The lepidopteran caterpillar, says physiologist David Moffett, is basically just a thin sheet of animal stretched over a big gut. The caterpillar s enormous gut reflects an enormous appetite, and indeed eating is its sole immediate purpose and occupation. But larval gluttony comes with certain physiological problems, which have long interested Moffett.
Plant leaves, the caterpillar s cuisine, are high in potassium and low in sodium.
In contrast, much of basic physiology depends on a model high in sodium and low in potassium. In fact, says Moffett, were a traditional medical physiologist to examine the blood of a caterpillar, she would declare it impossible. Moffett has explained the mechanism by which the midgut of the tobacco hornworm secretes potassium through its membrane. In the process, Moffett also got interested in another oddity of the larval midgut. It generates extremely alkaline conditions, again the opposite of what is considered the norm.