Sensory structure, smaller than the antennae and located more toward the midline of the body than the antennae
• (n.) A small antenna; -- applied to the smaller pair of antennae or feelers of Crustacea.
The short segmented appendages located between and below the eyestalks
(from the article `malacostracan`) ...of two branches. The eyes are usually pigmented and borne on movable stalks, but they are sessile on the sides of the head in isopods, amphipods, ...
An·ten'nule noun [ Dim. of antenna .] (Zoology) A small antenna; -- applied to the smaller pair of antennæ or feelers of Crustacea. 
a small antenna, esp. one of the foremost pair of a crustacean.