The Uniramia have only one pair of antennae on the head, a pair of mandibles, and uniramous (unbranched) appendages. They are distinct from phylum Chelicerata (which lack antennae) and phylum Crustacea (which have two pairs of antennae and biramous (twice-branched) appendages). There is no evidence that uniramiate limbs are derived from a branched condition.
Uniramia have segmented bodies and linearly segmented appendages, all enclosed in a hardened exoskeleton containing a nitrogen-rich polysaccharide, chitin. In embryos, the body is usually divided into segments along its length, although this is less obvious in adults due to segment loss or fusion. The nervous system consists of a brain situated antero-dorsal to the gut, linked to a ventral double nerve cord with paired segmental ganglia. The blood system is open, with a contractile dorsal vessel with paired segmental openings forming the heart. The body is formed of three sections - the cephalum (head), thorax (chest) and abdomen. Growth is accomplished by generation of a new, larger exoskeleton internally, followed by shedding the old one; hormones regulate this process. Larval forms often differ considerably from the sexually mature adult, so development often involves metamorphosis.
The Uniramia appear to have had their origin on land. Most are terrestrial and have internal gas exchange via spiracles (pores in the exoskeleton) and internal tracheae (thin-walled tubes that penetrate the tissues). The eyes may be compound or simple (single units that are non-image-forming). A pair of chitinous mandibles are used to make food accessible for ingestion, e.g. by cutting or crushing.