Chapter 3. Division of labor
3.1. Division of labor in social insects
Division of labor has been thought as a major factor for the
enormous success of social insects. Presumably, division of labor increases
the efficiency of an organization because it allows many different tasks to
be performed simultaneously by many different individuals. The most common form
of division of labor in social insects is reproductive division of labor: the
queen reproduces almost exclusively, while other non-reproducing members specialize
in the day to day operation and maintenance of the colony.
In many insect societies, there is a further division of labor among the non-reproducing members. This can be based on several mechanisms. In many species of termites and ants, some members are morphologically specialized to defend the colony and are called "soldiers," while the others are called workers. Workers in some ant species show large size variations and specialize in different tasks.
In many advanced social insects, division of labor is related to age: workers of different ages specialize in different tasks. This phenomenon is called "age polyethism" or behavioral development.
3.2. Basic description of age polyethism
In honey bees, workers typically perform brood rearing ("nursing") for the first week, engage in other hive maintenance duties (wax secretion, guarding, undertaking, nectar processing) when they are "middle-aged" (2-3 weeks old), and switch to foraging and colony defense when they are about three weeks old.
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Workers emerge as winged adults after 21 days of egg, larval and pupal stages. A newly emerged bee does not do much the first day, she cleans herself a lot, eats a lot of pollen and may clean empty cells to prepare for the queen's egg laying. |
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photo by Williamston |
The next 2-14 days, the bee would have her proper food glands developed and she would actively seek larvae to feed. She mixes the scretion from her mandibular glands (lipids) and hypopharyngeal glands (protein), to form a jelly for workers or queens.
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photo by Williamston |
The jelly for queen larvae or adult queen are called "royal jelly". Royal jelly contains less pollen and a higher ratio of fructose, compared to worker jelly. A queen larva resides in a larger cell that is vertically hang. This picture was shot from below the cell opening. A queen larvae was almost constantly tended by nurses.
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Queen laying (Apis cerana), photo by Zachary Huang |
When the queen comes by, usually looking for empty cells to lay eggs, a circle of bees would form around her, licking and antennating her. This is because the queen has a pheromone blend that is highly attrative to workers. However, foragers in a hive would tend to ignore the queen because they are not as responsive to the same chemical. |
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photo by Williamston |
When you are worker bee, middle age comes fast! Workers would have their wax glands developed around 4-18 days and they add saliva to wax, and using the wax to make new cell walls or caps. It takes about 8 pounds of honey to make 1 pound of wax. New beeswax is white as shown here.
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photo by Williamston |
Nectar foragers do not directly deposite nectar into cells, rather they pass their food to the receivers, who then deposite into cells. In this process, an enzyme is added that will break down sucrose into fructose and glucose. This enzyme is also produced by the hypopharyngeal glands in foragers. Water is also removed by fanning and p1lacing a droplet on tongue to increase the drying surface. Nectar contains 60-90% water, while honey contains only 16-18%, so there is much effort put into removing water.
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Photo by Scott Camazine |
While almost all workers go through nursing and foraging, only some workers (about 5-10%) become guards. Guards sit near the hive entrance and smell other incoming workers to see if they belong to the same family. If not, they are rejected by being bitten or even stung. Guards can tell how is foreign because everyone at home smells alike due to genetics (all are from the same mother) and a homogeneous environment. |
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Photo by Kirk Visscher |
Undertaking is another rare task (even less worker perform this than guarding). Undertakers have a lower threshold to smell released by dead workers and proceed to carry the dead bee outside the hive (usually dropping it about 2 to 30 meters away). Because she has to take off with something her own body weight, her take-off mimics that of an airplane -- she can only increase altitude slowly, rather than vertically. One can design "dead-bee" traps to frustrate the undertakers and keep-track of the number of dead bees. |
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Photo by Scott Camazine |
Quite recently (1994), another group of workers were identified as "soldiers", they are similarly aged as foragers but have not started foraging yet. They specialize in defending the colony against large mamals such as bears and humans. I think they eventually become foragers although there is no data yet. |
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Apis dorsata foraging, Photo by Zachary Huang |
Workers become foragers around 21 days old in Apis mellifera. Foragers bringing pollen (as shown here) as protein source, nectar as energy source, water to dillute honey or to cool the hive (when it is >35 degrees° C), and propolis to seal cracks or to embalm a dead animal that is too big to be removed from the colony (mice, for example). |
The following slide provides a summary of activities shown above.
Question for discussion: is it more adaptive to let older workers forage? why or why not?
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