How does flower-tube length affect pollination in honeysuckle species?

In the genus Lonicera (honeysuckle), the length of the flower tube acts as a selective mechanism that determines which pollinators can successfully access the nectar.1

Many honeysuckle species possess long, tubular flowers that restrict access to their nectar. Because the nectar is located deep within these tubes, it is inaccessible to most insects. Consequently, these species rely on specialized pollinators with long mouthparts, such as hawk moths, which are capable of reaching the nectar while hovering. For example, the perfoliate honeysuckle (Lonicera caprifolium) features long flower tubes that specifically facilitate pollination by night-feeding hawk moths, as other insects are physically unable to reach the nectar source.12

This relationship illustrates how floral morphology, specifically tube length, serves as an evolutionary adaptation to ensure effective pollination by specific visitors.21