No. 85: Jan-Feb 1993
Actually, the singing caterpillars are not particularly tuneful. They really generate a vibration that is transmitted through the material they are resting on. You and I cannot hear caterpillar songs, but some ants can, and they are attracted to these insect sirens.
The singing caterpillars belong to the Lycaenidae, which include such butterflies as the hairstreaks and blues. It is not only the singing or vibrating of this group of caterpillars that makes them remarkable, it is the complexity of their symbiotic relationships with several species of ants and a plant. Since both the ants and the caterpillars favor the Croton plant, they could well meet by chance, but the caterpillars' singing serves to accelerate contact. Once met on the Croton plant, a fascinating triangle is completed.
Player 1. The Croton plant provides nourishment to the caterpillars through both its leaves and specially evolved nectaries (nectar-producing organs), but receives nothing in return. The ants also dote on the nectaries, but they at least protect the plant from all herbivorous insects except the singing caterpillars.
Player 2. The ants get food from both the Croton plant and the caterpillars. The latter have evolved extrudable glands called "nectary organs." For their part of the bargain, the ants protect the caterpillar from predatory wasps, just as they defend the Croton plant from its enemies.
Player 3. The caterpillars, though seemingly benign, are the heavies in this menage-a-trois! They get both leaves and nectar from the plant for nothing. They do supply the ants with nectar in exchange for protection, but subtle subversion prevails here! First they attract the ants with their songs; then, they seduce them with nectar that is much more nutritious and attractive than that produced by the Croton plant. Finally, they chemically force the ants into defensive postures against predatory wasps by spraying them with a mesmerizing substance from special "tentacle organs" near their heads.
Why is all this subversive on the part of the caterpillars? It appears that the caterpillars have invaded and undermined the normal ant-plant symbiosis -- a very common, mutually beneficial arrangement. The ants have been seduced into letting the caterpillars feast on the Croton plant, although the ant-plant compact originally required that the ants repel all herbivorous insects.
What makes this tale of subterfuge so remarkable is that the caterpillars had to evolve three separate organs in order to accomplish it: (1) their vibratory papillae; (2) their nectary glands; and (3) their mesmerizing tentacle organs.
(DeVries, Philip J.; "Singing Caterpillars, Ants and Symbiosis," Scientific American, 267:76, October 1992.)