Insects have to manage a delicate balancing act while breathing. According to John Terblanche and his colleagues Berlize Groenewald and Steven Chown from Stellenbosch University, South Africa, and Stefan Hetz from Humboldt University at Berlin, Germany, insects have to supply sufficient oxygen to meet their metabolic demands while managing carbon dioxide waste removal. And they do all that without drying out the delicate fluid-filled tracheoles that transport oxygen directly to their cells. Explaining that insects tackle the problem using a variety of tactics, the team adds, ‘The most controversial of the proposed adaptations is the use of discontinuous gas exchange cycles’.
During discontinuous gas exchange, insects minimise the amount of time that the respiratory system is open to the atmosphere. They only take in air to supply oxygen during the second phase of the cycle (known as the flutter phase) and release carbon dioxide during the final phase (the open phase) when they finally open the spiracles that initially sealed the respiratory network at the start of the cycle (the closed phase). Yet, how and why insects evolved this complex respiratory mechanism is unclear. Explaining that there are several competing theories, ranging from protection from oxidative damage to prevention of dehydration, Terblanche, Groenewald, Hetz and Chown decided to find out more about how desert locusts use this particular form of respiration (p. 2301).
‘Desert locusts are big and easy to instrument’, says Terblanche, who explains that this is a particular advantage when taking pressure measurements inside the insect's tracheal system. ‘It is extremely challenging to insert tiny tubes into the locust's spiracles’, he adds. Having travelled to join Hetz in Germany, Groenewald measured the carbon dioxide release patterns, the pressure in the tracheal system and the thorax movements of 12 resting locusts as they performed discontinuous gas exchange.
Analysing the insect's movements, the team noticed that the insects pump the thorax in two ways: telescoping the thorax along its length and compressing the top and bottom. Also, instead of pumping during the open phase – when other insects pump to expel carbon dioxide – the locusts were pumping during the closed phase. The team suspects that instead of expelling gases from the body, the insects pump the thorax to increase the pressure in the tracheal system, reducing the evaporation of water into the air to reduce the insect's water losses during the open phase of the cycle. Alternatively, the pumping action could improve gas exchange between the tracheoles and the tissues that they supply with oxygen.
Turning their attention to the pressure and CO2 recordings, the team was surprised to see that the locusts were releasing CO2 during the closed phase, despite the pressure dropping in the trachea. Explaining that no other discontinuously ventilating insects release CO2 during the closed phase – when they usually consume the oxygen held in the trachea while storing the resulting CO2 in their body fluids – Terblanche says, ‘This suggests considerable complexity; perhaps constriction of spiracles in one part of the body while not in another part of the tracheal system, or possible cuticular CO2 leakage’.
Having shown that discontinuous ventilation varies widely amongst individual locusts and that they probably use this mode of ventilation to avoid dehydration and supplement gas exchange while at rest, the team is continuing to investigate the evolution and mechanism of this intriguing ventilation pattern.