dimerous insects are those that have two joints in all their tarsi.
So in fuchsias, a very common deviation consists in a trimerous and rarely a dimerous symmetry of the flower.
Darwin afterwards gave a doubtful explanation of this, and concluded that the ovary is dimerous.
Thus fig. 38 shows a pentamerous symmetrical flower, with dimerous pistil.
dimerous and tetramerous symmetry occur also among dicotyledons.