Photo: Jeff Gruber & Nicolas Gompel
The plastron of the long-toed beetle, Helichus striatus, does not form spontaneously when the beetle submerges. The beetle actively creates a plastron in a process that requires at least a half hour and more typically takes a couple of hours.
A beetle without a plastron will climb a stick so that only its mouthparts break the surface of the water. Air is trapped in the space between the mouthparts and the thorax when the beetle submerges….
Source: Living With Insects Blog