One constraint on the function of respiratory siphons is that the tips of siphon tubes must be arranged such that air can move into the siphon while water is excluded. Commonly, siphons have hydrofuge hairs associated with their spiracles. When their spiracle is submerged the hairs close over the spiracle and water is kept out of the siphon. At the surface, the hairs are separated by surface tension forces, allowing air to pass to and fro within the siphons. There is typically a funnel shape at the surface.
Respiratory siphons have certain shortcomings.
The siphons tend to limit insects to the upper levels, or strata, of water, so that they are close to the surface. Insects require other adaptations to achieve some independence from the surface strata. Some insects manage to exploit deeper reaches of water by carrying air stores along with them during their dives. This strategy is well developed in the predacious diving beetle Dytiscus . By keeping air stores in contact with spiracles during a dive, these beetles have their trachael systems plus air stores to draw upon to meet oxygen requirements.