Flagellomere

Antennae (singular: antenna), sometimes referred to as "feelers," are paired appendages used for sensing in arthropods.

Antennae are connected to the first one or two segments of the arthropod head. They vary widely in form, but are always made of one or more jointed segments. While they are typically sensory organs, the exact nature of what they sense and how they sense it is not the same in all groups. Functions may variously include sensing touch, air motion, heat, vibration (sound), and especially smell or taste. Antennae are sometimes modified for other purposes, such as mating, brooding, swimming, and even anchoring the arthropod to a substrate. Larval arthropods have antennae that differ from those of the adult. Many crustaceans, for example, have free-swimming larval forms that use their antennae for swimming.

The common ancestor of all arthropods likely had one pair of uniramous (unbranched) antenna-like structures, followed by one or more pairs of biramous (having two major branches) leg-like structures, as seen in some modern crustaceans and fossil trilobites. Except the chelicerates and proturans, which have none, all non-crustacean arthropods have a single pair of antennae.

Crustaceans bear two pairs of antennae. The pair attached to the first segment of the head are called primary antennae or antennules. This pair is generally uniramous, but is biramous in crabs and lobsters and remipedes. The pair attached to the second segment are called secondary antennae or simply antennae. The second antennae are plesiomorphically biramous, but many species later evolved uniramous pairs. The second antennae may be significantly reduced (e.g. remipedes), apparently absent (e.g. barnacles), or modified to such an extent that they no longer resemble antennae (e.g. spiny lobsters and slipper lobsters).

...
Wikipedia