Insects that pollinate these flowers do not escape from the sac until they pass under the receptive stigma and then out through one of two rear apertures. This escape pathway forces the insect to contact one of two dehiscent anthers fixing either sticky, amorphous lumps of pollen or whole pollinia to the dorsum of the insect's thorax or head. With a few important exceptions ,bees of various sizes dominate the pollination of Cypripedium spp.