CHAPTER FOUR - ORIGIN AND GROWTH OF DISCS

Summary

Insect imaginal discs are barely visible to the naked eye, so detailed observations had to await the invention of adequate magnifying lenses. Discs were first described by the great naturalist Jan Swammerdam (1637– 1680), a contemporary of Leeuwenhoek's, who applied his training in human anatomy to the study of insect morphology. In his Book of Nature (printed in English in 1758), Swammerdam waxes lyrical about the metamorphosis (which he calls “mutation”) of appendages (“horns” are antennae) in hymenopteran larvae (“worms” of bees):

The wings, horns, and other parts which worms without legs seem to acquire about their chests at the time of their mutation are not truly produced during the period of mutation, or, to speak more agreeably to truth, during the time of the limbs shooting or budding out, but … have grown there by degrees under the skin, and as the worm itself has grown by a kind of accretion of parts, and will make their appearance in it upon breaking the skin on its head or its back, and thereby give it the figure of a nymph, which it would afterwards of itself assume.

Hence it is, that we can with little trouble produce [by dissection] the legs, wings, horns, and other parts of an insect, which lie hid under its skin while in the shape of a naked worm, which has neither legs nor any other limbs.… […]