Science of Hearing-II (Hearing in Invertebrates)

Animals are classified into microscopic and macroscopic animals. The microscopic animals like bacteria, amoeba etc have a simple cell structure. The cell contains different vacuoles. These vacuoles perform specific function. Thus they carry out all the physiological activities.

The macroscopic animals are classified into invertebrates and Vertebrates. The Vertebrates are the animals with the well-developed spinal column, Brain and Nervous System. Invertebrates do not have vertebral column but they have nerve cells with the help of which all the sensory activities are carried out. Most of the invertebrates do not have a well developed brain. Therefore, the invertebrates cannot recognise sound or interpret sound in their environment. So we understand that the brain plays a vital role with respect to hearing.

Among the multicellular animals, Sponges are invertebrates without nervous system. They respond to the sense of touch by contraction of body. There are no sensory cells for hearing.
Hydra is a multicellular invertebrate animal. They have Nervous system characterised by a network of neurons, concentrated near the mouth. No sensory activities are performed by this organism except eating.
Jellyfish and Sea anemone have interconnected nerve cells all over their body with the help of which they are able to respond to the various types of stimuli of their surroundings.Earthworm has a segmented body. It has brain. They have sense of touch, light and vibration receptors present all over their body. They do not have cells for hearing.Star fishes cannot hear. There is no specific sense organ for hearing. There is no brain also. The nervous system is characterised by nerve ring that surround the mouth. But they have eyespots at the tip of each arm. The eyespot contains light sensitive pigments that allow them to identify shadows and changes in the brightness of the light.Worms like flat worms, round worms, tape worms etc are not capable of hearing or seeing. They move in their environment with help of vibration receptors. They feel the vibrations in their surroundings with the help of these receptors.

Insects are the only organisms among the invertebrates that are capable of hearing. Their ears are located at their head. Their ear drum is a thin part of cuticle sensitive to air vibrations. Sensitive receptors are located either on the trachea or on the ear drum.

Insects do not have ears like those of human beings, but are able to hear through tympanal organs, which work like our ear drum. Insects use sounds to communicate with other insects, and to navigate in their environment. Some even listen to the sounds of their predators in order to avoid being eaten. Sound is produced by vibration, and insects hear by collecting and interpreting the vibrations in their environment with the help of brain. 

Ants don't have ears. Ants "hear" by feeling vibrations in the ground through their feet. Ants can detect sounds from other ants through the air, using hair-like sensors at the tips of their antennae. They can sense vibrations from the nest, ground, and surrounding leaves using their antennae. Using this sound, they can locate lost family members and their prey.  Many ants produce squeaky sounds by rubbing one body part against another. They also produce sounds by jaw slapping.