Ant colonies > Internal structure of ants > Circulatory system

Circulatory system

The Circulatory System

Janet (1902) has studied this system in Myrmica. It comprises, as in other insects, the heart, aorta, haemo­lymph, or blood plasma, amoebocytes, or blood-corpuscles, and several ductless glands of very simple structure. The heart (Fig. 15, ht, Fig. 24, c) is a tube lying in the mid-dorsal region of the gaster and pre­senting five dilatations corresponding with the first to fifth gastric (fourth to eighth abdominal) segments, and each of these metameric regions is pierced by a pair of osteoles provided with valves. The wall of the tube is only a single cell-layer in thickness and the cells of its two halves are in pairs, indicating that they arise from the pairs of embryonic cells which I have called cardioblasts (18y3). There is no layer of muscles enveloping the tube, but very contractile muscle fibrillae are differentiated in the cytoplasm of the cells themselves. The tube is held in place by numerous suspensory filaments and five pairs of so-called aliforl muscles, belonging to the first to fifth gastric seg­ments. These muscles are fan-shaped, with their broad ends meeting and uniting in the middle line below the cardiac tube and their pointed ends inserted on the supero-lateral walls of the gaster. Anteriorly the heart is continued through the slender abdominal pedicel and into the thorax as the aorta, a slender non-contractile tube which opens into the head cavity.

The blood, as in other insects, is a colorless liquid filling the body cavity or spaces between all the internal organs and containing very small, colorless, amoeboid and nucleated corpuscles. Circulation is effected by the systole and diastole of the heart, the ptllsations of which proceed in a wave from its posterior to its anterior end. These move­ments are described as follows by Janet, with the aid of the accompany­ing diagram (Eig. 24, B) : "During systole the aliform muscles (alit), the suspensory filaments (sf ) and the heart (c) occupy the positions represented by the unbroken lines. In contracting, the aliform muscles shorten, and owing to this shortening, they recede in the middle region from the dorsal integument and take the position represented by the dotted lines. This movement draws down the suspensory filaments attached to the muscles and changes the direction of those attached to the dorsal integument. As these filaments can be but slightly elongated, the changes of position here described are produced, so to speak, entirely at the expense of the elasticity of the cardiac.wall, which dilates consid­erably. With this dilatation the valvules move away f rom the points to which they were applied and the blood Stl'ta111S through tile osteoles and fills the heart. The blood, propelled by the contracting heart, pours into the head, bathes all the organs and then leaves it through the neck to traverse the whole thoracic cavity in an antero-posterior direction. After having passed through the much constricted peduncle of the petiole and postpetiole, it enters the gaster and flows through two passages, separated by a diaphragm that divides the body cavity into a ventral, or neural, and a dorsal, or visceral, sinus. One current descends through the dorsal, another through the ventral sinus, following the latter to the tip of the gaster. The dorsal sinus, which is very large and supplies the heart with the blood it propels into the head, is thus

FIG. 24. Transverse section through heart of Myrmica rubra. (Janet.) A, Through region of rectum ; c, heart; sf, suspensory filaments; am, aliform muscle; ¢c, peri­cardial cells; f, fat-cells; u, urate-cell ; oe, oenocyte ; ch, dorsal integument ; r, dorsal wall of rectum. B, Diagram to illustrate position of heart, suspensory filaments and aliform muscle during systole (continuous lines) and diastole (dotted lines).

supplied simultaneously by the posterior portion of the postpetiole and the posterior portion of the ventral sinus of the gaster."

Connected with the circulatory system are some four different kinds of cells, which are suspended either singly or in clusters in the blood current. These are the pericardial cells (Fig. 24, pc), the oenocytes (oe), the adipocytes (f ), forming the fat body, or corpus adiposum and the urate cells (u). The pericardial cells are of small size and are attached to the suspensory filaments and aliform muscles. In the living insect these cells have an acid reaction. They probably function as ductless glands, taking certain substances from the blood, transforming them and returning them to the circulation in such a form that they can be absorbed and excreted by the Malpighian vessels (Cuenot). Some authors are of the opinion that these pericardial cells also give rise to the amoebocytes, that they constitute, in other words, a 1laemato­poetic organ. The oenocytes are glandular cells which arise in seg­mental clusters from the ectoderm of the embryo just behind the tracheal invaginations.

In the ants these cells are very small and in the adult scattered about among the fat cells. They are very conspicuous in the young larva and still occupy their embryonic position, but in aged ants, according to Janet, they disappear. Like the pericardial cells they are probably ductless glands, producing some unknown but physiologically important internal secretion. The fat cells form large masses or packets, often filling out all the spaces of the body cavity between the viscera, especially during the larval and pupal stages. As the name indicates,

FIG. 25. Longitudinal sections to show valve and method of closing the tracheae in Myrmica rubra. (Janet.) A, Last abdominal trachea open ; B, closed; o, stigmatic orifice; a, anterior stigmatic chamber.; b, occluding chamber; c, fixed insertion of occluding muscle ; d, mobile insertion of same ; e, mobile insertion of opening muscle; f, occluding muscle; g, opening muscle; h, stiffened portion of trachea; i, stigmatic or main tracheal trunk.

these cells have their cytoplasm filled with fat globules, which are often so numerous that the nucleus is reduced to a stellate or irregular body. Unlike the oenocytes, the fat cells are of mesodermal origin. The tirate cells are found singly or in clusters among the fat cells. They are large and opaque, owing to a mass of urate crystals stored in their cytoplasm. They are most easily seen in larvae and pupae and may be regarded as a very primitive form of kidney adapted for storing instead of excreting the products of tissue metabolism.