Apart from a brief overview of ant phylogeny in chapter one, there is no separate discussion of evolution — rather, evolutionary ideas pervade the entire work. Similarly, there is no separate chapter on ant anatomy, which is dealt with where relevant: the various glands, for example, are covered in the chapter on communication. And ants are connected throughout to more general biological theory, most notably sociobiology.
"Ant castes are exceptionally well suited for optimization studies in sociobiology. ... individual worker ants are full organisms with ordinary, whole patterns of social behavior, yet they are also clearly specialized for particular well-defined tasks. ... Thus well-defined anatomical structures and behavioral acts can be more readily assayed with reference to the four elements of optimization models; we really are able to specify a restricted and relatively easily defined state space, a set of strategies, testable fitness functions, and measurable constraints."
It is not a major focus, but aspects of the human and economic significance of ants are touched on. The dominant animals in many habitats, they are sometimes a major constraint on agriculture — and not always a negative one.
"Records from southern China show that weaver ant nests have been gathered, sold, and placed in selected citrus trees to combat insect pests for approximately 1,700 years. ... the oldest known instance of the biological control of insects in the history of agriculture."And extracts from travel narratives, or the more personal stories of myrmecologists, are used in a few places to illustrate aspects of ant behaviour.
A few opening pages on the importance of ants are followed by the least digestible chunks of the work: nearly 80 pages of taxonomic key and 60 pages of line drawings of the major genera. The other specialist material is scattered through the remainder of the book, in the form of tables showing the taxonomic distribution of particular traits or features.
The bulk of the book falls roughly into two parts, with the chapters in the first covering general features of ant life cycles, ecology, and so forth and those in the second different ant specialisations. The following summary does no justice to this at all.
A typical ant colony life cycle consists of the nuptial flight and mating, followed by colony foundation, growth, and movement; there are, however, many species that don't fit this pattern.
"By almost any conceivable standard, the single most important feature of insect social behavior is the existence of the nonreproductive worker caste." The debate over the origin and persistence of this and associated altruism, of eusociality, has seen kin selection first downplayed as an explanation and then revived.
A critical factor in ant social behaviour is the extent to which ants can recognise kin, other ants of the same colony, and different brood stages. But "the pattern of diversity in recognition systems of ants is still mostly unknown".
Many ant species exhibit polygyny, which can arise through founding queens remaining together, later adoption of new queens, or fusion of colonies. The number of queens is a critical feature of a colony, among other things influencing conflicts between queens and workers over the proportion of investment in new queens and males.
"The typical ant is a walking battery of exocrine glands", with more than ten organs varying in form and use among different ant groups — the six most important are "Dufour's gland, the poison gland, the pygidial gland, the sternal glands, the mandibular glands, and the metapleural glands". The pheromones from these are central to ant communication, lending themselves to ritualization and modulatory communication, but ants also use auditory and visual modes. Communication underpins alarm signalling, recruitment, adult transport, trails and border marking, food-sharing, and corpse removal, among other behaviors.
Ergonomic analysis of ant colonies reveals a range of tasks and roles, allocated between different castes. As well as males, workers, and queens (or an ergatogyne reproductive caste), there are often subdivisions of workers into minors, medias, and majors, with the latter often highly specialised.
"The greatest size variation of nestmates ever recorded in ants occurs in the Asian marauder ant Pheidologeton diversus. The minor worker depicted in this scanning electron micrograph has a head width exactly 1/10 that of the major on which it sits, and a dry weight only about 1/500 that of the larger ant."And there are often temporal castes, with roles changing over individual lifetimes. Caste determination is complex, but colony demographics are clearly adaptive.
While ants are capable of some learning — but don't play — it is often helpful to consider the ant colony as a superorganism, with its own homeostatic responses and flexibility in behaviour. Notable examples are in thermoregulation of nests and control of humidity.
A "temperature-humidity" envelope constrains the daily cycle of ants: with very few exceptions "they function poorly below 20 degrees Celsius and not at all below 10". Foraging strategies also face a tradeoff between predation and energy optimisation. Competition is another factor in population regulation: it produces overdispersion of colonies and a range of different territorial strategies.
And this applies between species as well: "Competition is the hallmark of ant ecology. Many, perhaps most, ant species employ aggressive techniques up to and including organized warfare." Coexistence can involve differences in niche, density, and size, with ants displaying "a startling variety of mechanisms that appear to adjust species to one another and hence to organize local communities".
Two hundred pages cover some of the specialisations of ants. There are three chapters on ant "symbioses", covering the full range from mutualisms to parasitic relationships, and five on trophic specialisations.
Ants exhibit many kinds of social parasitism, of which slave-taking is the best known, though "no verified examples are yet known of mutualism, in which two species cooperate to the benefit of both." Hölldobler and Wilson analyse the different stages of inquilinism (where the parasite spends its entire life cycle in the host nest) and their possible evolution.
When it comes to symbioses with other arthropods, individual ants have predators and parasites, but in some cases it is useful to see the ant colony as an ecosystem for symbionts. Myrmecophilous insects include beetles and the larvae of many lycaenid butterflies, with relationships ranging from mutualistic to parasitic; perhaps the most startling are the trophobiont homopterans (aphids) "herded" by some ant species.
Relationships with plants are equally varied: ants protect plants, plants shelter or feed ants, ants feed plants, ants disperse plants, and so forth. There are also fungi and microorganisms that are parasitic on ants.
There are ants specialised for a vast range of prey — arthropod eggs, termites, cockroaches, collembolans, and more. The evolution of feeding specialisations can be correlated with social behavior: the trend within the Dacetine ants has been a "shift from open foraging on the ground and low vegetation to an increasingly cryptic, subterranean existence. ... associated with a reduction in the variety of arthropods taken as prey".
Army ants are among the best known — if not infamous — ants. As well as the well-studied swarm raider Eciton burchelli and other swarm and column raiders in the same genus, there are African driver ants Dorylus and many others that combine migration and group predation. The origin of legionary behaviors remains unclear, however.
Among the most complex colonies are those of the leaf-cutter fungus growers Atta, with up to seven castes performing 29 distinct tasks.
"The ultimate size reached by the Atta nests is enormous. ... [one] A. sexdens nest, 77 months old, contained 1,920 chambers, of which 390 were occupied by fungus gardens and ants. The loose soil that had been brought out and piled on the ground by the ants during the excavation of their nest ... occupied 22.72 cubic meters and weighed approximately 40,000 kilograms."
The harvesting ants are the source of classical tropes about the industry and diligence of ants. And the weaver ants Oecophylla use silk produced by their larvae to glue leaves together to make nests, an adaptation that has helped them to become one of the most abundant social insects.
Hölldobler and Wilson end with some brief tips on collecting and farming ants. There's a sixty-five page bibliography and — more importantly for the non-specialists — a ten page glossary.
Replete An individual ant whose crop is greatly distended with liquid food, to the extent that the abdominal segments are pulled apart and the intrasegmental membranes are stretched tight. Repletes usually serve as living reservoirs, regurgitating food on demand to their nestmates. Also used as an adjective, e.g., replete worker.
The Ants has an excellent range of black and white photographs, backed up by sixteen separate pages of dramatic colour photographs. Effective use is also made of line diagrams, graphs, maps, plans, and so forth. It is a large format work, nearly 30cm square and weighing 3.5 kilograms, and on the expensive side, so it was never going to be a bestseller. Any university library should have a copy, however.
Hölldobler and Wilson have also written a much shorter and more accessible popular book, Journey to the Ants. This omits most of the fascinating biological detail as well as the reference material and technical apparatus, focusing on some of the more dramatic and unusual ants. It also includes some autobiographical and travel details not in the larger work.
May 2004
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