Gaseous exchange in insects e.g., grasshopper takes place across a system of tubes penetrating into the body known as the tracheal system. The main trachea communicate with atmosphere through tiny pores called spiracles. Spiracles are located at the sides of body segments;
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Two pairs on the thoracic segments and eight pairs on the sides of abdominal segments. Each spiracle lies in a cavity from which the trachea arises. Spiracles are guarded with valves that close and thus prevent excessive loss of water vapour. A filtering apparatus i.e. hairs also traps dust and parasites which would clog the trachea if they gained entry. The valves are operated by action of paired muscles.
Mechanism of Gaseous Exchange in Insects
The main tracheae in the locust are located laterally along the length of the body on each side and they are interconnected across. Each main trachea divides to form smaller tracheae, each of which branches into tiny tubes called tracheoles. Each tracheole branches further to form a network that penetrates the tissues. Some tracheoles penetrate into cells in active tissue such as flight muscles.
These are referred to as intracellular tracheoles. Tracheoles in between the cells are known as intercellular tracheoles. The main tracheae are strengthened with rings of cuticle. This helps them to remain open during expiration when air pressure is low.
Adaptation of Insect Tracheoles for Gaseous Exchange
The fine tracheoles are very thin about one micron in diameter in order to permeate tissue. They are made up of a single epithelial layer and have no spiral thickening to allow diffusion of gases. Terminal ends of the fine tracheoles are filled with a fluid in which gases dissolve to allow diffusion of oxygen into the cells. Amount of fluid at the ends of fine tracheoles varies according to activity i.e. oxygen demand of the insect.
During flight, some of the fluid is withdrawn from the tracheoles such that oxygen reaches muscle cells faster and the rate of respiration is increased. In some insects, tracheoles widen at certain places to form air sacs. These are inflated or deflated to facilitate gaseous exchange as need arises.
Atmospheric air that dissolves in the fluid at the end of tracheoles has more oxygen than the surrounding cells of tracheole epithelium’. Oxygen diffuses into these cells along a concentration gradient. ‘
Carbon (IV) oxide concentration inside the cells is higher than in the atmospheric . Air and diffuses out of the cells along a concentration gradient. It is then removed with expired air.
Ventilation in Insects
Ventilation in insects is brought about by the contraction and relaxation of the abdominal muscles. In locusts, air is drawn into the body through the thoracic spiracles and expelled through the abdominal spiracles. Air enters and leaves the tracheae as abdominal muscles contract and relax. The muscles contract laterally so the abdomen becomes wider and when they relax it becomes narrow.
Relaxation of muscles results in low pressure hence inspiration occurs while contraction of muscles results in higher air pressure and expiration occurs. In locusts, air enters through spiracles in the thorax during inspiration and leaves through the abdominal spiracles during expiration. This results in efficient ventilation.
Maximum extraction of oxygen from the air occurs sometimes when all spiracles close and hence contraction of abdominal muscles results in air circulating within the tracheoles.
The valves in the spiracles regulate the opening and closing of spiracles. Observation of Spiracle in Locust Some fresh grass is placed in a gas jar. A locust is introduced into the jar. A wire mesh is placed on top or muslin cloth tied around the mouth of the beaker with rubber band.
The insect is left to settle. Students can approach and observe in silence the spiracles and the abdominal movements during breathing. Alternatively the locust is held by the legs and observation of spiracles is made by the aid of hand lens.