B

Figure 8.9 Use of the limbs of Potamodytes as hydrofoils to accelerate water above and to the center of the bubble. a: As viewed from the side, the tibiae of the beetle are in position to impart an upward acceleration to water flowing past the beetle. b: As viewed from the top, the tibiae are held at acute angles to the body. In this position they impart a centripetal acceleration to water flowing past the beetle.

Figure 8.9 Use of the limbs of Potamodytes as hydrofoils to accelerate water above and to the center of the bubble. a: As viewed from the side, the tibiae of the beetle are in position to impart an upward acceleration to water flowing past the beetle. b: As viewed from the top, the tibiae are held at acute angles to the body. In this position they impart a centripetal acceleration to water flowing past the beetle.

in the lee of the obstruction. Fluids are accelerated around pebbles too, though, and suction pressures on the beetles' bubbles will be magnified by the enhanced velocity over the pebble.

The Winter Web of Argyroneta

We began this chapter with the diving bell spider, and the use of woven structures as gills. Let us now return to this problem, starting with the aquatic spider, Argyroneta, which, as noted above, uses its web as a conventional bubble gill. In fact, the spider only uses its web in this way during its active season, when it must get about and forage in its territory. In the winter, however, Argyroneta weaves a very different web. Where the summer web is thin and lacy, the winter web is a thickly woven mat of silk, and sometimes it encloses a rigid bracing structure like a leaf, a twig, or a snail shell. Sealed within its cocoon, the spider sits and waits out the winter. We can now see clearly that the winter web of Argyroneta is a candidate for a structural plastron gill. The thickly woven web and its bracing are structurally more resistant to collapse than the spider's rather diaphanous summer web. If the winter web sustains the pressure differences needed for a plastron gill to operate, it will, in accordance with the design principle, allow the spider to sit out the winter in its little house, underwater but dry and snug, breathing the oxygen that seeps in across the walls.

The Cocoon of the Ichneumon Wasp Agriotypus One of the more bizarre examples of a woven plastron gill is built by a wasp that parasitizes caddis fly larvae. Caddis flies belong to an order of insects (the Trichoptera) whose larvae are predominantly aquatic. Caddis larvae themselves are architects of sorts—they construct little tubular houses for themselves from which they capture whatever prey wanders close enough. Typically, the caddis larva weaves a silken tube, open at both ends, which it then decorates with bits of leaves, tiny pebbles, even pieces of its prey. Sometimes, these decorations add structural strength to the caddis house (Fig 8.10).7

As every homeowner with a mortgage will understand, having a house restricts the movements of the caddis larva away from it. Being a "homebody" makes a caddis larva vulnerable to predators, just as a homeowner is more frequently harassed by telemarketers. Among the predatory dangers faced by caddis larvae are nasty little wasps known as ichneumons: the parasite in the movie Aliens was loosely based on an ichneumon wasp larva. The ichneumon lays its eggs on a caddis larva. The wasp larva, when it hatches, has the endearing habit of consuming its living host, eventually leaving only an empty husk.

7. Caddis houses have been extensively studied by behavioral and evolutionary biologists because their variation provides a useful indicator of a behavior and its evolution. Their construction is an example of "frozen behavior" alluded to earlier in the book, a durable record of an instinctual behavioral program. See vonFrisch and vonFrisch (1974) andDawkins (1982).

Figure 8.10 a: Adult of a caddis fly, about 2 cm long. b: Caddis house, built from flakes of plant material glued to a silken sheath. c: Caddis larva, showing spines it uses to anchor its house in place and sensory bristles around the thorax and head. [From von Frisch and von Frisch (1974)]

Figure 8.10 a: Adult of a caddis fly, about 2 cm long. b: Caddis house, built from flakes of plant material glued to a silken sheath. c: Caddis larva, showing spines it uses to anchor its house in place and sensory bristles around the thorax and head. [From von Frisch and von Frisch (1974)]

Our example concerns pupae of the caddis flies Silo, which are parasitized by larvae of the ichneumon wasp Agriotypus. Silo construct fairly rigid houses coated with tiny bits of gravel. During the initial stages of its parasitic life, the Agriotypus larva breathes through spiracular gills and so lives quite happily in the water-filled house of its host. During its last larval instar, though, just before pupation, the spiracles open and the Agriotypus larva becomes an air breather. Its habit of parasitizing aquatic insects now becomes a bit of a liability, because if it remains in water, it will drown. Unfortunately, it is not really in a position to leave the water for air, either. The newly air-breathing wasp larva therefore weaves itself a heavy web, which closes the open ends of the caddis house. By some process still unknown, the wasp larva then evacuates the water from the now-closed cocoon, opening up an air space inside.

This is a perfect situation for a plastron gill, and in all likelihood, the simple closed structure would behave as one, just as the winter web of Argyroneta does. The Agriotypus cocoon has an unusual feature, how ever; a long ribbon, woven from silk by the wasp, extrudes four or five centimeters from the end of the larval case (Fig. 8.11). Remarkably, this ribbon appears to be a plastron gill. The ribbon's threads resist wetting, so that the ribbon encloses an air space that is continuous with the air space in the cocoon. Furthermore, the ribbon is crucial to the wasp larva's survival—if it is pinched off, the larva suffocates.

Even though the Agriotypus ribbon meets these criteria for a plastron gill, it seems to fall short in another. Consider a "conventional" plastron gill, held in place by a mat of hydrofuge hairs. These bubbles seem to be well-designed gas exchangers, according to the dictates of Fick's law. They are thin (x is small, a few micrometers) and they cover a fairly large area of the beetle (A is large, a few square millimeters). The ratio A/x is therefore very large, and Fick's law dictates a rapid oxygen flux. Well and good. Now consider the air-filled ribbon of an Agriotypus larva and try to fit that into the good design principles of Fick's law. It fails on two counts. Oxygen must travel a great distance along the ribbon before it gets to the larva, 5 cm or so (x is large). The ribbon is also narrow in cross-section (A is small). If Agriotypus is using Fick's law as a guiding

Figure 8.11 Parasitism of caddis larvae by Agriotypus. a: External view of a caddis house parasitized by an Agriotypus larva, showing the characteristic silk ribbon. [From Thorpe (1950)] b: Cross-sectional view of a parasitized caddis house. The Agriotypus larva is not shown, and only a portion of the ribbon is shown. [From Clausen (1931)]

Figure 8.11 Parasitism of caddis larvae by Agriotypus. a: External view of a caddis house parasitized by an Agriotypus larva, showing the characteristic silk ribbon. [From Thorpe (1950)] b: Cross-sectional view of a parasitized caddis house. The Agriotypus larva is not shown, and only a portion of the ribbon is shown. [From Clausen (1931)]

plastron thickness dependent flux plastron area dependent flux

Figure 8.12 Two-dimensional flow of oxygen through a flat plastron bubble.

spiracle

Figure 8.12 Two-dimensional flow of oxygen through a flat plastron bubble.

principle for its plastron gill, it seems to have developed a case of physiological dyslexia.

Occam's razor would, in this case, guide us to reject the hypothesis that the Agriotypus ribbon is a plastron gill. But let's see what Goldberg's lever can do for us. First, let us concede what is right: good design of a plastron gill must account for the limitations imposed by Fick's law, and indeed, we find that most plastron bubbles are both thin and capacious in area. Insect plastron breathers must fit this principle into their evolutionary legacy as air breathers, however, of which spiracles are an important feature. Thus, by dint of its ancestry, a plastron breather must convey oxygen from a bubble surface that covers a broad area of the insect and channel it to the very localized point of the spiracle (Fig. 8.12). This is rather more complexity than can be handled by the simple version of Fick's Law used so far in this book. It would do perfectly well if the entire surface of the beetle covered by the bubble could absorb oxygen—if, in the jargon of such things, the flow of oxygen across the bubble was one-dimensional. However, the flow of oxygen through a real plastron gill is not: it is two-dimensional. For oxygen to move from the water to the spiracle, it must flow across the bubble, perpendicular to the beetle's surface (one dimension), and then parallel along the surface to the spiracle (a second dimension).

If we are prepared to embrace complexity for its own sake, rather than avoid it lest it lead us astray, then some new principles for "good design" of plastron gills emerge. To illustrate, let us compare a "well-designed" plastron gill with a "poorly-designed" one. The plastron gill of an aquatic beetle, like Aphelocheirus, serves as an example. If we measure the oxygen partial pressures in the Aphelocheirus plastron, the ApO2 between the bubble and water varies only little between the edge of the bubble and the spiracle it serves (Fig. 8.13, solid curve). There is a slight dip right at the opening to the spiracle, but in general, the ApO2 changes little, even at points in the bubble far removed from the spiracle. What makes this a "well-designed" gill is the uniformity of oxygen partial pressure across the entire surface of the bubble. Because oxygen flow from the water into the bubble is proportional to the ApO2, it is sensible to keep this difference as large as distance from spiracle (cm) 0 1 2

average ApO2

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