CH5. The Social Insects PDF

Dr. Jazi Alenezi ▪ The social insects challenge the mind by the sheer magnitude of their numbers and variety. ▪ There are more species of ants in a square kilometer of Brazilian forest than all the species of primates in the world, more workers in a single colony of driver ants than all the...

Dr. Jazi Alenezi ▪ The social insects challenge the mind by the sheer magnitude of their numbers and variety. ▪ There are more species of ants in a square kilometer of Brazilian forest than all the species of primates in the world, more workers in a single colony of driver ants than all the lions and elephants in Africa. ▪ The "truly" social insects, or eusocial insects include all of the ants, termites, and the more highly organized bees and wasps. ▪ These insects can be distinguished as a group by their common possession of three traits : ▪ (1) Individuals of the same species cooperate in caring for the young. ▪ (2) There is a reproductive division of labor, with more or less sterile individuals working on behalf of fecund nestmates. ▪ (3 ) There is an overlap of at least two generations in life stages capable of contributing to colony labor, so that offspring assist parents during some period of their life. These are the three qualities by which the majority of entomologists intuitively define eusociality. ▪ Presocial refers to the expression of any degree of social behavior beyond sexual behavior yet short of eusociality. ▪ Within this broad category there can be recognized a series of lower social stages, which are defined in matrix form in Table 19-1. ▪ In the parasocial sequence, adults belonging to the same generation assist one another to varying degrees. ▪ At the lowest level, they may be merely communal, which means that they cooperate in constructing a nest but rear their brood separately. ▪ At the next level of involvement, quasisociality, the brood are attended cooperatively, but each female still lays eggs at some time of her life. ▪ In the semisocial state, quasi social cooperation is enhanced by the addition of a true worker caste; in other words, some members of the colony never attempt to reproduce. ▪ Finally, when semi social colonies persist long enough for members of two or more generations to overlap and to cooperate, the list of three basic qualities is complete, and we refer to the species (or the colony) as being eusocial. ▪ Precisely this sequence has been envisioned by Michener (1969) and his co-worker's as one possible evolutionary pathway taken by bees. ▪ The alternate sequence is comprised of the subsocial states. ▪ In this case there is an increasingly close association between the mother and her offspring. ▪ At the most primitive level, the female provides direct care for a time but departs before the young becomes adults. ▪ It is possible then for the care to be extended to the point where the mother is still present when her offspring mature, and they might next assist her in the rearing of additional brood. ▪ It remains only for some of the group to serve as permanent workers, and the last of the three qualities of eusociality has been attained. ▪ The subsocial route is the one believed by Wheeler (1923) and most subsequent investigators to have been followed by ants, termites, social wasps, and at least a few groups of the social bees. ▪ Once a species has crossed the threshold of eusociality, there are two complementary means by which it can advance in colonial organization : 1. Through the increase in numbers and degree of specialization of the worker castes. 2. Through the enlargement of the communication code by which the colony members coordinate their activities. ▪ Social insects are best exemplified by all termites (Isoptera; sometimes Blattodea) and ants (Formicidae) and by various bees and wasps (Hymenoptera). ▪ Social insects are differentiated in structure, function, and behaviour into castes, the major ones: ▪ The reproductives (e.g., the queen): Besides carrying out the basic function of reproduction, the members of the reproductive caste generally select the site for a new colony and excavate the first galleries ▪ The steriles (workers and soldiers): The workers care for the eggs and larvae, collect food for other members of the colony, and construct and repair the nest, while the soldiers defend the colony against predators. ▪ Wasp, any member of a group of insects in the order Hymenoptera, suborder Apocrita, some of which are stinging. ▪ Wasps are distinguished from the ants and bees of Apocrita by various behavioral and physical characteristics, particularly their possession of a slender, smooth body and legs with relatively few hairs. ▪ Wasps also generally are predatory or parasitic and have stingers with few barbs that can be removed easily from their victims. ▪ Similar to other members of Apocrita, wasps have a narrow petiole, or “waist,” which attaches the abdomen to the thorax. ▪ Wasps are subdivided into two groups: ▪ Solitary wasps: live alone. ▪ Social wasps: live in colonies. ▪ Of the tens of thousands of species of wasps that have been described, the vast majority are solitary in habit. ▪ The social wasps are confined to about 1,000 species within the family Vespidae (superfamily Vespoidea) and include the hornets and yellow jackets (yellowjackets). ▪ They differ from other wasp families in having their wings folded longitudinally when at rest. ▪ The social wasps within the family Vespidae are among the best-known species of wasps. ▪ In their societies they have a caste system consisting of one or several queens, a few drones (males), and sterile females called workers. ▪ The queen, a fertilized female, begins the colony in the spring by building a small nest and laying eggs that hatch into workers. ▪ The latter enlarge the paper-like nest, which is composed of chewed dry plant material, usually wood, that has been mixed with saliva and regurgitated. ▪ Depending on the species, the nest may be found in cavities in the soil, in tree trunks, or hanging from leaves, branches, or the eaves of buildings. ▪ Among the more primitively eusocial vespid wasps are the paper wasps of the genus Polistes. ▪ This species has an annual life cycle, with each colony lasting only for a single warm season. ▪ In the colder parts of the United States, the only individuals to overwinter are the queens: after being inseminated by the short-lived males in late summer and fall, they take refuge in protected places such as the spaces between the inner and outer walls of houses and beneath the loose bark of trees. ▪ In the spring the ovaries begin to develop several weeks before nest initiation, and during this time queens often aggregate in sunny places. ▪ Then, presumably when their ovaries reach an advanced stage of development, the queens begin to sit alone on old nests and future nest sites, where they react aggressively to other females who come close. SOCIAL INSECTS: ANTS ▪ Ants are in every sense of the word the dominant social insects. ▪ They are geographically the most widely distributed of the major eusocial groups, ranging over virtually all the land outside the polar regions. ▪ They are also numerically the most abundant. ▪ The reason for the success o f these insects: 1. The innovation, as far back as the mid –Cretaceous period 100 million years ago, of a wingless worker caste able to forage deeply into soil and plant crevices. 2. Primitive ants began as predators on other arthropods and were not bound to a cellulose diet and to the restricted nesting sites that place colonies within reach of sources of cellulose. 3. The ability of all of the primitive species and most of their descendants to nest in the soil and leaf mold, a location that gave them an initial advantage in the exploitation of these most energy-rich terrestrial microhabitats. ▪ This behavioral adaptation was made possible in tum by the origin of the metapleural gland, the acid secretion of which inhibits growth of microorganisms. ▪ It may be significant that the metapleural gland (or its vestige) is the one diagnostic anatomical trait that distinguishes all ants from the remainder of the Hymenoptera. ▪ Ants are social insects which form small to large colonies. ▪ A typical colony contains an egg-laying queen and many adult workers together with their brood (eggs, larvae and pupae). ▪ Workers are by far the most numerous individuals in the nest. ▪ They are responsible for nest construction and maintenance, foraging, tending the brood and queen, and nest defence. ▪ All workers are female which are sterile and do not lay eggs. ▪ Winged queens and males are present in the nest for only a short period. ▪ After emerging they leave the nest to mate and establish new nests. ▪ Queens are generally similar to the workers, differing primarily in having larger bodies. ▪ In some species, fully winged queens are lacking and egg-laying is undertaken either by typical workers or by individuals which are morphologically intermediate between typical queens and workers (these are called ergatoid queens). ▪ Males are generally about the same size as the workers or smaller, and have smaller heads with large ocelli, very short scapes and small mandibles. In many cases males look more like wasps than ants. ▪ Workers in a single nest can all be the same size or they can vary greatly in size. ▪ When all are of the same or a similar size they are said to be monomorphic. ▪ In some cases the variation in size can be so extreme that large workers are twice the size of small workers. ▪ If variation between small and large workers is continuous, the workers are said to be polymorphic. ▪ If there are only two distinct size classes of workers, they are called dimorphic. ▪ The workers are the most commonly seen caste, especially as they forage on the surface of the ground or when they are disturbed under rocks or other objects. ▪ Ants show a strong division of labour, where different workers perform different tasks within the nest, and in some cases the specific tasks undertaken will depend on the age of the ant. ▪ It is common for young, newly emerged workers to remain in the nest and tend eggs, larvae and pupae. ▪ As the workers age, they may shift their activities away from tending brood and begin to undertake nest construction and excavation. ▪ Finally, later in life they may become foragers, leaving the nest to search for food. ▪ Some workers may perform the same activities throughout their lives, or in other cases, all workers may undertake all activities of the colony, performing any given activity for a few days before switching to another. ▪ In many dimorphic and polymorphic species, the size of the worker will influence its activities. For example, major workers may only be found in or near their nests while only minor workers forage away from the nest. ▪ This cooperation and division of labour, combined with their well-developed communication systems, has allowed ants to utilise their environment in ways approached by few other animals.