There are two species of larkspur that have two different size flowers; delphinium decorum has large, fully opened flowers and delphinium nudicaule has small bud, like flowers. these two species are both fertilized by insects, but they cannot be fertilized by the same type of insect because the smaller flower of d. nudicaule will not permit larger insects to reach the nectar of the flower and the larger flower of the d. decorum is typically fed on by larger insects. which type of isolating mechanism would this be?

Variations in flower structure may impede pollination (fertilization). Even if the pollinators of one species visit flowers of the other, pollination cannot occur because the pollen does not come into contact with the style of the alternative species. Mechanical isolation is very common in plants. For example, flowering plants that do not have the correct shape for a pollinator will not receive a pollen transfer, and will therefore not be fertilized. In this case, the shape is the barrier and this can prevent breeding among even closely related species.