A feeler; especially, one of the jointed sense organs attached to the mouth organs of insects, arachnids, crustaceans, and annelids; as, the mandibular palpi, maxillary palpi, and labial palpi. The palpi of male spiders serve as sexual organs. Called also palp. See Illust. of Arthrogastra and Orthoptera.

(pl. Palpi): One of a pair of appendages on the front of a butterfly's head which protect the feeding apparatus, the proboscis.

The usually 3-5 segmented appendage of the maxilla or labium. In the Lepidoptera the maxillae and labium are normally atrophied beyond easy recognition and only portions remain– such as the palpi and the galeae. When palpus is used without a modifier (i.e. maxillary or labial) it refers to the labial palpus. In Lepidoptera either both palpi are visible or atrophied, or only the labial is present.