The sub-œsophageal ganglia correspond in the main, as we have already stated, with the medulla oblongata of vertebrate animals, and their fusion with the thoracic ganglia in the Arachnida, as well as in the Crustacea and Myriapoda, confirms the view held by some anatomists, that the medulla should be regarded as a prolongation of the spinal cord, rather than as an integral part of the brain.
Fig. 6.—Nervous System of an Insect (Acrida viridissima).
The nervous system of insects varies not only among different classes and orders, but even in the same individual, in different stages of its development. The larva, or caterpillar, of a butterfly, for instance, presents a nervous system not very different from that met with in the centipede; while in the imago stage, or perfected insect, the same system has undergone some remarkable changes, leading to
