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. As the mouthparts repeated break the surface and submerge, the bubble expands and pushes backward along the sides of the beetle. Brush-like hairs on the front legs move air from the bubble into the hairs of the plastron on the head and thorax. Movement of the pronotum moves air backward under the elytra.
Eventually all the legs and body regions move to distribute the air in the bubble into the plastron. These behaviors cease once the plastron is complete. How does the beetle sense that its plastron is missing or that its plastron is complete? Only the beetle knows for sure.
*Hilda T. Harpster. An Investigation of the Gaseous Plastron as a Respiratory Mechanism in Helichus striatus Leconte (Dryopidae). Transactions of the American Microscopical Society, Vol. 60, No. 3 (Jul., 1941), pp. 329- 358.