Antenna (anatomy)
The antenna was a biological feature of the Andorian species and the Aenar subspecies. Both had a set of two antennae, capable of movement, on the top of their head. The twenty-second century Andorian Shran once demonstrated that antennae were moved by voluntary muscles that could be controlled separately from one another. ()

Function
At least in the case of Andorians, the antenna aided in balance. If one was lost, an Andorian became partially disabled in the short term, although they would start compensating within days. The antennae would eventually grow back, within nine months if left untreated, and in half that time with the use of electrical stimulation and brisk cranial massage. All in all, the loss of an antenna was not considered critical, although it might be considered humiliating. ()

Placement and shape
The placement and shape of Andorian antennae could vary significantly. Most Andorians in the 22nd century pre-Federation Imperial Guard sprouted antennae from their frontal skull bones. This Arrangement was also seen in the Aenar. Ambassador Shras and his delegation sported antennae which sprung further back, off their parietal bones. () Yet others had longer and thinner antennae. (; )

Other information
In 2154, Captain Jonathan Archer cut off one of Thy'lek Shran's antennae during an Ushaan, rendering him defenseless and thus respecting the tradition without killing him. "What good is a Guardsman without two antennae?", Shran lamented afterwards. He did say that it would eventually grow back. ()

Thelev, an  Orion operative altered to look like an Andorian to infiltrate the 2268 Babel Conference, had a transceiver hidden inside his artificial antenna. ()

When Lisa Cusak was a junior officer stationed on Andor, she met a man that kept pointing his antennae at her whenever she walked through his office. At first, she thought it rude, but she ended up dating him for six years. ()

In 2372, Quark tried telling Morn a joke of which the punchline was "That's not my antenna." Morn did not laugh. ()

Appendices

Background information 
Some other species also had antenna-like features, such as the Xindi-Insectoid, Species 6339, and the Alfa 177 canine, but antenna were only ever pointed out and discussed by name in reference to Andorians.

A pair of Andorian antennae seen in Enterprise was sold off on the It's A Wrap! sale and auction on eBay. 

The Andorian antennae seen in "Journey to Babel" were sculpted out of clay by makeup artist Fred Phillips using the ends of thread spools for the tips while the main stems were made of a rigid material and attached to the white wigs. (Star Trek: Aliens & Artifacts, p.49)

For more detailed information on the Antenna makeup (and the changes in appearance through the years), see the background section of the article on Andorians.

Apocrypha 
In the  novel Avatar, the Andorian Shar keeps picking up signals through his antennae during a visit to Deep Space 9 in 2376. While in Quark's, he is able to focus on one of the signals and detect a pair of shrouded Jem'Hadar soldiers.

External links 
 
 
Anatomy