Many night-pollinated flowers close during the day, to prevent thieves from getting at their nectar and pollen. Many daytime-pollinated flowers close at night for the same reason. Flowers pollinated at night are usually white or pale yellow and very fragrant. This helps to announce the flowers' presence. Flowers also assist the pollinator in finding where the pollen or nectar is stored.
Shapes are specific to pollinators and, in some cases, also keep out unwanted pollen collectors.
Some insects, like ants, love nectar, but are not good pollinators. Plants have developed protective devices, like closed throats or sticky hairs, that are designed to keep ants and other robbers out. Other plants exude a sweet liquid on their stem to sidetrack unwanted visitors. Hiding nectar in tubes and closed blossoms has many advantages. If the nectar is hard to get at, usually only designated pollinators can find it.