The limb of an arthropod is said to be uniramous when it has only one branch. Arthropod limbs are typically biramous, with two branches: a sturdy lower walking limb, with a feathery upper breathing limb (also called a gill limb). But many groups of arthropods have lost the gill branches on some or all of their limbs, and these limbs which have only the walking branch are described as uniramous. Occasionally the term is applied to an entire organism with uniramous limbs, so one might say "Insects are uniramous".
For a time, possession of uniramous limbs was believed to be a shared, derived character, so uniramous arthropods were grouped into a taxon called Uniramia. It is now believed that several groups of arthropods evolved uniramous limbs independently, so this taxon has been abandoned.
