As you can see from this centipede from Napa County, California, all uniramians have unbranched appendages. In other arthropods, legs are biramous unless otherwise modified - they have an outer branch, which often forms a feather-like gill, and an inner branch that is used for walking or modified for some other function.
Adult uniramians lack external gills; although aquatic insect larvae often have gills, adults have a set of internal tubes, the tracheae, that open to the air through small holes known as spiracles. This morphological feature has permitted the enormous radiation of uniramians, in particular the insects, into terrestrial habitats.
Drawings and information about a wide range of uniramians, mostly insects, are available from the University of Illinois Department of Entomology. We have used some of them in this exhibit; our thanks to David Lampe for making them available to us.