Biology Reference
In-Depth Information
in the world, which is approximately equivalent to all the known species of birds and
mammals. The staggering natural history of social insects can be illustrated by the
following small sample of facts. In terms of size, a colony of African driver ants (
Dorylus
wilverthi
) may contain up to 22 million individuals weighing a total of 20 kg. In terms
of communication, the honeybee dance language, in which successful foragers tell
other worker bees about the direction and distance to a food source, provide a rare
example of a communication system in a wild animal where an abstract code (the speed
and orientation of the dance) is used to transmit information about remote objects (see
Fig. 14.14). In terms of feeding ecology, the diets of social insects include seeds, animal
prey, fungus grown in special gardens on collections of leaves or caterpillar faeces and
the excreta ('honeydew') of tended herds of aphids. The impact of this feeding can be
great - for example, in some areas of Latin American tropical forest, leaf-cutter ants are
the main herbivores, eating over 5% of the leaves produced each year (Leigh, 1999).
Social insect colonies are often populated by individuals specialized to perform
different tasks (so-called castes). Sometimes castes have bizarre morphological
modifications to help them carry out their jobs (Fig. 13.2). For example, the head of
soldier termites of the species
Nasutitermes exitiosus
is modified into a 'water pistol' used
for squirting defensive sticky droplets at enemies, while the head of soldiers in the ant
species
Cephalotes varians
has a disk on top which fits neatly into the nest entrance to
keep out intruders.
In the following sections, we will first describe the life history of one example of a
eusocial insect to provide a background, then discuss the various genetic and ecological
factors which have been put forward to account for the evolutionary origins of sterile
castes, before going on to also discuss how conflicts are expressed and resolved within
colonies.
Diversity of social
insects
Specialized
worker castes
The life cycle and natural history
of a social insect
Lasius niger
is a species of ant commonly found in Europe in woodlands, farmland and
gardens (Fig. 13.3). It builds a nest, the chambers of which are excavated in the ground
underneath flat stones or in open soil. The nest is started by a single fertile queen. She is
fertilized during a 'nuptial flight' in July or August in which very large numbers of winged
reproductive females and males swarm around in the air and copulate (only the sexual
forms can fly, and then only at this stage in their life cycle). The queen stores the sperm
obtained on this nuptial flight to use throughout her life, which may be many years. After
the nuptial flight, the queen loses her wings and spends her first winter sealed inside the
nest chamber, which she has built by excavating a hole in the ground. During the
following spring, eggs which she has laid develop into larvae and mature into adult
workers (the typical ants one sees scurrying around near ants' nests) before the autumn.
Up to the time of maturity of the first clutch of 5-20 workers, the queen survives on her
own fat reserves and breaks down her flight muscles (which are not needed anymore) to
provide the proteins to produce trophic eggs that the developing larvae eat. When the
workers mature they start to care for their younger siblings and collect food for them and
their mother. The workers are female, but they are sterile. They never develop wings,
In
Lasius niger
a single queen
founds the colony
In the
Hymenoptera
workers
are female
