Ant colonies > Internal structure of ants > Brain


The Brain

I agree with those authors, who, restrict the term "brain" to the supraoesophageal 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, OH) 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 are: first, the in fero-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 tritocerebrtlm 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 oesophagus, 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.) aclio, Antennary chordotonal organ; cn f, connective of frontal ganglion; art, antennary articulation ; nans, superior antennary nerve ; nani, inferior antennary nerve; n f, funicular nerve ; nsc, nerve to scape ; nlr, labral nerve ; soplti, sense-organs of pharynx ; mph, inferior dilator muscle of pharynx ; no, nerves to ocelli ; hcs, hypocerebral gang­lion; mann, adductor muscle of mandible; lg, labial sympathetic ganglion ; In, labial sympathetic nerve ; nl, labial nerve ; sol, labial sense-organs ; nmx, maxillary nerve; nin, mandibular nerve ; s, portions of salivary gland ; cn, connective between sub­oesophageal 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), RablRuckard (1875), Lrandt (1876), Dietl (187G), hlogel (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 antennae 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-Ruckard expressed a similar opinion.

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 Flogel 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 (i894) 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 fuliginosits, drawn to the same scale 0904). 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 horel. In a similar series of Foriyiica 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, rr the complete absence of the latter, but I would interpret this greater volume as an embryonic char­acter

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

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. Fore] was probably influenced in his interpretation by the view, 5o long accepted, but now abandoned by myrmecologists, that the queen ant is a degenerate creature like the queen bee. I shall have occasion to show the untenability of this supposition in the light of recent observations.