The segmented body plan is one of the hallmarks of arthropod structure. Morphological segments are formed during embryogenesis, through a complex procedure involving the activation of a series of gene regulatory networks. The segments of the arthropod body are organized into functional units known as tagmata, and these tagmata are different among the arthropod classes (e.g. head, thorax and abdomen in insects; prosoma and opisthosoma in arachnids). Recent work has shown that the process of segmentation varies for different tagmata within the same embryo, and that these differences appear to be consistent across arthropods. For example, in many insects, gnathal and thoracic segments are generated simultaneously during an early developmental stage, whereas abdominal segments are generated sequentially from a posterior growth zone during the germband stage. I will focus on work on Oncopeltus fasciatus, but also compare to work carried out on additional species. Looking back at the arthropod fossil record with this embryological insight, allows us to understand the functional changes underlying major transitions in the evolution of the arthropod body plan.