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the grown-up butterfly, its impressive wings swathed in a variety of unique patterns and colours, always makes for a pleasing sight as it gracefully flutters among flowers, scooping up the nectar with its unfurled trunk. These majestic creatures share the same insect order of Lepidoptera with the closely related moths, which evolved more than 200 million years ago. Butterflies rely heavily on their visual sense to survive. Their eyes allow them to detect predators, navigate their environment and find suitable nectar sources from which to drink. This article covers some fascinating details about the number and structure of the butterfly’s eyes, as well as their visual quality.
How many eyes does the butterfly have?
The answer to the question changes depending on the life stage of the butterfly. Once it leaves the egg, caterpillar stage has 10 to 14 (though usually 12) small eyes arranged in a semicircle around the face. However, these very simple eyes only have the ability to identify light and dark rather than precise images or colors. The caterpillar navigates its environment almost completely blind, using its other senses to find its way.
After undergoing the transformation process of metamorphosis, the adult butterfly will have two large and fully developed compound eyes, located on either side of the head. While these are actually fewer eyes than a caterpillar, the adult eyes generally have much better vision. The butterfly is not unusual in this regard. Two eyes are a common number for most adults insects. It is actually a misconception that insects have more than two eyes, like spiders, crabs and other arthropods.
What people may be confused about is the structure of the eye. Unlike the simpler lenses of most animals, the insect’s compound eye is composed of many thousands of different individual photoreceptors. Each receptor actually has its own individual cornea and lens, but still works as part of a single eye. In total, each butterfly eye can contain up to 17,000 of these receptors (although many species have fewer).
Another important part of the eye are the photoreceptor cells, which distinguish color and brightness. While in a human eye there are many light-sensitive cells that work together to form an image (as well as several other mechanisms for controlling the amount of light entering the lenses), each individual butterfly photoreceptor contains only a few cells for the light to focus on. This actually has a negative effect on the visual quality of the butterfly, but compensates for it in other ways.
What does the butterfly vision look like?
The compound eye of the butterfly has some serious pros and cons compared to your average non-compound eye. The main advantage it offers is the ability to look in many different directions at once. This means that the insect eye stays in place, cannot move and still has a better field of view than your standard animal eye. For a small, vulnerable animal like a insect, allowing it to act almost immediately the moment it detects a threat.
In contrast, the main drawback of a compound eye is loss of acuity (meaning quality of vision). Even with many individual lenses, the insect eye cannot achieve the same visual acuity as a non-compound eye with a particularly dense arrangement of cells. It is estimated that the acuity of the human eye is about a hundred times better than even the highest quality insect eye (thought to be the dragonfly with about 30,000 lenses per eye). Compound eyes also lack the ability to focus on objects by controlling the shape of the lens, meaning the insects have to get closer or further away to produce much the same effect.
Butterflies make up for this deficiency in other ways. For example, compared to most insects, seeing colors is a very important aspect of the butterfly’s visual repertoire. It allows them to identify and distinguish the right types of flowers to feed on in the wild. Butterflies are also better able to distinguish fast-moving objects than humans, which can be very useful in flight. Another important feature is the ability to see beyond the normal range of human vision, including both ultraviolet and polarized light. Ultraviolet light, a strong component of sunlight, is just outside the visible blue and violet end of the color spectrum, while polarized light is often present when the atmosphere scatters sunlight in a certain way. It has been suggested that some insect species (including fruit flies and honey bees) may use ultraviolet and polarized light to navigate the air in flight.
The key to his wide field of view may be the types of photoreceptors in his eye. A human eye has only three different types of photoreceptor cells, although this is enough to see millions of different colors. Butterflies, on the other hand, have at least four different types of receptor cells, and many species have at least six; this allows them to distinguish certain aspects of color and light that humans cannot.
Some butterfly species have truly extraordinary color vision. According to a study published in the journal “Frontiers in Ecology and Evolution,” the male version of the common bluebottle has 15 different types of photoreceptor cells (the female has not been studied, but it is believed to be similar). Only green light seemed to activate four different receptors, while blue light triggered three more. It’s not entirely clear why it evolved 15 different receptor cells, but one hypothesis is that it is related to mating. Being highly sensitive to any variation in the blue-green part of the color spectrum, the male is much more adept at distinguishing fellow blue-colored rivals that may be hard to spot against the blue sky.
What are the eyes on the wings?
Those black markings on the wings are called false eye spots. Many butterfly species have at least some version of it; the small blue species of the family Lycaenidae even have fake heads at the tail with large eye spots and false antennae. These markings are clearly not real eyes and cannot provide vision, but they do serve other important purposes. For a long time it was thought that these eyespots evolved to mimic the eyes of fearsome creatures in order to fool potential predators and make them think twice about approaching. Another possible hypothesis is that the eye spots redirect attacks away from the head and to less vulnerable parts of the body. These eye spots sometimes also have a secondary function to attract partners.
Next one: What does snake meat taste like?
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