Collins New Naturalist Library. M. Brian V.

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СКАЧАТЬ of wings). They can get out of their cocoons without help. These workers forage for prey and feed the brood. In spite of this help the queens go on working and more may join the group from time to time. Myrmecia is another primitive type; it constructs an open cell and forages regularly for nectar, only collecting insects when its eggs hatch. If deprived of prey it can make some growth so presumably it has a body source of food, perhaps its thoracic wing muscles. On a similar level is the European genus Manica of the myrmicine ants which excavates a small nest, leaves the entrance open and goes out to collect prey for its larvae. The workers the queen produces are considerably smaller than herself.

      Finally, a stage of evolution is reached which has been called claustral; in this the queen seals herself in a hole and produces the first brood from her own body reserves entirely. Such a limited food supply would favour the evolution through natural selection of small workers which can be produced both more quickly and more economically. However, a big caste divergence necessitates the establishment of effective methods of intercommunication, co-operation and, above all, control. Thus the general theme of ant evolution is from small groups of structurally similar, very versatile females to large groups of structurally dissimilar, highly specialized, interdependent females.

CHAPTER 2

       ANT STRUCTURE

      THIS is no more than a brief account of the main structural features of females (queens and workers) and their larvae. Males and pupae are only treated superficially. Myrmica rubra will be used as a type; it is a fairly unspecialized species whose structure was meticulously studied by Charles Janet at the end of the nineteenth century. Some of his diagrams have been adapted and reproduced here.

      EXTERNAL FEATURES

      Like all insects, ants have a hard external covering. This cuticle is many-layered and chemically complex; apart from protecting the body mechanically and providing a strong basis for muscle attachment it also screens out dangerous solar rays and reduces the amount of water vapour that escapes. Water loss can be substantial even in quite humid conditions and this last function is very important to terrestrial animals. Flexibility is achieved by joining rigid segments by a supple connecting membrane folded and tucked inside for protection. Although ants are by no means as hairy as bees they do have a great many simple and usually rather short setae scattered over most parts of the body. Spines also occur in some groups.

      As in all insects the ant’s body is divided into three parts – head, thorax and abdomen. The unusual feature in ants (and most other members of this insect order, the Hymenoptera) is the evolution of a petiole towards the front of the abdomen. This is formed by reducing one or two segments to narrow tubes and articulating them. This separates a middle body (called the mesosoma) composed of the thorax and one abdominal segment from a hind body (the gaster) made up of the last seven segments (not all of these are visible). The number of petiole segments is two in the sub-family Myrmicinae (fig. 1) and one in other ants; formicine ants usually have a scale on their petiole (fig. 7). Most of the hinging motion occurs between the mesosoma and the first petiole segment and lies in a vertical plane; the gaster can be brought round under the mesosoma until its tip reaches up to near the jaws. In this way an enemy or prey can be located with the antennae, grasped with the jaws and immediately stung or sprayed with toxin. Damage caused by the bite probably helps the venom to penetrate. In a similar way eggs in the genital aperture can be brought round to the mouth and picked up in the jaws. The ventral side of the gaster can also be bent under the thorax and can be used to stop small objects, principally brood, from slipping backwards as they are being manipulated by the forefeet and mouthparts.

      The head is very flexibly articulated on to the thorax and the actual junction protected above by a stiff collar which projects forward from the thorax. The head can be moved downwards and rotated sideways but cannot be lifted very much. However it is arranged in the long axis of the body so that the jaws protrude, an obvious advantage in defensive situations.

      The most important sense organs are the antennae (figs. 6, 7). They are hinged so that they can be both extended well in front of the jaws or, in dangerous situations, folded back so that they lie close to the head itself. This hinge occurs at the junction of a long, thin, basal segment (called the scape) and a highly-subdivided club (called the funiculus). These organs undoubtedly help the ant to understand the size and shape of objects in its environment: they can be moved through a wide arc independently or the tips can be brought together on to small objects much less than a millimetre in diameter. A wide variety of chemicals can also be detected and recognized by the antennae and they may also respond to sounds and vibrations in the substratum.

      Most ants have the usual insect compound eyes but they are never well-developed, even in males, and in those which spend most of their time underground or following scent trails above ground they may be quite absent. Each eye is formed of a group of small visual organs – ommatidia; in some insects like dragonflies there may be as many as thirty thousand of these but even in flying male ants one thousand is more like the maximum. In one of the wood ants, Formica cunicularia, there are 460 ommatidia in the worker caste; in the common black garden ant, Lasius niger, there are only 120 and in the related but soil-dwelling Lasius flavus a mere 45. Males and sexual females have three small, light-responsive organs on the top of their head (ocelli); their use is not fully understood but it seems likely that they play some part in activity, particularly flying.

      The mouth, as in all insects, is surrounded and enclosed by several pairs of articulated appendages. The simplest and most prominent are the jaws (mandibles). These are hollow but have thick, tough walls and they hinge firmly on the front of the head in such a way that they can be either opened wide or closed tightly together (fig. 3). Their sharply serrate margins enable a strong grip to be taken on small objects, such as the leg of a fly. Not all the teeth are of equal size; the front two are larger and can pierce insect cuticle. The back ones are shorter and more frequently used in gripping the brood when it is being carried about. All these teeth wear quite blunt as the worker ages and one or two of the bigger ones may chip away. Between the jaws there lies another pair of jointed structures that carry small palps which are almost certainly used in tasting and feeling food. These maxillae, as they are called, can fold back and cover the mouth completely without interfering with the free movement of the jaws; this is an obvious advantage in fighting. Finally, there is a tongue (the labium) that can be ejected by blood pressure (fig. 2). It is quite manoeuvrable and is frequently used as a rasp since its tip is covered with fine striations. Liquid food flows up the tongue towards the mouth in saliva but before entering it must pass over a silt trap, known as the infra-buccal pocket, which takes out solid, largely indigestible granules from the food (fig. 2). This pocket is also used as a depository for dust from the body in general; dust is collected by rubbing the middle and hind legs over the body surface; combs on the front legs (fig. 6) are then used to remove it and these are then cleaned by passing them through the mouthparts. A lot of dust is collected by licking other ants and when the pocket is full its contents are thrown away into the rubbish area.

      The mesosoma is the motor part of the body and carries the six legs and, in sexuals, the two pairs of wings. Each leg has five main joints but the outermost one is finely subdivided to make a flexible foot that ends in two curved claws. The articulation and manoeuvrability of each leg is remarkable; they are perfectly constructed for movement over rough and irregular ground and up vertical СКАЧАТЬ