In insect anatomy the arista is a simple or variously modified apical or subapical bristle, arising from the third antennal

Antenna (biology)

Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....

 segment. It is the evolutionary

Evolution

Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

 remains of antennal segments, and may sometimes show signs of segmentation. These segments are called aristameres.

The arista may be bare, sometime appearing no more than a simple bristle, pubescent - covered in short hairs, or plumose - covered in long hairs.

The presence of an arista is a feature of the Diptera

Fly

True flies are insects of the order Diptera . They possess a pair of wings on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax...

 suborder

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 Brachycera

Brachycera

Brachycera is a suborder of Diptera. It is a major suborder consisting of around 120 families. The most distinguishing characteristic of the suborder is reduced antenna segmentation...

and may be especially well-developed in some species.