Collins New Naturalist Library. M. Brian V.
Чтение книги онлайн.
Читать онлайн книгу Collins New Naturalist Library - M. Brian V. страница 6
Название: Collins New Naturalist Library
Автор: M. Brian V.
Издательство: HarperCollins
Жанр: Зарубежные любовные романы
isbn: 9780007406470
isbn:
FIG. 3. Worker of Myrmica rubra, head section from a. to b. of fig. 2 (after Janet).
Water-dissolved substances are carried through the long, thin oesophagus to be stored in a dilatation called the crop which lies dorsally in the front of the gaster. This can be distended to a huge size for it is used to carry nectar and honeydew back to the nest. There is a valve between the crop and the rest of the gut which prevents a good deal of loss from the crop but not all. In some ants this is held closed by muscular sphincters; in others it closes passively when the fluid pressure in the crop rises. It is thought that this second method of control is more efficient since it uses no energy. The crop contents are mostly very liquid and can be regurgitated to other adults and to larvae. This is probably done by telescoping the gastral segments inwards but perhaps by reversing the pharyngeal pump mechanism.
Once fluid has passed back into the midgut it can no longer be shared by other ants. The thick walls of the midgut (fig. 4) secrete digestive enzymes and absorb the small molecules into which the food is broken. Waste material passes back through another tube into a thin-walled hindgut which stores indigestible residues and probably extracts some water and water-soluble substances. Finally, the liquid waste is ejected in a special part of the nest some way away from the brood rearing zone, along with the refuse. Foragers probably eject their own residues outside the nest. It includes waste substances extracted from the body fluids by long, thin, hollow, thread-like tubules; they join the gut just after the midgut. Much of this excretion is synthesized into uric acid which appears as white granules. In workers the main storage tissue is in the gaster. Oils, glycogen and proteins are all stored there though most of it is oil.
FIG. 4. Queen of Myrmica rubra, gaster sagittal section showing main organs (after Janet).
A very important part of the gaster is occupied by the genital system (fig. 4). This consists of thin-walled egg tubes (many more in queens than in workers) that start just below the heart at the front of the gaster and pass backwards and downwards to the genital opening below the anus and the sting. Eggs start as small cells at the tip and move down, growing and maturing as they go. Part of the material for growth comes from nurse cells which are broken down and incorporated into the egg tissue but some is extracted from the blood by an enveloping layer of follicle cells. When the egg has been fully formed it is enclosed in a special skin (the chorion) and the egg passes into the oviduct to be fertilized. The oviduct is a simple tube with a thicker wall than the ovary. In queens it has two pouches on its dorsal side, one near the outside which received sperm during copulation and the other a bit farther in that stores the sperm alive for a decade or more. How it gets from one to the other is unknown. These pouches are not present in workers unless the ant is a very primitive species.
It remains only to mention two glands that open into the sting (fig. 4). One is the poison gland which consists of two closed tubes feeding a thin-walled storage vesicle and which contains a watery solution of mixed venoms. The other is a smaller gland called after Dufour which contains an oily material secreted by cells which surround the storage space. The poison glands vary quite considerably in different families of ant, depending upon whether they synthesize a thin liquid for squirting or injecting with a sting, or a sticky fluid for smearing.
In general the ant body is neatly divided for separate functions: the head carries the main sense organs and the brain and has in front a mechanism for food capture and food pre-treatment, the mesosoma is specialized for locomotion and the gaster has a region of absorption and food storage but also contains the reproductive organs. Defensive apparatus is disposed both in front and behind but the body is flexible enough for both to act forwards in concert.
LARVAL STRUCTURE
Ants’ eggs are oval and white; each egg weighs only ·00005 gm and is less than a millimetre long. In most of them a legless grub develops. This is of course no bigger than an egg at first and breaks its way out by means of its sharply pointed jaws. It is almost hairless and transparent so that a small, residual blob of egg yolk in its midgut can easily be seen. Though its cuticle is inelastic rather like polythene it is wrinkled and folded so that a good deal of room for expansion is allowed. There are three stages separated by moults, when the old skin is cast off and a new one, folded and wrinkled to allow for growth, is built underneath. Only the head has a firm cuticle that will not expand at all so that its growth only takes place during the moult between successive stages. The second stage has a good many more hairs than the first and the third and last stage is very hairy indeed. It has some hooked hairs as well as the simple ones which enable the grubs to interlock and so be carried about in one group. It is tempting to believe that the workers are able to distinguish the stages by feeling the degree of hairiness with their antennae, but again this has not been tested by experiment.
The head carries jaws with pointed, hardened tips. These are capable of piercing eggs and can masticate soft tissues a little. There are several sense organs of a very simple kind, including minute, unjointed, conical antennae; several other similar ones occur around the mouth and are probably used by the larva to feel and taste what it is eating, as well as to enable it to lock on to the worker’s mouth whilst it is being fed regurgitated liquid. The head, like that of the adult, contains a pharynx innervated by muscles (fig. 5). It is a pump that only sucks inwards; larvae cannot regurgitate food. The ‘brain’ consists of a dorsal ganglion and a ventral ganglion with nerve trunks running round the oesophagus to connect them. The ventral ganglion is the first of a long, chain-like series that runs down the body near the ventral wall.
FIG. 5. Larva of Myrmica rubra in third instar, longitudinal sections to show main organs: a. slightly off mid-line, b. on mid-line (sagittal).
Most of the body contents are connected with the digestion and storage of food. A very large, sac-shaped midgut consists of a single layer of large, glandular cells. This is connected to the pharynx by a thin-walled oesophagus and where this and the midgut meet there is a structure for secreting and moulding a thin membrane which encloses the food and is thought to protect the delicate gut cells from abrasion. However, there is no way through from the midgut into the much smaller hindgut and all the food residues collect and condense into a black pellet surrounded by food membranes which are ejected at the end of the larval stage. The hindgut is very thin-walled and collects the liquids extracted by excretory tubules that lie freely in the body fluids. It thus acts as a bladder by storing urine and, if the diet is rich in protein, white insoluble granules of uric acid. The bladder is emptied by a longitudinal contraction of the body; this usually occurs only in response to a light touch on the skin in the region of the anus so that normally there is a worker nearby to collect it up and carry it away. The larva has a single but large pair of tubular digestive glands that start in the front, pass backwards and then enlarge into thin-walled reservoirs which go forwards again well into the first body segment just behind the head before sending tubes that join and run to the lower lip. A great deal of fluid that is rich in protein-splitting enzymes is stored in these reservoirs, ready to be shed on food: as they are thin-walled and quite close to the body wall they are really a СКАЧАТЬ