1640s, "feeler or horn of an insect or other arthropod," from Latin antenna, antemna "sail yard," the long yard that sticks up on some sails, which is of unknown origin, perhaps from PIE root *temp- "to stretch, extend." In the entomological sense, it is a loan-translation of Aristotle's Greek keraiai "horns" (of insects). Modern use in radio, etc., for "aerial wire" is from 1902. Adjectival forms are antennal (1815), antennary (1833), antennular (1853).
-ae
occasional plural suffix of words ending in -a (see a- (1)), most of which, in English, are from Latin nominative fem. singular nouns (or Greek ones brought up through Latin), which in Latin form their plurals in -ae. But plurals in native -s were established early in English for many of them (such as idea, arena) and many have crossed over since. Purity now would only breed monsters.