C1139Social Insects. The Individual. Lecture 3

The Individual

Aims

1. To examine the role of non-adults and males in hymenopteran societies, the roles of adult females, and stages in the loss of the reproductive ability of workers.

2. To understand how morphologically-distinct queens and workers are reared.

Objectives

1. To learn the basic features above including examples.

Big Picture

Insect societies are functional units composed of distinct individuals, unlike the cells in your body which are fused. The individuals come in various types (e.g., queen and worker), and there are also different types of workers whether due to just behavioural and physiological differences, or morphological differences as well. The types and their roles vary among taxa.

Reproductive division of labour

This is the key defining feature of eusociality. Colonies have individuals that are specialized for either reproduction or working.

Hymenopteran societies

Hymenoptera are holometabolous insects. Eusocial hymenopteran societies are female and adult dominated.

Role of brood

In Hymenoptera, brood (non-adult individuals) have little role other than to grow. Larvae are passive, legless individuals but in a few cases they perform something useful to the colony such as helping in digestion or nutrition (Vespinae wasps) or in producing silk for nest construction (some ants).

In termites the working individuals are non-adults (nymphs). Termites are hemimetabolous insects with egg, nymph, and adult stages.

Role of males

In eusocial Hymenoptera, the father male or males is present only as sperm stored in the queen’s sperm-storage organ, the spermatheca, and never co-founds the nest with his mate. Offspring males are present at certain times of the year. Males arise from unfertilized eggs and are haploid. They seldom perform any work and are generally considered powerless in influencing colony reproduction because they are not involved with brood rearing. In a few cases they may be able to promote their own reproductive interests, such as in honey bees where the drones do not leave the parental nest, with its abundant supply of food, to join a swarm. In Cardiocondyla ants there are two types of males. Normal winged males that mate away from the nest, and worker-like males with large mandibles that mate young sister queens in the nest. (Unusually, in this species there can be mating among siblings inside the nest.) The worker-like males fight and the winged males smell like females to avoid being attacked.

Role of females

Hymenopteran societies are essentially societies of females. In most species workers and queens are morphologically distinct. This arises during development, usually in the larval stage. Because queens are normally larger, the adult workers can control whether a larva can develop into a queen or a worker by controlling the food supply. In this way they can prevent an excess of larvae developing into queens. In the honey bee, Apis mellifera, queens are reared in special queen cells on special food called royal jelly. The queen cell stimulates the workers to provision the larva in it with royal jelly. The royal jelly causes the larva to follow the queen developmental pathway by switching on the appropriate set of genes. In honey bees and almost all eusocial Hymenoptera, any female larva can become either a queen or a worker. Beekeepers can rear additional honey bee queens by transferring one-day-old female larvae from worker cells to queen cells. In Melipona stingless bees, the queen and worker castes are of the same body size, though very different morphologically, and are reared in the same cells. Because of this the adult workers cannot control the caste fate of the female larvae and an excess of larvae develop into queens. They are killed shortly after emerging from their cells.

There are many differences between worker and queen castes in relation to differences in the need to work and communicate. In the honey bee, new colonies are founded by a swarm consisting of a queen and thousands of workers. The queen is never on her own. Honey bee queens have therefore lost the ability to work. They do not have pollen baskets, never forage, never make communication dances or the Nasonov pheromone (an attractant pheromone which serves to help guide bees to the hive entrance) nor the alarm pheromone (which causes other workers to respond defensively to intruders). Conversely, queen bumble bees found the nest on their own and retain pollen baskets and the ability to do all tasks. Most ant and Vespinae wasp queens can also found a nest on their own.

Four stages in the loss of worker reproductive potential in the Hymenoptera

Worker insects are sometimes referred to as neuters or sterile. Although workers in some species are completely sterile because they have vestigial or non-functional ovaries, in most species they are not. In eusocial Hymenoptera there are several stages in the loss of worker reproductive potential (see table below). These stages are often convergently evolved, inasmuch as ants, bees, and wasps each evolved eusociality independently. The same occurs in termites. In higher termites (Termitidae) the workers are a terminal caste of non-adult nymphs. In lower termites workers can metamorphose into reproductives and so are not sterile, even if few of them actually do become reproductives.

queen-worker morphological differences? / caste determination / potential worker reproductive capacity / examples
1. no / behavioural, in adult stage / equal to queen (no loss of reproductive potential) / ants: none
wasps: Stenogastrinae, Polistinae, Microstigmus
bees: Halictidae, most Allodapini
2. yes / developmental, in immature stage / partial loss of reproductive capacity, but retaining ability to mate / ants: some Ponerinae
wasps: some Epiponines (Polistinae)
bees: some Allodapini
3. yes / lower than queen; workers retain ovaries but have lost the ability to mate and can lay only unfert. eggs / ants: most species
wasps: Vespinae
bees: most eusocial Apidae
4. yes / zero; have vestigial ovaries and so cannot reproduce directly / ants: a few genera, e.g. Solenopsis, Pheidole, Monomorium
wasps: none
bees: some Trigona bees (Meliponinae)

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