Page:Popular Science Monthly Volume 9.djvu/740

712 increased size of the cerebral ganglia, and also to further development of some of the ganglia pertaining to the ventral cord, with concentration or even suppression of others.

In such insects as butterflies, bees, and dragon-flies, in which the visual organs are enormously developed, and in which the power of vigorous and sustained flight is correspondingly increased, the nervous system attains its maximum of development among the Arthropoda. The brain of these creatures differs from that existing in all other members of the class by. reason of the great development of those portions of it in relation with the visual organs. A ganglionic swelling is frequently found where the nerve joins the brain (Fig. 6, B,)and in some insects there are also small ganglionic swellings at the corresponding parts of the antennal nerves.

As in spiders, the œsophageal ring is very narrow, owing to the greatly-diminished size of the œsophagus in the imago forms of higher insects. The double upper or cerebral ganglion is, however, connected in all insects with a separate sub-œsophageal ganglion, from which nerves are given off to the mandibles, the maxilæ, and the labium, though in spiders, crustaceans, and myriapods, as I have before stated, this part has no existence separate from the thoracic ganglia.

In insects the three thoracic ganglia also often preserve a separate existence (Fig. 6), though in such higher types as I have named above



these ganglia are more frequently fused into a single, lobed mass. The eight abdominal ganglia, which are always much smaller than the thoracic, continue to have a separate existence among some of the less developed types of insects, though it is more frequent for some, or even all, of them to disappear.

The stomato-gastric system also attains considerable complexity