C1139 Social 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 spermstorage 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
1
C1139 Social Insects. The Individual. Lecture 3
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?
1. no
2. yes
3. yes
4. yes
caste
determination
potential worker reproductive
capacity
behavioural, in
adult stage
equal to queen (no loss of
reproductive potential)
developmental,
in immature
stage
partial loss of reproductive
capacity, but retaining ability to
mate
lower than queen; workers
retain ovaries but have lost the
ability to mate and can lay only
unfert. eggs
zero; have vestigial ovaries and
so cannot reproduce directly
2
examples
ants: none
wasps: Stenogastrinae, Polistinae,
Microstigmus
bees: Halictidae, most Allodapini
ants: some Ponerinae
wasps: some Epiponines (Polistinae)
bees: some Allodapini
ants: most species
wasps: Vespinae
bees: most eusocial Apidae
ants: a few genera, e.g. Solenopsis,
Pheidole, Monomorium
wasps: none
bees: some Trigona bees (Meliponinae)