Antennae (singular antenna) are paired appendages connected to the front-most segments of arthropods. In crustaceans, they are biramous and present on the first two segments of the head, with the smaller pair known as antennules. All other arthropod groups, except chelicerates and proturans which have none, have a single, uniramous pair of antennae.
Antennae are jointed, at least at the base, and generally extend forward from the head. They are sensory organs, although the exact nature of what they sense and how they sense it is not the same in all groups, nor always clear. Functions may variously include sensing touch, air motion, heat, vibration (sound), and especially olfaction (smell) or gustation (taste).
In insects, olfactory receptors on the antennae bind to odour molecules, including pheromones. The neurons that possess these receptors signal this binding by sending action potentials down their axons to the antennal lobe in the brain. From there, neurons in the antennal lobes connect to mushroom bodies that identify the odour. The sum of the electrical potentials of the antenna to a given odor can be measured using an electroantenogram.
The three basic segments of the typical insect antenna are the scape (base), the pedicel (stem), and finally the flagellum, which often comprises many units known as flagellomeres. Muscles are only present in the two first segments, the scape and pedicel. The scape is surrounded by a membranous region of the head. It pivots on a single marginal point called the antennifer, allowing it to move in any direction. The number of flagellomeres can vary greatly, and is often of diagnostic importance. True flagellomeres have a membranous articulation between them, but in many insects, especially the more primitive groups, the flagellum is entirely or partially composed of a flexible series of small annuli, which are not true flagellomeres. In many beetles and in the chalcidoid wasps, the apical flagellomeres form a club, and the collective term for the segments between the club and the antennal base is the funicle (or funiculus); for traditional reasons, in beetles it is the segments between the club and the scape, but in wasps, it is the segments between the club and the pedicel. In the groups with more uniform antennae (for example: Diplopoda), all segments are called antennomeres. Some groups have a simple or variously modified apical or subapical bristle called an arista (this may be especially well-developed in various Diptera).
Crustaceans bear two pairs of antennae. The first pair are uniramous and are often referred to an antennules, while the second pair are biramous, meaning that each antenna is composed of two parts, joined at their base . In most adults, the antenna are sensory organs, but they are used by the nauplius larva for both feeding and swimming. In some groups of crustaceans, such as the spiny lobsters and slipper lobsters, the second antennae are enlarged, while in others, such as crabs, the antennae are reduced in size.
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A spiny lobster, showing the enlarged second antennae
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The large flattened plates in front of the eyes of a slipper lobster are the modified second antennae
The crab Cancer pagurus, showing its reduced antennae