The anatomy of the supra-oesophageal ganglion is very complex; it has been recently investigated by Viallanes[[38]] in the wasp (Vespa) and in a grasshopper (Caloptenus italicus). The development and complication of its inner structure and of some of its outer parts appear to be proportional with the state of advancement of the instinct or intelligence of the Insect, and Viallanes found the brain of the grasshopper to be of a more simple nature than that of the wasp.
Fig. 65.—Brain of Worker Ant of Formica rufa. (After Leydig, highly magnified.) Explanation in text.
Brandt, to whom is due a large part of our knowledge of the anatomy of the nervous system in Insects, says that the supra-oesophageal ganglion varies greatly in size in various Insects, its mass being to a great extent proportional with the development of the compound eyes; hence the absolute size is not a criterion for the amount of intelligence, and we must rather look to the complication of the structure and to the development of certain parts for an index of this nature. The drone in the honey-bee has, correlatively with the superior development of its eyes, a larger brain than the worker, but the size of the hemispheres, and the development of the gyri cerebrales is superior in the latter. In other words, the mass of those great lobes of the brain that are directly connected with the faceted eyes must not be taken into account in a consideration of the relation of the size and development of the brain to the intelligence of the individual. The weight of the brain in Insects is said by Lowne to vary from 1⁄150 to 1⁄2500 of the weight of the body.
Figure 65 gives a view of one side of the supra-oesophageal ganglion of the worker of an ant,—Formica rufa,—and is taken from Leydig, who gives the following elucidation of it: A, primary lobe, a, homogeneous granular inner substance, b, cellular envelope; B, stalked bodies (gyri cerebrales), a, b, as before; C, presumed olfactory lobes, c, inner substance, d, ganglionic masses; D, ocular lobes, e, f, g, h, various layers of the same; E, origin of lateral commissures; F, median commissure in interior of brain; G, lower brain (sub-oesophageal ganglion); H, ocelli; J, faceted eye.
Fig. 66.—Stomato-gastric nerves of Cockroach: A, with brain in situ, after Koestler; B, with the brain removed, after Miall and Denny: s.g, supra-oesophageal ganglion; o, optic nerve; a, antennary nerve; f.g, frontal ganglion; oe, oesophagus; c, connective; p.g, paired ganglia; v.g, crop or ventricular ganglion; r, recurrent nerve.
Besides the brain and the great chain of ganglia there exists an accessory system, or systems, sometimes called the sympathetic, vagus, or visceral system. Although complex, these parts are delicate and difficult of dissection, and are consequently not so well known as is the ganglionic chain. There is a connecting or median nerve cord, communicating with the longitudinal commissures of each segment, and itself dilating into ganglia at intervals; this is sometimes called the unpaired system. There is another group of nerves having paired ganglia, starting from a small ganglion in the forehead, then connecting with the brain, and afterwards extending along the oesophagus to the crop and proventriculus (Fig. 66). This is usually called the stomatogastric system. The oesophageal ring we have already spoken of.
By means of these accessory nervous systems all the organs of the body are brought into more or less direct relation with the brain and the ganglionic chain.
Our knowledge of these subsidiary nervous systems is by no means extensive, and their nomenclature is very unsettled; little is actually known as to their functions.