|Henbit, long tubular corolla|
The henbit has a long tubular corolla. Thus, the length from petal to stamen limits the number of insects which can pollinate the flower.
Charles Darwin puzzled over this phenomenon and concluded that the evolution of flowers with deep tubular corollas was a "race" with insects who evolved long noses and tongues to reach the nectar. The longer nosed/tongued insects avoided flowers with short corollas and so put these flowers at a disadvantage. But what is in it for the flower? First, the longer tubed flowers are at an advantage with specialized insects. Second, the tubes help to protect the stamens from the elements and so aid in propagation of the plant seeds.
The head of the flower has two lips. The upper lip is covered in fine cilia to catch the pollen from pollinating insects. Notice the dark purple lines on the interior of the lips which act as landing guides for insects.
The next question is what insect pollinates the henbit? Don't know. Fortunately for the henbit, it can be self-pollinating. Maybe, the race is over.