SOCIAL INSECT REPRODUCTION

Social Insect Reproduction

Reproduction has two distinct phases in eusocial insects. Mating occurs between female offspring that become new queens (these females are called gynes) and males (which in honey bees are called drones). Within a species, colonies typically produce gynes and males in synchrony, so that the reproductives have the opportunity to find mates from other colonies; this prevents inbreeding, which has catastrophic effects in the ants, bees and wasps. For example, in Western harvester ants, Pogonomyrmex occidentalis, the emergence of reproductives is timed to coincide with rains, which are relatively infrequent in arid western habitats. Scientists mimic the effect of rain by pouring water onto the nest; this stimulates a flight of reproductives.

Once the males and females have found one another and mated, the next phase of reproduction is nest establishment. In the eusocial ants, bees and wasps starting a new nest is typically the province of the mated gyne. In termites the male and female work together to establish a nest. In some species a mated gyne is joined by workers--the queen and her workers collectively are called a swarm. In addition to honey bees (see below), army ants, stingless bees, and many social wasps establish new colonies by swarming. The advantages of swarming are obvious; a queen has a much better chance of survival if she has workers who will defend the colony, construct the nest, and collect food. The major disadvantage of swarming is that the large investment of workers in swarms limits the number that can be produced. Swarming is expected in species which have already saturated a habitat and when competition for nesting sites is keen (it is a classic K-selected strategy). In species that occupy relatively open habitats (such as early stages of ecological succession) the best reproductive strategy is likely the production of many gynes, each of which attempts to establish a nest without help from workers.

In swarming species, many more males than gynes are typically produced, leading to intense competition among males for mates. In honey bee species (genus Apis), an evolutionary extreme has been reached, in which the male's genitialia are ripped from his body during mating, remaining in the gyne's genital tract, where they serve as a copulatory plug. This probably originally evolved as a way of preventing gynes from mating with more males, but subsequently gynes have evolved the ability to remove the plug and to mate with many males.

Many eusocial insects, including the honey bee, Apis mellifera, which is illustrated here, reproduce by swarming. Honey bee colonies swarm in the spring, sending the old queen and about half the colony's workers in search of a new nest. The old nest is inherited by one of the queen's daughters. Swarms generally settle on a tree branch near the old nest; scout bees then go out in search of a new nesting site. As many sites may be found, a method of choosing the best one is necessary. Scouts dance on the surface of the swarm, indicating the location of the sites.Over time, a consensus emerges among the bees concerning the best nesting site, as scouts who have found poorer sites stop dancing, while ever-increasing numbers of bees dance for the better sites.

A variation of swarming occurs when several gynes join together to establish a nest. Under this circumstance, an uneasy truce exists among the individuals; each "hopes" to become the primary reproductive. Multiply founded nests are known in social wasps in the genera Polistes and Ropalidia, in carpenter bees, sweat bees, and Australian eusocial bees called allodapines, and in a variety of ants. Reproductive skew theory has been used to analyze the evolutionary costs and benefits for gynes in joining groups (versus trying to establish a nest on their own). Generally speaking, it is not necessary for the gynes to be related in order for them to find a potential evolutionary benefit from belonging to the group, but groups of related gynes have a better chance of surviving and reproducing. This is because the effects of kin selection can augment the benefits of belonging to a group, if the group members are related.

Ants display two rather distinct patterns of nest establishment. In some species, the queen is fully equipped to build the nest and to forage for food while she rears her first group of workers. In other species,which are called claustral ants, the queen does not leave the nest to forage. Instead she uses energy and nutrients from metabolically breaking down her flight muscles to provide food for her worker larvae. Claustral founding protects the queen from the risks of foraging.

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