Monk'shood Aconitum napellus is an uncommon British native wild flower whose natural distribution is confined to south-east England, but it makes an excellent plant for the herbaceous border anywhere - provided you bear in mind that it is very poisonous. You can read about the horrific symptoms of monk'shood poisoning, as described by by herbalist John Gerard in 1597, by clicking here.
The plant is an absolute magnet for bumblebees and depends on them for pollination, and the strange hooded flowers have evolved to give them exclusive access to the nectar supply.
The nectaries are located up inside that hood petal in the flower, so when the bee arrives ...
...... it has to crawl into the narrow entrance and then climb high up inside the flower to reach the nectaries, becoming dusted with pollen from the stamens clustered near the entrance. When the bees are visiting the flower they disappear entirely inside, then have to back out when they leave - there's no room inside to turn around.
When they leave the flower their tongues are still extended from straining to reach the nectar deep in the flower .....
.... and it's only long-tongued bees that can exploit it. You can see the bee's tongue here, still extended and curved under the insect's thorax.
Bumblebees don't seem to be able to tell whether the flowers have already been visited and drained of their nectar, because when I was watching this afternoon I saw the same flowers visited on numerous occasions in quick succession......... something that's obviously very beneficial to the plant but involves the bees in a lot of effort and wear and tear during fruitless visits.