Evolution

Cave environment is hard and selective and a very few organisms have adapted by undergoing specific morphologic and metabolic modifications. Changes aren’t immediate, but come as the result of an evolution that can take up to millions of years, starting from species that lived outside and that for many reasons ended up trapped and isolated in underground cavities. Species that have been used to living longer in underground conditions are the ones that show more specific modifications.
In the absence of light, eyes are a useless instrument: species that have lived in caves the longest can be recognized by the fact that they are eyeless, while a species that is in the process of adapting to this type of environment still has eyes but they are very small or underdeveloped. Others have eyes at the time of birth but then they disappear as the animal reaches adulthood. To move in the dark, to feel the presence of predators or other animals of their same kind, they develop other senses: cave organisms generally have long legs, long antennas, hairs and bristles that function as tactile organs as well as a highly developed olfactory sense.
In total darkness, even the ability to fly is useless: cave animals that descend from species that were originally able to fly (such as insects) have all lost this capacity, with the subsequent atrophy and loss of their wings.
In the dark colours disappear too: cave fauna shows a depigmentation phenomenon, animals are scarcely coloured and have mostly light colours such as beige and yellow tones (the colour of chitin), as is the case of many insects, or are completely pale or transparent, such as shrimp or certain fish. It is interesting to notice that species that have adapted only recently, when exposed to the light they tend to regain their colour, while species that have totally adapted to cave life no longer have this capacity and often are killed by the intensity of solar radiation.
The lack of light has an influence also on chitin production (the substance that makes up insects’ and crustaceous’ exoskeleton), so in general these have a lighter and finer exoskeleton which makes them more vulnerable to predators and dehydration. Some beetles have evolved by developing a particular elytra welding, that creates a cavity inside the abdomen that can contain a small quantity of “reserve” liquids: this is why many cave insects have a large spherical abdomen.

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