excess consumption Figure 9.13 Playing the margins of leaf photosynthesis.
margin—which can be diverted to fueling the hyperplasia the gall insect depends upon. The situation is reversed at temperatures higher than the optimum: net photosynthesis declines because metabolism starts to outstrip gross photosynthesis. Sugar may then be "im
ported" from other leaves, but because galled leaves will be warmer, the diversion of energy to them may be greater. And in all cases, sitting there in the middle of these streams of energy is the gall insect, skimming its take.
First thou shalt arrive where the enchanter Sirens dwell, they who seduce men. The imprudent man who draws near them never returns, for the Sirens, lying in the flower-strewn fields, will charm him with sweet song; but around them the bodies of their victims lie in heaps. —circe's warning to odysseus, homer, the odyssey, book xi chapter ten
Twist and Shout!
Social beings that we are, we tend naturally to think of communication as an interaction: good communication is a two-way street, a clear exchange of thoughts, desires, or emotions between two or more parties. We want to be very sure we understand a contract before we sign it, we want to be very sure of a rival's intentions before we deal with him, we want to be very sure we will not be caught up in someone else's hidden agenda when we enter into a relationship with her. The interaction, spoken and unspoken, that humans use to ascertain both meanings and motives is prolonged, complex, and subtle. Despite the effort we put into it (or perhaps because of it), human communication is maddeningly prone to failure, to the enrichment of no one save lawyers and country music singers.
Communication between other animals is usually more straightforward. These messages are generally not intended to convey truth, understanding, or co-mity—they are intended to manipulate other animals' nervous systems to elicit from them particular responses that will, in the end, increase the likelihood that the sender's genes are passed to its offspring. Communication is judged solely by how reliably it accomplishes this goal. By this logic, Homer's Sirens were great communicators;using their songs to zero right in on and activate mate-seeking behavior in the sailors that heard them, they got the desired response without having to waste time on the protracted, subtle, and delicate negotiation that we call courtship.
For the physiologist, communication is an exchange of energy between the sender and recipient. Generally, the energy involved is deducted from the sender's metabolic energy budget. Premiums should accrue, therefore, to animals that communicate efficiently— that is, with minimum expenditure of energy but consistent with conveying the sender's message widely and clearly. Fireflies, for example, communicate with potential mates by converting ATP energy into light. The light signal should be sufficiently bright to attract the attention of potential mates. However, the signal should activate seeking behavior only in potential mates that are programmed to receive them. Consequently, signals between potential mates are usually modulated in some way, as by color or flash rate.
Communication presents an interesting problem to the physiologist, because it is very difficult to separate what goes on inside two communicating animals from the physical transmission of energy through the medium separating them. At what point in the chain of signal transmission does communication stop being physiology and start being physics? Let us put the question more concretely, staying with the example of fireflies. The eye of a firefly consists of two elements, sensory and optical. The sensory eye consists of cells that absorb photons and convert them into electrical signals suitable for transmission to the firefly's brain. Before the photons get to these cells, though, they pass through an array of refractive lenses and light guides that make up the optical eye. The functioning of the optical eye is governed by the same physical interaction of light with matter that governs inanimate optical systems. When the optical eye manipulates light, is it physics or is it physiology? In my opinion, the distinction is a false one: I would have difficulty arguing that anything that modulates, modifies, or transmits the energy in a message is not part of a physiological process of communication. And this leads us to an interesting quandary: if the built-in parts of sensory systems, like corneas, lenses, and irises, are properly organs of communication, why should structures that do the same thing, but are constructed outside the animal, not be considered the same?
In this chapter, we will turn to structures, built by crickets, that aid in transmitting auditory signals to prospective mates. We will first explore the question of why crickets should need such structures in the first place. As part of the answer, we must also familiarize ourselves with the acoustical principles underlying the production of cricket song, and how the structures they build make them more effective disseminators of sound. We will then turn to two remarkable examples of crickets that build external structures that modulate, amplify, or direct the sounds they produce. Along with the argument I wish to convey, I also have a hidden agenda, which, in the interests of good (human) communication, I now reveal: the physiology of communication, more than any physiological process of which I am aware, demolishes the artificial boundary that limits physiology to the organism.
Crickets, like Sirens, are superb acoustic communicators. Males produce sounds and use them to broadcast their identity and location to prospective mates flying about looking for them. A female who intercepts this sound is guided by it to its source, just as we are drawn to the pleasant smells emanating from a kitchen. The messages are simple, direct, and clear.
Anyone who has sat outside on a warm summer's evening is aware of how well the cricket's sounds carry. This is a sensible thing for a mating call to do: a sound that travels far is more likely to be heard by a potential mate than a call that dies out quickly. Generally, making a sound travel far means making a loud sound. The curious thing is that by most principles of acoustical physics, crickets should only be able to produce relatively soft sounds, and inefficiently at that. Below, we will delve into the reasons why this is so, but first let us learn more about cricket songs and how they are produced.
A cricket song is a series of "chirps" (Fig. 10.1), each chirp consisting of a series of discrete tones, each lasting a few milliseconds, interspersed with short periods of silence. The number of tones in a chirp, which de-
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