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 (Cuénot). Some authors are of the opinion that these pericardial cells also give rise to the amœbocytes, that they constitute, in other words, a hæmato'poëtic organ. The cenocytes are glandular cells which arise in seg'mental 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,

f

B

FIG. 25. Longitudinal sections to show valve and method of closing the trachea 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 cenocytes, the fat cells are of mesodermal origin. The urate 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 larvæ and pupæ and may be regarded as a very primitive form of kidney adapted for storing instead of excreting the products of tissue metabolism.

The Respiratory System.-The trachea of ants are not unlike those of many other insects, as shown by Janet's studies (1902) of Myrmica and other genera. In all ant-larvæ there are ten pairs of stigmata or tracheal orifices occurring on the meso- and metathoracic and first to eighth abdominal segments. These stigmata also persist in the adult ant as small, round openings. According to Janet the metathoracic pair is closed in the Myrmicinæ (Myrmica), but remains open in the Camponotinæ (Formica) and Dolichoderinæ (Tapinoma). Each stigmatic orifice leads into a short stigmatic trunk which is furnished with a very interesting valve by means of which it can be closed (Fig. 25). The stigmatic trunks of the thorax and gaster bifurcate in an anterior and posterior direction and the two branches fuse on each side of the body to form a continuous longitudinal trunk. This is very large in the gaster, but much more tenuous in the thorax, where a second pair of more dorsal longitudinal trunks is formed, which, in the queens and males, supplies the wing muscles with air. The gastric trunks dilate and contract with the so-called respiratory movements of the external skeleton and in this manner the air is pumped into and out of the finest ramifications of the trachea. The gastric trunks are united by ventral, transverse, anastomosing trachea and also give off segmental dorsal branches which break up into finer and finer ramifications to supply the various viscera.

The Muscular System. For an account of this system in ants the reader must be referred to the articles of Janet, Nassonow, Berlese and Lubbock, as the subject is one of too great complexity and detail to be treated within the limits of this work. Still there is an ontogenetic change in the muscular system of the adult queen ants, which cannot be passed over, as it is of no little ethological importance. I have often observed that aged, deälated queens will float when placed in water or alcohol, and that when the thorax of immersed specimens is pierced with a needle, large bubbles of air escape, showing that the wing muscles must have atrophied. Janet (1906, 1907a, 1907b) has studied the histological changes, which lead up to this peculiar condition in Lasius niger, and finds that the muscles, which in the virgin queen fill up most of the thoracic cavity and are well-developed and beautifully striated till the marriage flight occurs, are completely broken down within a few. weeks after deälation (Fig. 26). He maintains that this sarcolysis is not due to phagocytes devouring the muscles piece-meal, but that the blood corpuscles (amœbocytes) which creep in among the fibrillæ take up spontaneously the dissolving muscle substances and convert these. within the cytoplasm into fat globules and albuminoid granules. Thus the amœbocytes become adipocytes and replace the muscle fibrillæ (Fig.

26, B). Somewhat later the amoebocytes discharge the fat globules and albuminoid granules from their cytoplasm into the blood plasma, which from being a limpid liquid assumes a more granular appearance as it becomes charged with more and more of the metabolized products of sarcolysis. Eventually nothing remains of the muscles but their sheaths, and the thoracic trachea become greatly enlarged, which accounts for the floating of the insect in liquid and the emission of air bubbles when the thorax is pricked under water (Fig. 26, D). The fatty and albuminoid substances derived from the histolyzed wing-muscles are carried in the blood to the abdomen, where they are taken up by the

FIG. 26. Wing muscles of Lasius niger queen, to show their degeneration after nuptial flight. (Janet.) A, Sagittal section of thorax and petiole of queen immediately after nuptial flight; B, ten months later; C, transverse section through mesothorax on day of nuptial flight; D, same five weeks later; m, longitudinal vibratory muscles; n, transverse vibratory muscles; b, blood coagulated and charged with the products of muscle dissolution; t, trachea.

ovaries and, no doubt, contribute greatly to the growth of the eggs. The queen ant thus resembles the salmon, in which, according to Miescher, there is at the time of sexual maturity a conversion of part of the trunk musculature into substances that are appropriated by the reproductive cells and further their growth and maturation.

66

CHAPTER IV.

THE INTERNAL STRUCTURE OF ANTS. (CONCLUDED.)

It is certain that there may be extraordinary activity with an extremely small absolute mass of nervous matter; thus the wonderfully diversified instincts, mental powers, and affections of ants are notorious, yet their cerebral ganglia are not so large as the quarter of a small pin's head. Under this point of view, the brain of an ant is one of the most marvellous atoms of matter in the world, perhaps more so than the brain of man."--Charles Darwin, "The Descent of Man."

The Nervous System.-The structure of the central nervous system is best considered in connection with the primitive segmentation as this is revealed in the embryonic ant. As stated in a previous chapter, the body of the ant, like that of all other true insects (Pterygogenea), consists of a series of twenty metameres, or segments. The first and last of these are peculiar in certain respects and have been called the acron and telson respectively. In the embryo the ectoderm of the mid-ventral portion of each segment (except the telson) thickens and gives rise to a pair of ganglia that soon split off from a thin surface layer of cells which then become the ventral integument. The ganglia of each segment are closely approximated and connected with each other by a pair of commissures, while the ganglia of successive segments are united by pairs of connectives which therefore run longitudinally. Later these connectives lengthen, and as the body grows more rapidly. than the ganglia, we find the latter forming a chain extending through the ventral region of the head, thorax and abdomen. Not only do many of the ganglia thus become rather widely separated from one another, but there is also a tendency for some of them to fuse together and make larger masses. Thus the ganglia of the first (acron), second (antennary) and third (intercalary) segments, known respectively as the proto-, deuto- and tritocerebrum of Viallanes, fuse to form the brain, or supraœsophageal ganglion. As the latter term indicates, this mass is dorsal to the oesophagus, and therefore preoral. This is true, however, only of the protocerebrum of the embryo, the two other pairs of ganglia being postoral at first, but moving forward and becoming preoral before the hatching of the larva. The ganglia of the mandibular, maxillary and labial segments also unite to form a single mass, the subœsophageal ganglion, which, as its name implies, lies behind the gullet. This ganglion is united to the brain by means of a pair of circumœsophageal connectives. The pro- and mesothoracic ganglia

remain distinct and lie in their respective segments even in the adult ant. The first (mediary) and second abdominal ganglia, however, are drawn up into the metathorax and fused with the metathoracic ganglion, and the ganglion of the third abdominal segment comes to lie in the petiole (second abdominal segment) (Fig. 13, ag3). The fourth, fifth, sixth and seventh abdominal ganglia retain their independence, but the latter two are close together and are immediately succeeded by the fused eighth to tenth, which constitute a single ellipsoidal mass, terminating the chain and in the adult ant lying some distance in front of the posterior end of the gaster (Fig. 13, ag8-11). The central nervous system of the adult ant therefore presents only eleven ganglionic masses, formed by condensation of the primitive nineteen. For convenience in description, this system may be divided into the brain and ventral cord, and these, with their ganglia and peripheral nerves, may be briefly considered before we take up the sympathetic nervous system and the sense organs.

The Brain. I agree with those authors, who, following RablRückard (1875), restrict the term "brain" to the supracesophageal ganglion, although it must be admitted that in ants and other Hymenoptera the circumoesophageal connectives are so short and robust that the supra- and suboesophageal ganglia seem to form but a single mass perforated by the gullet. Leydig called this whole mass the brain; Janet suggests for it the term "encephalon." The three primitive pairs of ganglia, constituting the proto-, deuto- and tritocerebrum, though intimately fused, can still be recognized in the adult brain, at least by their innervations, but the three apparent segments indicated by the outline of the organ do not correspond to the primitive segments. The protocerebrum is the largest single pair of ganglia in the central nervous systems and differs markedly from all the others in form and complexity of structure. It is broadest in the middle where it is continued on each side into the optic nerves (Figs. 28-30, on) to the compound. eyes. The portion between the optic nerves may be called the midprotocerebrum. It is flanked on each side by an optic ganglion (og) of complicated structure and projects anteriorly as a pair of rounded frontal lobes (pb). From the notch between these, nerves are given off to the three stemmata, or ocelli (oc), when these organs are present. As the median stemma has two nerves, it must have been a paired structure originally. The deutocerebrum is represented by a pair of rounded protuberances known as the olfactory lobes (ol), which are morphologically behind, though apparently somewhat in front of the other brain segments. According to Janet, each antenna is supplied with six nerves which arise close together from each olfactory lobe (Fig. 27). These