What Is Plastron In Insect Eggs

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Figure 8.6 Plastron gills in insects. a: A sheet of air is held in place by a mat of hydrofuge hairs (shown in cross-section as filled circles), which span a space between the insect's body (dark shading) and the water (light shading). b: Detail of a plastron gill of a blowfly egg. [From Hinton (1963)]

Figure 8.6 Plastron gills in insects. a: A sheet of air is held in place by a mat of hydrofuge hairs (shown in cross-section as filled circles), which span a space between the insect's body (dark shading) and the water (light shading). b: Detail of a plastron gill of a blowfly egg. [From Hinton (1963)]

equation 8.5, that is pushing outward at a force equal and opposite to the ApO2. To give you an idea of the magnitude of this force, the ApO2 in a typical plastron gill is about 5 kPa.

The most common way beetles stiffen the bubble involves using the hydrofuge to manipulate the surface tension forces. The hydrofuge consists of very closely spaced hairs; individual hairs are separated from their neighbors by distances of a micrometer or less. Because chitin is not wettable (that is, it resists absorbing water), these mats of hair resist penetration of water into them, by a sort of "reverse matric potential." In this case, water between the hairs form tiny meniscuses—crescent-shaped surfaces of water like those discussed in Chapter 7. The surface tension of each meniscus pushes outward on the water, resisting the excess hydrostatic pressure that ordinarily should collapse the bubble. This outward force can be quite large—the plastron can hold an air space against a hydrostatic pressure of 500 to 600 kPa, roughly five to six times atmospheric pressure and roughly a hundred times more forceful than the usual ApO2 of 5 kPa or so. Thus, the bubble is "stiffened," in this case, by the surface tension forces that push the water away from the hydrofuge hairs.

With this design principle in hand, we can now recognize plastron gills popping up in circumstances we would never have considered before. For example, anyone who has been neglectful about taking out the household trash (as I have been on occasion) has confronted the problem of maggots, writhing around in liquefying rotting meat. Maggots, of course, have to come from somewhere, and this means there had to have been eggs laid in that fluid. If you can choke back your disgust for a moment, a question might occur to you—how do those eggs in there breathe? It turns out, in fact, that many insect eggs, particularly eggs that are laid in liquid, like a stream or a lake or a rotting puddle of carrion, are coated with woven mats of fibers, supported by complicated arrangements of pillars (Fig. 8.6). The woven mats exert outward "reverse matric" forces, just as the hairs in a hydrofuge do, and the pillars resist the collapsing force of the excess hydrostatic pressure. Clearly, the eggs are structured to be a plastron gill.

The Dynamic Bubble Gill of the Water Beetle Potamodytes A particularly interesting example of a novel plastron gill is found in an aquatic beetle common to western

Africa, Potamodytes. Living in swift-flowing streams, Potamodytes faces into the current, clinging to rocks or sitting just behind them. These beetles often carry with them a prominent bubble, which encloses their legs and streams out behind them as it is pulled by the current (Fig. 8.7). In still water, the bubble carried by Potamodytes is a conventional bubble gill: it eventually shrinks and disappears and the beetle must come to the surface to capture a new one. Yet, in flowing water, the beetle is capable of maintaining its bubble and staying underwater apparently indefinitely, just as it could if the bubble were acting as a plastron gill. Under certain conditions, the beetle can even grow a bubble directly out of the water. Yet Potamodytes has none of the specialized structures, like hydrofuge hairs, that other plastron breathers use to stabilize their bubbles. Rather, these beetles maintain the plastron gill by using kinetic energy in the environment (or, as Mr. Spock might say, "from pure energy").

The beetle maintains its bubble by exploiting a behavior of flowing fluids known as the Bernoulli princi bubble

Figure 8.7 A Potamodytes beetle with its dynamic bubble gill. The outline of the bubble is indicated by the dotted line. [From Stride (1955)]

ple, named for the eighteenth-century physicist Daniel Bernoulli.6 The Bernoulli principle expresses how different types of energy in a fluid interact. For example, flowing water has mass and velocity, which imparts to it kinetic energy. This can be precisely quantified:

where KE is the kinetic energy (joules), p is water's density, about 1,000 kg m-3, and vis velocity (m s-1). If this fluid is brought to a halt, as it would be if it encountered a solid wall, this kinetic energy would not disappear, it would be converted to a potential energy, which in fluids is expressed as a pressure. Bernoulli's principle simply states that the relationship between a fluid's kinetic energy and its potential energy are constrained by the First Law:

where P is the fluid's pressure and k is a constant. Thus, the total energy in a parcel of water can exist either as potential energy or kinetic energy, and these can interchange, as long as their sum remains constant.

To illustrate, suppose, for example, a parcel of water is abruptly brought to a dead stop. This means the kinetic energy in the parcel falls to zero, because now v = 0. But the First Law tells us that the energy cannot simply disappear. Rather, it is converted to potential energy, in other words, a pressure. Indeed, the pressure will be equal to the kinetic energy in the parcel when it was flowing at its unobstructed velocity; that is, P = 1pv2. Such pressures, called dynamic pressures, are a part of common experience—it is harder to run

6. For the purposes of this discussion, I am presenting a simplified version of Bernoulli's principle, which in full is expressed as P + pgh + 2 pv2 = k. The terms are designated the head pressure, P, the hydrostatic pressure, pgh, and the dynamic pressure, 2pv2. What I have left out, of course, is the hydrostatic component of the pressure.

into the wind than with it, and you can even be knocked off your feet when a sufficiently vigorous wind is brought to a halt by your body.

Dynamic pressures are important in helping Potamodytes maintain an ordinary bubble as a plastron gill. In this case, the dynamic pressures arise from water being accelerated around the beetle and its bubble. Equation 8.6 tells us that an increase in velocity must result in an increase in the water's kinetic energy. The pesky demands of the First Law insist that the energy come from somewhere, and the only place it can come from is from pressure, that is, the pressure must drop to less than P.In other words, a dynamic suction pressure must arise that will pull outward on the obstacle to the flow. If a Potamodytes beetle carrying a bubble faces upstream, water must inevitably be accelerated around it. In accordance with Bernoulli's principle, there should be a suction pressure acting on the bubble, and this suction pressure should increase with the square of the velocity of the water. This is indeed the case (Fig. 8.8).

It is clear now how swift currents can make the bubble behave like a plastron gill. The outward force of the dynamic suction pressure opposes the forces that would normally collapse the bubble. Remember that the suction force need not be very great, on the order of a few hundred to a few thousand pascals or so. What normal plastron breathers accomplish by structure—hydrofuge hairs that use surface tension to resist the bubble's collapse—Potamodytes accomplishes by clever use of kinetic energy in the physical energy stream.

The mere presence of the beetle in a flowing stream is sufficient for the Bernoulli inflation of its bubble, but Potamodytes takes the principle a couple of steps further. For example, one curious anatomical feature of Potamodytes is the flattening of the proximal segments of the limbs, those closest to the body (Fig. 8.9). The beetle holds these limbs out to its sides, and the flattened surfaces are held at an angle pointing downward toward the substratum. The flat segments therefore act as hydrofoils, which accelerate the water up-

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