Crabs have two black eyes which sit above its head on stalks. The eyes can fold into sockets in the shell for protection. Having eyes on stalks means crabs have a better field of vision. Crab also have compound eyes just like insects. Each eye is made up of thousands of tiny units, each with its own lens. Though compound eyes do not focus well, they are very sensitive to movement and can see colors.
Sense of Smell
The antennae on crab are called chemoreceptors. They allow the crab to taste and smell, to find food and mates. Crab smell things they eat underwater by detecting chemicals from food with the tufts of hairs on the tips of their antennas. When a crab detects these chemicals, it starts to search for food. Between the eyes are two pair of feelers. Taste-sensitive hairs are located on the mouth parts, along the pincers, and on the crab’s feet.
Below a crab’s feelers is its mouth. The mouth of the crab is made up of hard pairs of mouth parts which have different uses. One pair of jaws holds the food; other mouth parts break the food into small bits and put them into the crab’s mouth. A flap on one pair of these mouth parts pumps water over the crab’s gills, so the crab can breathe while eating.
Sense of Touch
In spite of their hard shell, crab have a good sense of touch. This is because growing through the cuticle are tiny, but very sensitive bristles. These bristles are scattered all over a crab’s body, legs and feet. Other types of hair on crab are sensitive to vibrations traveling through the water. These hairs are found mostly along the pincers.