In Lake Malawi, billions of insect larvae descend every day to depths of more than 200 meters. This migration seems incompatible with their respiratory system filled with air. A study nevertheless shows that their internal reserves withstand even higher pressures, thanks to an adapted biological structure.
The larvae belong to the species Chaoborus edulis, an aquatic fly present in this large lake in East Africa. During the day, they reach a deep layer almost devoid of oxygen. Few fish can remain there for long. At night, they return to the surface to feed.
Lake Malawi, also known as Lake Nyasa in Tanzania and Lago Niassa in Mozambique, is an African Great Lake and the southernmost lake of the East African Rift system, located between Malawi, Mozambique and Tanzania.
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The researchers tracked these movements with a sonar installed at the bottom of the lake. They observed massive, regular vertical migrations. The larvae nevertheless cross an area occupied by predatory fish. Their deep dive therefore constitutes a temporary refuge, despite the physical constraints imposed by the water.
In insects, the respiratory system relies on air-filled tubes called tracheae. In these larvae, part of this network forms two pairs of small sacs. These function like a submarine's ballast tanks. Their volume determines whether the animal rises, descends or remains at a given depth.
The wall of these sacs notably contains resilin, a highly elastic protein often compared to biological rubber. The larvae locally modify the acidity, and therefore the pH, of this wall. The latter expands or contracts, adjusting the air volume and buoyancy without producing new gas.
Tests in a pressurized chamber measured the actual resistance of the sacs. The most developed larvae withstood a pressure corresponding to more than 500 meters deep. They therefore had a significant margin compared with their usual dives, generally limited to 200 meters.
The researchers now want to understand how the wall retains its resistance over daily cycles. Resilin is also of interest to biomimetics, which draws inspiration from living organisms to create materials. An artificial device sensitive to pH could, for example, change its shape or buoyancy without a mechanical system.