Page:Jardine Naturalist's library Entomology.djvu/182

176 optic nerves, which are the largest in the whole body, the most considerable are the lateral cords uniting the two ganglia in question; their length is in proportion to the diameter of the esophagus, and as that is least in sucking insects, the two ganglia in these approximate almost to touching, but they are remote in gnawing species, and the uniting nerves are proportionately elongated. The sub-esophagial ganglion (the cerebellum of Burmeister, or rather what is called the medulla oblonga by Mr. Newport, who regards the true analogue of the cerebellum to be entirely wanting in insects,) is commonly heart-shaped, or inclining to transversal, and lies at the base of the cavity of the skull. Anteriorly it gives birth to several nervous stems on each side, which supply branches to the mandibles, maxillæ, labium, and the neighbouring muscles; and, posteriorly, it gives origin to the long nerves which form the ventral chain. The latter is composed of either one or two equal nervous cords, and the maximum of ganglia which it contains is eleven, in which case one is allotted to each segment. Frequently, however, they are concentrated in the thorax, and the abdomen entirely deprived of them. The mode of their distribution is most regular in larvæ. Each of them sends off nervous filaments, commonly amounting to three pair on each side; those in the thorax supply the wings and legs, those of the abdomen the muscles with which its cavity is furnished, and the hinder abdominal one transmits branches to the organs of generation. When ganglia do not exist