The Open Door Web Site
Part XX: How Organisms Communicate
The Effect of Stimuli on the Behaviour of an organism
Lobsters have chemical receptors on their mouthparts,
Insects detect chemicals in the air using their
CHEMICAL EMITTER AND RECEPTOR ORGANS
Large animals which use chemical stimuli have special glands which are open to the surface of the animal's body. In insects these glands are usually found in the abdomen, although ant trails are made by secretions from a gland in an ant's mouth. In vertebrates the gland is either located somewhere on the head or near the openings of the sex organs at the rear of the animal.
Chemical Receptor organs
Small aquatic animals and small land animals with wet body surfaces have chemical receptors all over their bodies. Larger invertebrates have chemical receptors in various parts of their bodies. Spiders have chemical receptors at the tips of their legs while most crustacea have them on their mouthparts, tail and antennae, as well as on their legs.
Insects detect chemicals in the air using their antennae and palps on their mouthparts. Snails and slugs are able to detect chemicals with all parts of their bodies but they are both more sensitive around the mouth, on the tentacles and at the front of the foot. It is the tentacles which detect the odours from foods.
Most vertebrates have developed a special structure, the nose, which samples the air as they breathe.
It is no accident that the nose is found at the front of the head. It is in a good position to detect chemical stimuli as soon as possible. Some reptiles, such as snakes, use their tongues to pick up chemicals in the air. This is why they constantly flick their tongues in and out. The tongue of a snake is forked at its end and, when the tongue is in the mouth, the two tips are placed in small sensory pits in the roof of its mouth. This is how the snake 'smells' its environment. The taste organs of fish are not only found near the mouth but also on the fins and, in some cases, on the tail!
Mosquitoes find their human victims by sensing the presence of lactic acid, carbon dioxide and moisture on the skin. The mosquitoes are also attracted by body heat and movement.
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