Eusocial Insects

Honey bees (Apis mellifera) are all part of what is termed a superorganism. Superorganisms are defined as a social unit of Eusocial animals, specialized labor is divided up, and the individuals can not exist outside of the group on their own for any extended period of time.

The term Eusocial refers to animals that exhibit the following qualities:

  1. division of labor, with a caste system involving sterile or non-reproductive individuals assisting those that reproduce
  2. co-operation among colony members in tending to the young
  3. overlap of generations capable of contributing to colony function

Examples of these type of groups are found typically in insects such as Ants, Bees, Wasps, and Termites. There is one example of mammals being Eusocial, and that is the Naked  Mole-Rat (Heterocephalus glaber) found in East Africa.

The division of labor involves a caste system ( Lesson 5 – The Honey Bee Caste System ). This caste system is comprised of a restricted reproductive unit (Queen), non-reproductive members that assist reproducers (Workers), and either Soldiers (as in the case of termites and ants) or Drones.

In haplodiploid genetic systems, the queens control the sex of their offspring. They either release stored sperm fertilizes haploid eggs which develop into diploid female offspring. Unfertilized eggs produce male offspring.

Members within different castes often differ behaviorally. In the case of Honey bees, individuals perform different tasks at different stages in life. This is termed polyethism.

Female castes are dimorphic which means they are different in appearance. Generally, the Queen is larger than the workers as in the example of Yellow Jackets (Vespula alascensis ) below:

vespulacastes

 

Caste differentiation is trophogenic. This means it is determined by the quantity and quality of larval food. In the case of Honey Bees, Queens are fed an exclusive diet of Royal Jelly which is secreted from the hypopharynx of the nurse bees. Worker bees are fed Royal Jelly for the first 3 days and then a mix of Royal Jelly, pollen, and honey called Worker Jelly.

Reproduction in the colony is limited to the queen due to the QMF (Queen Mandibular Pheromone). One function  (or inclusive chemical) inhibits ovary development in the all female worker caste.

Nest construction in Eusocial insects varies by species. In the example of the Common Wasp (Vespula vulgaris), nest selection and construction begins every spring with an overwintered Queen.

In the Autumn, the Queens mate, and then select a resting site to “hibernate” until Spring. During her resting period, even though she has mated, she goes through a diapause. Her ovarioles  remain undeveloped. When Spring arrives, she leaves her temporary resting site in search of food and a proper nesting site. She feeds on Sap and Nectar, and this feeding initiates the development of her ovarioles.

Once she locates a suitable nesting site, she begins construction. The queen only forages for construction material to begin the nest. Once the first workers emerge, the colony begins to expand.

Wasps in nest cavities undergo thermoregulation. Heating and cooling is regulated by a combination wing beating (fanning) and evaporation applied to the pupal cells.

As the end of Autumn approaches, the males and future potential queens (gynes) are produced. The old queen fails and dies, and the males and gynes emerge, mate away from the hive, and the Queen seeks a place for hibernation.

There are a few similarities and a few differences for Honey Bees. Once the current hive becomes too crowded in the Spring, the bees decide to split off the colony. This is called swarming (See Colony Swarms for more detail). The founder queen and a large percentage of the current hive seek a new nest site. Scouts are sent out in search of a new cavity. Once a new cavity is selected, construction begins. Unlike wasps who forage for construction materials, Honey bees use wax as construction material. The wax is produced by the workers wax_glandsfrom their abdominal wax glands. This wax is mixed with saliva to help with manipulation, and they begin construction of hexagonal cells. Another difference is cell orientation. Wasps hang the cells downward, whereas honey bees build vertical comb from the wax, and build the hexagonal cells in a slightly upward 13º angle. This helps keep the honey in the cells.

The nursery is in the lower section of the nest cavity, as well as storage for pollen. Pollen is not as easily moved from place to place so it is typically stored near the brood for ease of access. Nectar or Honey is stored above the nursery and around the peripheral of the larvae.

Bees also thermoregulate the hive. They keep the brood around 95°F (35°C). Similarly to wasps, they use evaporative cooling to assist in cooling the temperature. To lower temperature, a certain number of bees may go to the outside of the hive. This is a condition called Bearding.

The bees continue to build up populations, store nectar and pollen, and get ready for winter. As winter closes in, and the temperature drops, the bees cluster together. Egg laying declines (or in many cases ceases). Winter bees have a different composition (see The Winter Cluster) then do Summer bees. Summer bees live a total of 4-5 weeks, whereas Winter bees may live up to 5 months.

When Spring begins to roll around, egg laying begins again, and once the population gets high enough, that colony will swarm to repeat the cycle above.

Eusocial societies have to be altruistic in nature to survive. This altruism is more genetic than behavioral. Since most Eusocial groups are haplodipoid, as sister’s, they share 75% of the same genetic material, whereas,  humans are diploid, and sister’s will only share 50% of the same genetic material (I will write a future blog to explain the math). This sharing of a high percentage of genetic material is likely to be the factor explain this unique trait.

 

References:

  1. https://en.wikipedia.org/wiki/Superorganism
  2. http://es.rice.edu/projects/Bios321/eusocial.insect.html
  3. https://www.nature.com/scitable/knowledge/library/an-introduction-to-eusociality-15788128
  4. https://en.wikipedia.org/wiki/Ploidy
  5. http://faculty.uca.edu/benw/biol4415/presentations/lect7b.pdf
  6. https://youtu.be/xD62suykD6o

2 thoughts on “Eusocial Insects

  1. Pingback: Honey Bee Genetics

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