Introduction
There are no special respiratory organs in unicellular and aquatic organisms such as Amoeba and Paramecium. Oxygen that dissolves in water diffuses into the body while carbon dioxide goes out of the body through the entire body surface. The cell membrane acts as the respiratory surface. The concentration of oxygen in water is higher than that inside the body, hence it is will diffuse into all parts of the body. On the other hand, carbon dioxide inside the body is higher than that of the water, hence it will diffuse out of the body. The process of gaseous exchange in unicellular organisms is made possible by simple diffusion. These organisms have large surface area to volume ratio hence diffusion through the body surface alone is enough to satisfy its gaseous exchange needs.
Insects
Insects generally use the tracheal system for gaseous exchange. The insects have tiny openings in their abdomen called spiracles through which oxygen diffuses into the body and carbon dioxide diffuses out the spiracles lead into certain tubes called tracheae (singular, trachea). These tracheae further divide into smaller tubes called tracheoles and these tiny tubes get into the tissues and cells through the body fluid
Insects such as cockroach, grasshopper, etc. use the tracheal system. The insects perform breathing movements by compressing its body dorso-ventrally (i.e. inwards from both sides along its entire body) and then relaxes it. When it compresses its body, it becomes flattened and air is expelled from the tracheae through the spiracles. When it relaxes its body air enters into the tracheae.
During the process of compression and relaxation of the body, oxygen in the air taken in dissolves in the body fluid at the end of the tracheoles from where it enters the various cells. Carbon dioxide from the cells diffuses into the body fluid then into the tracheae and finally comes out of the insect body through the spiracles…
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