Social Insects

DEFINITIONS

Caste	-	A specialized segment of the population of social insects, castes have different functions within the society and sometimes different morphologies. Castes have distinct divisions of labor
eusocial	-	Social systems characterized by parental care of young, overlap of generations, and reproductive division of labor. True sociality.
Homeostasis	-	The maintenance of a functional steady state in an organism or superorganism
Polyethism	-	Behavioral differences among castes
Polymorphism	-	Caste members are radically different in appearance, us. results from environmental (food) differences
Social Insects	-	Insects that live cooperatively in colonies and exhibit a division of labor among distinct castes. ex. termites, ants, bees, some wasps.
Superorganism	-	A social insect colony described as a multicellular animal, individual members of the colony are similar to individual cells in an animal

1) Parental care of young (young couldn't survive without parental care)

2) Overlap of generations (essential for 1)

3) Reproductive division of labor, i.e.. there are egg-laying females and other females, may be other castes

example: Honey Bees

• Reproductive Castes - queen and drone
  • Queen - produces eggs to maintain the colony.
  • Drones - mate with new queens.
• Worker Caste - sisters, all daughters of the queen
  • Care for the eggs, larvae, queen and drones.
  • Maintain and defend the hive, and forage for food.

Three Castes of Honey Bees

Sociality

Most insects are not social, some aggregate or contact other members of their species for short periods to mate or for other functions. Some even dispense with mating and reproduce asexually.

Only a few groups are truly social.

All termites (Isoptera), some Hymenoptera (all ants, honey bees, stingless bees, bumble bees, and some members of other bee groups, and at least one wasp sp.).

True social insects, esp. the ants and termites, are dominant ecological groups.

ADVANTAGES & DISADVANTAGES OF SOCIALITY

	Solitary Insects	Social Insects
Advantages	Hide from predators	Colony productivity increased
	No competition with others of your species	Group defense and alarm
	Live in small spaces	Food gathering
	Exploit small food resources	Nest building
		Care of young
Disadvantages	Lack of social benefits	Intense predation, parasitism, disease

For societies to persist, they must survive and reproduce more successfully than solitary individuals.

CASTES

• Reproductives - queen and drones
• Workers
• Soldiers
  • May be distinct morphological types, esp. in ants.
  • Lacking in wasps and bees.

Three Castes of Termites

Comparison of Isoptera and Hymenoptera Caste Systems

	Workers and Soldiers
	Sex	Stage	Male Reproductives (Drones, Kings)
Termites	M/F	Immature or adult	Permanent attendant of the queen
Hymenoptera	F	Adults	Die after mating

 
Unlike bees, in termites the male is not a drone. Workers may be males and the king has functions other than mating. The queen may live for more than 10 years. But if she is killed or her egg production declines, secondary queens replace her.

Caste members may be radically different in appearance from one another or polymorphic and castes may have subcastes that differ in appearance and function. This usually results from environmental (food) differences not genetic differences. Behavioral differences among castes are called polyethism.

WHO RULES THE HIVE?

The Queen Bee

•  Lays all the eggs and regulates sex of offspring (parthenogenesis).
  • Unfertilized eggs -> males
  • Fertilized eggs -> females
• All members of the hive are the queen's progeny.
• The queen's pheromones identify hive members.

The Worker Bees

•  Workers determine type of egg laid by queen.
  • Large cells receive unfertilized eggs that develop into males -- males haploid.
  • Smaller cells receive fertilized eggs that develop into females -- females diploid.
• Workers determine whether a female egg develops into a reproductive or worker.
  • Workers receive royal jelly only their first three days.
  • Queens receive royal jelly throughout the larval stage.

DOES TRUE SOCIALITY EXIST IN MAMMALS?

• Wolves, killer whales, lions, man
• SW African naked mole rats

SUPERORGANISMS

Steps in evolution of animal size and complexity

•  Single celled animals
• Colonial animals
  • Clusters of cells capable of independent life
• Multicellular animals
  • Groups of interdependent cells
    • Cellular division of labor
    • Cells not capable of independent life
    • Cellular communication and coordination essential
    • Most cells no longer reproducing new individuals
  • Superorganisms
    • Groups of interdependent, individual insects
      • Individual division of labor - castes
      • Individuals not capable of independent life
      • Colony individual to individual communication essential
      • Most individuals are not reproductive.

Peculiarities of Insect Societies

SLAVERY

A biological, not a cultural trait, that is wide-spread among ants. Most ant battles you see are actually slave raids. Ant slavery is unique because ant slavery is usually between species, unlike human slavery.

Slave making ants

• Capture larvae and pupae of another species.
• Carry them back to there own nest where:
  • They acquire the nest odor.
  • Develop into adults and act as workers for their new colony.

Some slave making ant species are incapable of surviving without slave workers. They are no longer able to collect food or feed their immatures or themselves.

WARFARE

Embodies restless aggression, territorial conquest, and genocidal annihilation of neighboring colonies. Ants war with their own and other species and use a variety of tactics.

Imported Fire Ant, Solenopsis invicta vs. the Woodland Ant, Pheidole dentata

The fire ants have colonies hundred times larger than the woodland ant and whenever they discover a woodland ant colony they completely destroy it. Yet woodland ant colonies are abundant around fire ants. Whenever, a woodland worker discovers a fire ant scout soldiers are so rapidly deployed that the scout rarely makes it back to its colony. The soldiers do not sting or spray poisons like many ants but rely on large mandibles to cut their opponents into pieces. If despite this the woodland nest is discovered the soldiers fall back to form a short perimeter around the nest which keeps the invading fire ants at bay temporarily. The colony evacuates the nest and after the battle and the fire ants have departed, they will return and reclaim their nest.

FARMING

Many ants keep insect livestock in the order Homoptera. Commonly seen in our area are ants tending aphids. The ants herd the aphids and protect them from predators and parasites, in turn, the aphids reward the ants by providing with droplets of sweet and nourishing honeydew. Besides aphids, scale insects, other Homoptera, are farmed and some insects in other orders.

Other ants and some termites are gardeners. They collect plant material, bring it into their nests, compost it, and use it to grow fungus which they feed on. Leaf cutter and parasol ants are examples.

AIR CONDITIONING

Some social insects are able to maintain steady state conditions in their colonies or nests, e.g. in temperature and humidity. This is called homeostasis and is essential for colony health.

Examples: Honey bees

• Ventilate their hives - if too hot, wax melts.
• Cluster to stay warm in the winter - if too cold, individuals die.

Termites

•  Soft bodied, very susceptible to desiccation.
• "Air conditioned" termite mounds - vent heat and retain humidity.

Links

Tree of Life: Hymenoptera ---- Isoptera (Termites)

Honey Bees

• Africanized Killer Bees, Jan. 3, 2001
• International Bee Research Association , Jan. 3, 2001
• Beekeeping: The Beekeepers Home Page, Jan. 3, 2001
• Iowa State Entomology Index: Beekeeping, Jan. 3, 2001

Sociobilogy - the influence of genetics on human and animal behavior. Developed from the study of insect societies.

• Sociobiology, Jan. 3, 2001
• Journal of Behavioral Ecology and Sociobiology, Jan. 3, 2001

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Last updated Jan. 2, 2000
Gary Brewer