Describe the three phases of discontinuous ventilation in insects and give two possible reasons for its adaptive significance

Question

What will be an ideal response?

Answer 1

Answer: a. Insects use a tracheal system for gas exchange. Air enters the tracheae (passively or through active ventilation) through spiracles that have the capacity to open and close. The air travels down the tracheae to tracheoles, and then oxygen diffuses into the tissues.
b. Discontinuous ventilation is a ventilatory pattern that is sometimes used by insects when they are at rest. Discontinuous ventilation has three phases. In the first phase (the closed phase), the spiracles remain shut to prevent gas exchange with the environment. This causes the PO2 in the tracheoles to drop, since the body is still consuming oxygen. The carbon dioxide produced by metabolism reacts with water in the interstitial fluid to form bicarbonate, and therefore the PCO2 in the tracheoles does not increase much. The decline in O2 without an increase in CO2 causes a slight decrease in the total gas pressure within the trachea. In the second phase (flutter phase) the spiracles open and close many times rapidly. The low gas pressure within the tracheae causes air to enter the tracheae during this time. Over time, CO2 will accumulate when it can no longer be stored as bicarbonate, and PCO2 will increase in the tracheae. At this point, the third phase occurs, when the spiracles open completely and CO2 is released.
There are three hypotheses listed in the textbook regarding the adaptive significance of discontinuous ventilation:
i. It may facilitate tracheal ventilation: By generating a low PO2 within the tracheae, it creates a greater PO2gradient between the tracheae and the environment. When the spiracles finally open, the oxygen will diffuse more readily into the tracheae. This could be particularly useful for insects that spend large portions of time in places where environmental PO2 is low and PCO2 is high (e.g., underground).
ii. It may help to minimize water loss across the tracheae by having the spiracles closed for longer periods of time.
iii. It may protect insects from the harmful effects of oxygen. Although oxygen is necessary for most animal life, it is also a highly reactive chemical that can damage tissues. When an insect's spiracles are fully open, fresh air can diffuse deep into the tissue, and the PO2. At the ends of the tracheoles is much higher than the PO2h tat vertebrate tissues would experience. This exposure could result in tissue damage. During discontinuous ventilation, the PO2 would only be high intermittently.