How do yellow-faced bees belonging to the genus Hylaeus store pollen for transport?

In their social stomachs, also known as crops

The defining physiological characteristic differentiating yellow-faced bees, members of the genus *Hylaeus*, from many other familiar bee species is their method of pollen carriage. Instead of possessing external structures specifically evolved for holding pollen loads, such as the corbiculae (pollen baskets) found on the hind legs of honey bees or bumblebees, these small, dark bees utilize an internal storage system. They collect the pollen grains and pack them into their social stomachs, scientifically referred to as crops. This stored pollen is then transported internally back to the nest site, a key trait used for identification in the field, especially when contrasted with bees that display conspicuous external pollen masses.