Plastrons and physical gills are both the result of a layer of dense, hydrophobic hairs that, due to their inability to interact with water, are able to maintain a layer of air between the water and the insect's or spider's respiratory surfaces. However, they differ in that plastrons are non-compressible and permanent, while compressible gas gills are not, and must be replenished regularly (Thorpe 1950).

Physical gill and plastron respiration is only possible due to the fact that the gases exchanges during respiration, have certain chemical properties. For respiration to occur, there must be sufficient diffusion of oxygen into the bubble, and carbon dioxide must be absorbed out of the gill. This is indeed possible due to the fact that, while carbon dioxide is highly soluble in water, oxygen is not. This means that carbon dioxide is rapidly absorbed into the surrounding water, due to its high solubility, while oxygen’s absorption from the gill into the water is very slow due to its lower solubility (Heckman 1983).

The compressible gas bubble, or the physical gill, is not permanent. This is due to the diffusion of nitrogen gas out of the bubble and into the water. One factor that has a large influence of the lifespan of the compressible gill is the depth at which it exists because of the increase in water pressure that occurs as depth increases (Rahn and Paganelli 1968).

Rahn and Paganelli (1968) give a model equation for the lifetime of a compressible gill:

T = [1/(kN2A)] . [V1N2/ΔPN2]
Where:
T= lifetime of the gill in seconds
kN2= the N2 permeability of the gill
A= the surface area of the gas gill available for exchange, cm2
V1N2= initial N2 volume of the gill

Oxygen gas also plays a significant role in the function of a compressible gas gill. While the lifetime of the gill is largely influenced by nitrogen diffusion, the metabolic needs of the insect or spider play a large role in the movement of oxygen and carbon dioxide. The total nitrogen difference in the gill is affected by two factors: the drop in oxygen pressure inside the gill due to the metabolic needs of the organism, and water pressure (Rahn and Paganelli 1968).

At lower depths where there is minimal hydrostatic pressure, the rate of nitrogen loss is dependent exclusively on metabolic rate, and thus the life span of the gill is dependent on metabolic rate. However, in deeper waters, the water pressure is increased, causing a higher rate of nitrogen diffusion out of the physical gill due mainly to hydrostatic pressure, and less to metabolic rate (Rahn and Paganelli 1968). Overall, the lifetime of a gas gill is directly proportional to the initial volume of nitrogen and inversely proportional to the metabolic rate (Rahn and Paganelli 1968).

On the other hand, with plastrons, which are incompressible, the depth at which they exist does not affect the total pressure, and are in nitrogen equilibrium with the water (Rahn and Paganelli 1968).

One main limiting factor in aquatic insect and spider respiration is size. The larger an insect or spider is, the smaller the surface area to volume ratio of the plastron/ physical gill becomes. It is important that the surface area of the plastron be relatively large enough such that the diffusion of oxygen into the plastron satisfies the metabolic needs of the animal (Flynn and Bush 2008). Thus, plastron respiration is limited to only organisms of a relatively small size.

Belmert et al (2011) identifies another important factor in the life of the physical gill: the density and types of hairs on the arthropod surface, which can consist of larger hairs that are more spaced out (setae), smaller hairs located closer together (microtrichia), or both. Plastrons contain only small, densely spaced microtrichia, while compressible gills contain setae and mictrotrichia (Gorb 2009). At lower pressures, the large, sparse setae are able to keep the arthropod surface dry, while at higher pressures, the microtrichia function as the barrier that prevents the respiratory surfaces from becoming wet (Balmert et al 2011)

Image adapted from Figure 7 (Balmert et al 2011)

Left: At low pressure the setae keep the arthropod surface dry, microtrichia are dry; Right: Higher pressure, the denser microtrichia keep the arthropod respiratory surface dry

Balmert et al concluded that it was the density of these hairs that was the most important factor in the life span of the physical gill and the stability provided by this rather than the volume of the air that is stored in the gill (Balmert et al 2011).
However, Goodwyn et al (2008) reports that the length of microtrichia has an impact on the permanence of the gas gill. Long microtrichia of 7-9 μm long make up a compressible gill, while plastrons have microtrichia of about 5μm long (Goodwyn et al 2008).

Flynn and Bush (2008) identify biological characteristics of plastron breathers that affect the function of the plastron as a method of gas exchange. These factors include:

• Location- the plastron must be located on the area of the body where spiracles are located in order for respiration to take place
• Shape of the hairs- Hairs must be tilted so they are parallel to the insect/spider surface
• Spacing- Hairs must be equidistant from their neighbors.

However, studies have shown that there is some leniency in these requirements. Hairs can withstand pressure in large depths, and some studies have seen that hairs next to each other may overlap (Flynn and Bush 2008).

Representation of the ideal plastron surface, adapted from Figure 3(a) (Flynn and Bush 2008)