What we have just said with regard to flight is equally applicable to terrestrial locomotion, which also represents in itself many varieties. The typal insect possesses three pairs of feet, whence the name of hexapods, but there are particular species which drop a pair of feet, for instance, the _Lepidoptera_ of the genus _Vanessa_; in others, the physiological function of the foot varies; in the case of the carrion-beetle (a necrophagus coleopter) it serves as an instrument of tillage, to dig with; for the cricket, the third pair of feet are used for the purpose of leaping; for the _Dytiscus_, it serves as an oar, and so on. We must also bear in mind the curious fact that there exists in the larvæ of certain insects what are called supplementary feet, having only a transient existence and disappearing at maturity; the caterpillar, the larva of the butterfly, has five pairs of supplementary feet. These notable facts demonstrated by comparative anatomy, cannot fail to furnish us with valuable information concerning the functions of the complex organs found in the ganglia of the thorax. But this is not all. We have not enumerated all the contributions of comparative anatomy to the problem which we are now about to consider; we may make use of the method of comparison without bringing the different types into juxtaposition, but by viewing the nervous system of only a single animal in its entirety. We know in fact that the body of an insect is formed by a definite number of segments, all constructed on the same fundamental plan and arranged in a linear series. Each one of these segments is joined to a nerve-ganglion, which is all its own and supplies it with sensibility and motility, the two elementary properties of nervous activity. In the course of development, these ganglia have the power of changing their positions; and it is not uncommon to find that the greater number of the abdominal ganglia move up into the thorax; each one, nevertheless, retaining its nerve-relationship to its own segment. Now all the segments of an insect’s body are not called upon to play the same rôle; a division of labor has been effected among them with regard to the functions which they are found to exercise: as we have already seen, the ganglia of the thorax are essentially centres of locomotion; in the head, one of the ganglia, the sub-œsophageal, furnishes the nerves of the buccal portions; the other one, the brain, is connected with particular nerves and becomes the centre of the highest form of psychical activity of which the creature is capable. We have here a number of modifications superadded to the original plan. Yet the original plan should again be met with in the ganglia that have been least differentiated, such as those in the abdominal region; and the comparison between an abdominal and a thoracic ganglion, for instance, is well calculated to show what are the primal and fundamental structures, and