Why do some Dipsacales flowers have bilateral symmetry?

In the plant order Dipsacales, the development of bilateral symmetry—also known as zygomorphic symmetry—is primarily an adaptation related to specialized pollination mechanisms.12

While many members of the order possess regular, radially symmetrical flowers, others have evolved two-lipped flowers where one half is a mirror image of the other. This structural shift is often associated with advanced floral evolution, which typically involves a reduction in the number of floral parts and the fusion of petals into a corolla tube.12

These morphological changes allow flowers to interact with specific pollinators more effectively. For example, in the Caprifoliaceae family, variations in flower color and the length of the corolla tube correspond to different pollination strategies, ranging from bees to hawk moths and hummingbirds. By restricting access or guiding the pollinator's approach, bilateral symmetry helps ensure more efficient pollen transfer compared to the generalized approach seen in primitive, radially symmetrical flowers.12