You have probably heard someone tell you that flies have millions of eyes, which helps them to see far more, and perceive motion far faster than humans can.
However, this isn’t totally the reality – flies don’t have ocular systems like we do – they have two separate systems, one to sense light and once to sense motion.
In this article, we will take a closer look into the visual system of a fly, and answer once and for all ‘how many eyes does a fly have’. Read on to find out more about fly’s eyes.
How Many Eyes Does A Fly Have?
It is a common urban myth that flies have millions of eyes. However, this isn’t technically true.
If you have ever seen a fly up close, you will be familiar with their two, large bulbous and glittering ‘eyes’ on the side of their head.
These strange looking components are what’s known as ‘compound eyes’.
Essentially, this boils down to these eyes being made up of many tiny eyes. Each of these tiny eyes is called a facet, and there are hundreds of these facets in each fly’s head.
This means that they can see very well even though they have smaller eyes than us – which can make it difficult to catch a fly with your hands – as they can perceive motion far faster than us.
What Is A Compound Eye?
Insects have compound eyes. Compound eyes form a 3D image which is a combination of thousands of images received by thousand photoreceptors present within a single compound eye.
The resolution of the image created is quite low compared to a single aperture eye (which is the kind of eye that we have).
However, the compound eye gives flies and other insects a larger area of vision (so they can see motion in nearly their whole environment).
It also gives them the ability to detect movements around them much more quickly than would be possible with a single aperture eye.
This helps flies and other insects to respond quickly to any movement nearby. This makes it difficult to catch them, as they can detect even the slightest motion.
Flies are often a prey animal – eaten by other animals such as bats and birds – so these compound eyes have evolutionarily developed to give them an advantage that can help them survive.
Compound eyes are made up of lenses and receptors. There are basically two types of compound eyes: apposition, and superposition.
Apposition eyes form several inverted images, while superposition eyes form a single image (like our eyes).
Flies have two large compound eyes, which have been specially designed to provide them with good visions (especially when compared to the vision of other insects).
Overall – flies have only two eyes, and thousands of eyes at the same time. As they have two large compound eyes, the number of facets in each eye is equivalent to a thousand of human eyes.
Some flies also have ocelli eyes on the top of their head, which sense changes in light, rather than movement.
How Many Lenses Does An Average Fly Have?
We have two eye lenses, with each lens in each eye. Flies have a pair of compound eyes, and each compound eye consists of 4500-5500 individual lenses combined (although this is just an approximation).
In some senses, a fly’s compound eyes are much better than humans because they have more lenses, and can perceive motions and environmental changes far faster.
Compound eyes are very useful for insects because they help them see things better. They are made up of many tiny pieces called facets.
Each facet sees a different part of the light coming towards the eye. They combine the information received by each facet into an image of the whole scene.
Insects use this to determine direction, distance and speed.
How Many Pupils Does An Average Fly Have?
In a human, our iris (the colored part of our eye) can contract and relax to change the size of our pupil (the black hole in the center of our eye).
This alters how much light can reach our retina – which protects our eyes in bright situations, and helps us see better in dark situations.
However, flies don’t have irises, and therefore can’t change the size of the aperture in their eye. They also have no pupils.
They therefore don’t have any control over how much light they have exposed to their retina, and this can even result in damage to their eyes.
Overexposure to blue light can lead to eye damage and permanent blindness in older flies. This is known as futurity.
Houseflies: How Many Eyes Do They Have?
Houseflies have two types of eyes: compound eyes, and ocelli eyes. Their compound eyes are made up of thousands of tiny units called facets.
These facets are arranged in a hexagonal pattern – and can be counted as individual eyes, giving a housefly about 3000 to 6000 eyes.
Horseflies: How Many Eyes Do They Have?
While a common housefly has ocelli eyes in addition to their compound eyes, horseflies are a little different.
Unlike the common housefly, a horsefly doesn’t have these ocelli eyes, instead making do with larger compound eyes.
The eyes of a female horsefly are separated by a thin black line. Male horse flies’ eyes touch each other. So, horsefly eyes aren’t only for vision, but also for telling the sex of a fly.
Fruit flies: How Many Eyes Do They Have?
Fruit flies have two main eyes that each have 750 lenses. They use two-thirds of their brains to process visual information from these eyes.
Though most flies have black eyes, there is more variety when it comes to fruit fly eye color. You can find fruit flies with red, black, brown and white eyes.
In conclusion, flies have approximately 2,000 lenses in each of their compound eyes. These lenses are arranged in rows and columns, which create the illusion of depth perception.
They are similar to the lenses in our own eyes, but are slightly different in that they don’t have irises to control pupil size, and they are packed tightly in a hexagon pattern.
So, in a way – flies have both 2 to 6 eyes, and also thousands of eyes.