Besides having two physical castes, honey bees also have several temporal castes.
Physical castes are based on morphological differences—in other words, the structure of individuals in each group is different. For example, you only need to glance at a queen and a worker to see they are dissimilar.
Temporal castes, on the other hand, are based on temporary periods of highly specialized work. The work is so specialized, in fact, that it requires changes in the physiology of the individual. This phenomenon is called temporal polyethism.
For example, nurse bees and comb-building bees are both workers. Structurally, they look the same. But nurses have highly developed hypopharyngeal glands that allow them to feed the developing larvae. Comb building bees have shrunken hypopharyngeal glands and lose the ability to provide brood food but produce large amounts of wax instead.
The process is not well understood. On the surface it appears that a worker bee goes from one job to another in a more or less orderly fashion, with each temporal caste lasting a week or so. But in times of stress, bees will revert to previous jobs if necessary for colony survival.
Some researchers say that if a bee can go “willy nilly” from one job to another, there is no temporal caste system. Others point out that stimuli from inside the hive are causing the changes to occur—not individuals—and that the type of physiological changes that occur define temporal polyethism.
To make it all the more complicated, spring and summer bees have a very distinct division of labor, whereas winter bees tend to be generalists. We know that the physiology of winter bees is different from spring and summer bees, so perhaps temporal polyethism does not occur in winter bees at all. The debate is far from over.
Honey Bee Suite