A fascinating aspect of honeybee colonies is that they are eusocial. Eusociality is characterized by three distinct traits. The first is that eusocial species have cooperative brood care in which individuals care for offspring that are not their own. The second component of the eusocial society is an overlap of generations as the offspring assist the reproductives with colony tasks. The final trait of the eusocial society, is that it has a caste system, with most of the colony represented by individuals that do not reproduce and only one or a few reproductive individuals. Eusociality has evolved independently in several orders of insects. In addition, to the Hymenoptera including all termites, some species of thrips and aphids, and even with beetle species. Within the Hymenoptera, eusociality has evolved multiple times. All ants are eusocial along with some bees and wasps. The altruistic behavior that drives the evolution of eusocial societies, such as cooperative brood care, is more likely to arise if individuals share similar genes and are closely related. In Hymenoptera such close kinship maybe the result of the genetic phenomenon known as haplodiploidy. Haplodiploidy is the sex determination mechanism found in almost all Hymenoptera. In the Hymenoptera and certain other animals, males normally developed from unfertilized eggs, and are haploid, and have half the full complement of chromosomes. While females, developed from fertilized eggs, and are deployed. The diploid females, receive one of two possible sets of chromosomes from the mother. In addition, to the single set of chromosomes they all received from the father. If a queen bee mates with a single male, this results in sisters sharing 75 percent of their genetic code with each other. This is important when considered in the context of the genetic relationships between colony members. Full sisters are more closely related to each other than they would be to their own offspring who would only share 50 percent of their genome with them. This means that more copies of a daughters genes will be passed on to the next generation if she helps her mother produce more sisters and not create offspring of her own. As such haplodiploidy can help tip the evolutionary balance in favor of altruistic behavior among family members, which can lead to a eusocial lifestyle. Keep in mind that most Hymenoptera and queens actually mate with more than one male. This can change the genetic relationships within the colony and makes kinship calculations much more complex. Nonetheless, the underlying concept remains the same. Like other eusocial Hymenoptera, a colony of honeybees consists of two female casts that are specialized for different tasks, workers, and the reproductive queen. Male drones are also present. Workers are the most numerous members of a honeybee colony. All worker bees are non-reproductive females that perform duties such as foraging, brood care, and hive defense. Workers have small ovaries that they lack the necessary structures to mate or store sperm. This means, any eggs laid by workers will be unfertilized and we'll inevitably develop into males. In a healthy colony, this does not typically occur. A young newly emerged adult worker bee will start her life as a nurse bee, which can include a variety of colony tasks, including cleaning the hive, feeding larvae, attending to the queen, and building honeycomb. Some workers will then become guards to monitor and defend the hive entrance. When the workers are about 21 days old, they will begin to leave the hive and take on the important foraging duties. Foraging workers, use the position of the sun and visual landmarks to navigate towards food sources, which can be many kilometers from the hive. A foraging bee, can visit up to 2,000 flowers in a single day. Although, she will only produce one-twelfth of a teaspoon of honey in her lifetime. In order to care for the queen and her offspring, referred to as her brood, workers have specialized glands in their heads known as mandibular glands that convert pollen and nectar into nutritious food for the larvae. This larval food is called either worker jelly, which is made from fermented pollen, called bee bread and honey, or royal jelly, which has a nutritious fluid produced exclusively in the mandibular glands of workers. All larvae received royal jelly for the first three days after hatching. After which, larvae intended to become workers are fed worker jelly with submandibular secretions mixed in, and larvae intended to become queens are fed royal jelly. In addition to the mandibular glands, worker bees also have a variety of other important glands that serve many functions, which we will introduce later on. Workers guard the entrance of the hive from invading organisms such as predators, like bears, or honey badgers. In order to protect the valuable honey and the rest of the colony, workers can cooperate to sting attackers on mass. In times of resource scarcity, guard bees will defend their hives from foreign invader bees that may rate a nest and rob it's honey stores. These raids can involve violent encounters between many foreign and local hive members, and may result in thousands of dead bees. However, during times of resource abundance, guard bees may actually allow foreign workers from other colonies to enter the hive. As the foreign workers are likely drifters that do not pose a threat, and may actually come bearing gifts of pollen and nectar. Eventually, these foreign workers may be adopted into the colony. The queen is the only sexually reproductive female in the hive. Although under certain conditions, it's possible to find two queens in a colony. All other hive members are the queen's offspring and are sterile workers. During late summer, a newly emerged queen will take multiple mating flights from her hive called nuptial flights. For bees like some birds, mating occurs in midair. Queens take around 15 nuptial flights and will normally receive sperm from multiple males. There's not only maximizes the chances of her mating with drones from other hives, but also allows her to obtain enough sperm to maintain a colony. The sperm is stored in her spermatheca and will fertilize her eggs for three to four years, the average lifespan of a queen. The queen's enormous ovaries can produce about 1500 eggs every day during the summer, which translates to more than a million eggs in her lifetime. Queen bees are morphologically different from other bees. We've mentioned some of these unique features already such as larger abdomens, fully developed ovaries, and a functional spermatheca. Additionally, queen bees do not forage for pollen and nectar. So they have a short proboscis, no pollen baskets, and many of their glands are different compared to the worker bees and produce compounds important to maintaining the function of the colony. Unlike workers, queens can sting multiple times because the barbs on their stingers or very small. Worker bees are able to sting other insects multiple times. But the elastic skin of vertebrates catches the barbs on a worker stinger and tears it off when the bee tries to fly away. The queen's stinger is primarily used against rival queens that have been raised within the colony. It contains 2-3 times the venom of a worker stinger. Because a queen may need to sting many rivals in a short period of time. The last inhabitants of a hive are the male honeybees, which hatch from unfertilized eggs and are known as drones. A drone has one and only one objective in life, to mate. Males have several important adaptations to help them find and secure a mate. Unlike queen or worker bees, drones have large eyes for spotting a queen, along with sensitive antennae that have olfactory receptors tuned to the queen's pheromone. Male drones form large congregations in the air that virgin queens will come to for mating. Males are attracted to the queen and will compete with each other to be the first to reach the queen and mate in flight. Males have large broad wings and large flight muscles, and the strongest and fastest will usually reached the female first. Being the first to mate with a queen, will give that male sperm a competitive advantage over that of subsequent mates. Unfortunately for them, drones die after mating. This is because the phallus lodges breaks off within the female's reproductive tract. This pulls out the reproductive organs from the male after mating, leaving a large hole in his abdomen. The phallus functions as a temporary plug to keep the sperm inside the queen's reproductive tract. Other drones will later remove the phallus to facilitate another mating encounter. Male bees do not forage or have stingers. While the sex of a bee is normally determined by the number of sets of chromosomes it has, caste differentiation within the colony is driven by the nutritional components of food given to developing larvae. Larval queen cells are always larger and shaped differently compared to worker cells. Interestingly, there is no difference between the eggs laid in these cells. It is only once the larvae begin feeding that caste differentiation is initiated. The caste of a developing honeybee is typically determined by the fourth day of larval development. During the first three days, larvae have the potential to become either a worker or a queen, and they are all fed the same royal jelly produced by the mandibular glands of nurse bees. Larvae that are destined to become queens are housed in special queen's cells, and after day 3, are continually fed royal jelly. Well, after day 3, larvae that will become workers are fed bee bread and honey called worker jelly, mixed with some mandibular secretions. The nutritional profiles of royal jelly and worker jelly are different, and there are many different proteins and chemicals in these foods that cause the differentiation into a queen or a sterile worker. The queen larvae are also fed a larger quantity of food relative to the other larvae. Nurse bees visit queen's cells nearly 10 times more often than worker cells. A complex interaction between the food content and the developing larva occurs to determine the caste of the larva. There are compounds in royal jelly that seem to promote differentiation into a queen, and also plant compounds, found in the pollen and honey, fed only to worker larvae that contribute to their differentiation into a worker. Scientists are still working to understand these complexities. What is understood is that this difference in food provided to queen and worker larvae leads to a higher level of juvenile hormone production in developing queens compared to worker larvae, which is associated with the reproductive development of the queen's ovaries. In worker larvae, ovarian development is not activated, and the ovaries are instead degenerated due to a lower level of juvenile hormone. At the end of larval development, the quantity of juvenile hormone in both the queen and worker larvae will fall to the same level in preparation for metamorphosis. In this lesson, we have discussed how honeybees have complex use social societies with a caste system that determines the division of labor within a hive. While sex is determined based on the number of sets of chromosomes in an individual, caste determination among females is established by the type of food fed to the larvae, which influences endocrine control of caste determination. This basic knowledge of honeybee society provides a background for the next lesson, when we will discuss some interesting adaptations that have evolved in honey bee biology and behavior.
