Propodeum
The propodeum is a term that can refer to unrelated structures in insects or in mollusks.
Insects
The first abdominal segment in Apocrita Hymenoptera (wasps, bees and ants) is known as the propodeum. It is fused with the thorax to form the mesosoma. It is a single large sclerite, not subdivided, and bears a pair of spiracles. It is strongly constricted posteriorly to form the articulation of the petiole, and gives apocritans their distinctive shape. There may be a suture between the propodeum and the thorax, like in Symphyta or not, and the presence or absence of such suture can aid in identifying specimens.
Mollusks
Propodeum is the anterior (frontal) part of the foot of a mollusk.