects blue light, the other green to orange, centred on yellow. Red is right on the very edge of their vision.

Human skin has a strong orange-red colour, irrespective of pigmentation, and interestingly, mosquitoes respond to that part of the spectrum. In experimental conditions they flew towards orange, red and black, but only after having been primed to look for food by blowing some carbon dioxide into their experimental chamber.

Tsetse flies on the other hand, are attracted to the colour blue and black. These flies transmit deadly sleeping sickness, and knowing their colour preferences helps in the control of this disease.

Tsetse traps – such as this insecticide infused black and blue flag – is all that is needed to keep down their numbers.

Some bees and butterflies have an extra pigment in their compound eyes, that responds to colour all the way past the violet end of the rainbow – which is invisible to us. They can see in the ultraviolet (UV) range, and insect-pollinated flowers often have fancy patterns that are only visible under UV light. You can watch an interesting video on this.

If we combine the three colours that a bee can detect into ‘false colour’ pictures, we get an inkling of what flowers might look like to them.

Credit: J Trunschke et al.

In a previous article, ‘Insects and colour’ I wrote what colour means to insects, how they communicate with colour. Insects don’t just see other colours, they also look different, when UV light is taken into account. Wasp nests glow! Other animals too, including spiders and scorpions, geckos, chameleons, birds, or even mammals have strange markings or colourations, only visible in UV light. Oh the wonder of colour vision!

Four eyes, faux eyes, no eyes

In whirligig beetles, the compound eyes are split into separate top and bottom sections.

These funny aquatic beetles swim on the water surface, with half their body submerged. The top part of each eye is adjusted for vision in air, while the bottom half is adjusted for looking underwater.

Whirligigs zoom about in tight circles, using a hair-fringed ‘tail’ as rudder.

Male mayflies also have special divided compound eyes. Each eye on a male has a large, telescopic sections on top of the head, which allows them to spot a female in a large swarm. Females don’t have this fancy technology.

Male mayfly.

The difference between mayfly sexes can already be seen in the aquatic larvae (called naiads):

Mayfly naiads: male (left) with large, reddish, telescopic eyes, and a female (right) without.

Many insects, like many other soil-dwelling or cave-dwelling or deep-sea animals, have lost their eyesight. In the dark, eyes are quite useless.

Many termites are blind – at least the soldiers and workers, who live underground all their lives.

The royal kings and queens that emerge to swarm and mate, do have eyes. They need them as they leave their dark home, to dodge the birds waiting outside, and find a new home.

On this caterpillar of swallowtail butterfly, neither the large black ‘eye spot’ on its back, nor the large orange head capsule, are the eyes. Look closely, and you will notice a few dark spots on the orange heat, just above the mouth – those are the eyes (larval eyes are called stemmata).

Swallowtail caterpillar

Compound eyes miraculously grow during the pupal stage, when the caterpillar basically dissolves, and reassembles to form a butterfly, changing from one kind of creature into another, growing long stilt legs where before there were six short stubby things, growing wings where before there were none, a long straw mouthparts where before there were biting mandibles…

Moral of the story: insects are amazing, and things are not always as they seem. A lot more could be said on this topic. One could mention the complex binocular eyes on the hunting tiger beetle larvae that more resemble the human eye, or the ability of insects to see light polarisation which allows them to detect water surfaces, or the ability of some moths to see colour even by starlight, or the ability of dung beetles to navigate by starlight.

Let’s end with one final funny example: the stalk-eyed fly. Males are highly territorial, though not violent. They walk up to an opponent, head-to-head, and simply size each other up: the one with the longest eye stalks wins. The other walks away, defeated.

Yes, the eyes of stalk-eyed flies really are at the end of those stalks!

Author photo: Pat McKrill

About the author

Marlies is a biologist with a PhD in Epidemiology, and a Post-Graduate Diploma in Adult Education. She has worked for many years in health research and climate change. Though she did originally study Entomology, her love affair with insects is very personal. In her book What Insect Are You? – Entomology for Everyone, she shares that passion with young and old. Through the book and interactive events she wants to kindle in people of all ages enthusiasm and a deeper appreciation of nature and show them why and how they can make a difference. She started a non-profit organisation called EASTER Action which promotes awareness and action on biodiversity, climate change, and sustainable living.