are: first, the infero-internal sensory nerve (nani), second, the superoexternal sensory (nans), third, the chordotonal (to acho), fourth, the nerve to the anterior (adductor) muscles of the scape, fifth, the nerve to the posterior (abductor) muscles of the scape (nsc), and sixth, a nerve which supplies the little muscles in the funicular joints (nf). The tritocerebrum is so much reduced that it is represented only by a pair of small bodies, concealed under the olfactory lobes and connected. with each other by a slender commissure, which, however, passes under the esophagus, thus indicating the originally postoral position of this portion of the brain. Each tritocerebral lobe gives off a nerve which soon subdivides into two branches, one (Fig. 27, cnf) going to the

FIG. 27. Sagittal section of head of worker Myrmica rubra. (Janet.) acho, Antennary chordotonal organ; cnf, connective of frontal ganglion; art, antennary articulation; nans, superior antennary nerve; nani, inferior antennary nerve; nf, funicular nerve; nsc, nerve to scape; nir, labral nerve; soph, sense-organs of pharynx ; mph, inferior dilator muscle of pharynx; no, nerves to ocelli; hcs, hypocerebral ganglion; mam, adductor muscle of mandible; lg, labial sympathetic ganglion; In, labial sympathetic nerve; nl, labial nerve; sol, labial sense-organs; nmx, maxillary nerve; nm, mandibular nerve; s, portions of salivary gland; cn, connective between subœsophageal and prothoracic ganglion; mal, adductor muscle of labium. Remaining letters as in Fig. 13.

frontal ganglion (to be described below in connection with the sympathetic nervous system) the other again subdividing to innervate the labrum and the wall of the pharynx (nlr).

The minute structure of the brain, with its ganglion cells and fibers, the former comprising the deeply-staining, the latter the more achromatic portions, or "Punktsubstanz" of authors, is too intricate to be considered in the present work. For these details the reader must be referred to the papers of Dujardin (1850), Leydig (1864), Rabl

Rückard (1875), Brandt (1876), Dietl (1876), Flögel (1878) and Kenyon (1896). I cannot, however, omit consideration of two regions of the ant brain, namely, the frontal and olfactory lobes, which have frequently been compared with the cerebrum and olfactory lobes of vertebrates. The frontal lobes contain two pairs of extraordinary structures, the pedunculate, or mushroom bodies (Figs. 28-30, pb), each consisting of a cup-shaped mass of nerve-fibers, the calyx, with a stem formed of a stout bundle of similar fibers which run back into the mid-protocerebrum. The calyces are embedded in a dense accumulation of minute, deeply-staining ganglion cells, which form the bulk of the frontal lobes. and evidently give rise to the fibers of the calyces and their stems. Each olfactory lobe consists of a central fibrous portion containing peripherally a large number of round bodies of still denser fibrous structure and a cortical portion made up of larger ganglion cells. The round bodies have been called glomeruli from their resemblance to the wellknown structures in the olfactory lobes of vertebrates. Since the antennæ of ants are mainly organs of smell, the occurrence in the deutocerebrum of structures so much like those in the olfactory organs of vertebrates is not without interest.

FIG. 28. Heads of worker (A), female (B), and male (C), Lasius brevicornis, drawn under same magnification, with brain, eyes and ocelli viewed as transparent objects. (Original.) oc, Median ocellus; pb, pedunculate bodies; og, optic ganglion; on, optic nerve; ol, olfactory lobe; an, antennary nerve.

It has been customary since the time of Dujardin to compare the pedunculate bodies with the cerebrum of vertebrates and to regard them as an organ of intelligence. Dujardin based his opinion on the fact that these bodies are largest and most elaborately developed in the social Hymenoptera. Leydig and Rabl-Rückard expressed a similar opinion.

A

Forel (1874) first observed that these bodies are largest in worker ants, smaller in the queens and vestigial in the males, and as the worker was supposed to be the most, and the male the least, intelligent, this was regarded as additional evidence in favor of Dujardin's opinion. The condition described by Forel for the ants was affirmed by Brandt (1876) for the social Hymenoptera in general. More recently Kenyon (1896), after an elaborate study of the bee's brain, has reached a similar conclusion. He says: "All that I am able at present to offer is the evidence from the minute structure and the relationships of the fibers of these bodies. This seems to be of no inconsiderable weight in support of the general idea started by Dujardin. For in connection with what was made known by Flögel and those before him and has since been confirmed and extended by other writers, one is able to see that the cells of the bodies in question are much more specialized in structure and isolated from the general mass of nerve fibers in those insects where it is generally admitted complexity of action or intelligence is greatest." He also cites experiments of Binet (1894) which tend to show that in insects. "when connections between the dorso- and ventro-cerebron are destroyed, the phenomena afterwards observed are similar to those seen in a pigeon or mammal when its cerebral hemispheres are removed." In support of Dujardin's hypothesis, Forel has published a series

FIG. 29.

Heads of worker (A), female (B), and male (C), Formica fusca, drawn under the same magnification, with brain, eyes and ocelli viewed as transparent objects. (Original.) Letters as in Fig. 28.

of figures of the brain of the worker, female and male of the European Lasius fuliginosus, drawn to the same scale (1904). I here introduce a similar series of the American L. brevicornis (Fig. 28). Comparison. of these figures shows that the pedunculate bodies do, indeed, vary quite independently of other portions of the brain and in the manner

noticed by Forel. In a similar series of Formica glacialis (Fig. 29), however, there are no such striking differences in the three phases. The pedunculate bodies (pb) are as highly developed in the female as they are in the worker, and they can hardly be said to be vestigial in the male. In Pheidole instabilis (Fig. 30), too, the female and soldier have well-developed pedunculate bodies, though these seem to be insignificant in the male. While, therefore, the male brain in all these species, apart from the huge development of its optic ganglia and stemmatal nerves, is manifestly deficient, I doubt whether we are justified. in regarding the brain of the female as being inferior to that of the worker. It is true that the worker brain is relatively larger, notwithstanding the smaller eyes and stemmata, or the complete absence of the latter, but I would interpret this greater volume as an embryonic char

FIG. 30. Heads of soldier (4), worker (B), female (C), and male (D) of Pheidole instabilis, drawn under the same magnification, with brain, eyes and ocelli viewed as transparent objects. (Original.) Letters as in Figs. 28 and 29.

acter. The worker is, in a sense, an arrested, neotenic or more immature form of the female, and it is well known that the volume of the brain and of the central nervous system in general is much greater in proportion to that of the body in embryonic and juvenile than in adult. animals. Forel was probably influenced in his interpretation by the view, so long accepted, but now abandoned by myrmecologists, that the

queen ant is a degenerate creature like the queen bee. In future chapters of this work I shall have occasion to show the untenability of this supposition in the light of recent observations.1

The foregoing considerations do not, of course, invalidate Dujardin's hypothesis. It is also true that the conditions throughout the insect class point to a direct correlation between the development of the pedunculate bodies and the instinctive activities, but a study of these structures in other Arthropods is not so unequivocal. Turner, in a contribution from my laboratory (Zool. Bull., II, 1899, pp. 155–160), showed that the pedunculate bodies not only occur in Crustacea (Cambarus) and the king crab (Limulus), but also in annelids (Nereis, Lepidonotus, Polynoë), and that they reach their greatest development in the king crab. In this animal they are a much-branched mass, which forms the bulk of the brain, and as Turner says, “simulates in structure the vertebrate cerebellum." On Dujardin's hypothesis we should therefore expect the king crab to be the most intelligent of arthropods. But although no one will deny that this animal has had ages in which to acquire a high psychical endowment, it shows no signs of having profited by its opportunities. It would seem, therefore, that the pedunculate bodies must be subjected to a more critical morphological and physiological study before they can be accepted as the insectean analogue of the human fore-brain.

The Ventral Nerve-Cord.-Although the suboesophageal ganglion, like the brain, consists of three fused ganglia, these have become less modified and are clearly discernible in sagittal sections (Fig. 27). The rule that each ganglion of the central nervous system innervates only the segment in which it originated in the embryo also holds good of the subœsophageal ganglion. We find that it sends off three pairs of nerves, containing both motor and sensory fibers. The first pair (nm), which is stouter than the two others, innervates the sense organs and muscles of the mandibles, and the second (nmx) and third (nl) the corresponding parts of the maxillæ and labium respectively. The three thoracic ganglia, owing to the voluminous and complicated leg and wing muscles which they innervate, are much larger than the abdominal ganglia. Each gives off a pair of crural nerves to the legs and the prothoracic ganglion also supplies a chordotonal organ near its antero-ventral end (cho). From

1 Comparison of my figures of L. brevicornis with Forel's of L. fuliginosus reveals the fact that the female brain of the latter species is no larger than that of the worker, whereas in brevicornis there is a slight difference in size in the corresponding phases. It appears from recent observations of De Lannoy (1908) and Emery (1908) that the queens of L. fuliginosus are but little larger than the workers and are probably temporary parasites (see Chapter XXIV). This may at least partially account for Forel's finding the brain of the female ant inferior in organization to that of the worker.