href="@public@vhost@g@gutenberg@html@files@43579@[email protected]#Footnote_36_36" class="fnanchor pginternal" tag="{http://www.w3.org/1999/xhtml}a">[36]; but in many other insects numerous variations in this respect occur.—Thus in the stag-beetle the last internode is single[37]; in the caterpillar of the cabbage butterfly (Pontia Brassicæ) the five first are double, and the six last single[38]; in that of the great goat-moth (Cossus ligniperda) the three first only are double, but the others terminate in a fork[39]; in the cockroaches (Blatta) the four first, in Hydrophilus piceus the three first, and in Eristalis tenax the two first only are double, the rest being all single[40]. A singular variation takes place in Hypogymna dispar; all the internodes are single, except the second, the chords of which at first are separate, and afterwards united[41]; and, to name no more, in Clubiona atrox there is only one internode, which is single, with a longitudinal furrow[42]. In some, as in the louse, the grub of Oryctes nasicornis, and the cheese-maggot, there are no internodes, the spinal marrow being formed of knots separated only by slight or deep constrictions[43].

I must next say something of the ganglions[44]. Lyonet has observed that, in the caterpillar of the great goat-moth, these in one respect differ remarkably from the chords that connect them; in the latter the air-vessels or bronchiæ only cover the outside of the tunic, while in the former they enter the substance of the ganglion, which is quite filled with their delicate and numberless branches[45]. Every ganglion may be regarded in some degree as a centre of vitality or little brain[46], and in many cases, as well as the brain, they are formed of two lobes[47]. I shall now consider them more particularly as to their station, number, and shape.

1. With regard to the first head, their station, they are most commonly divided between the trunk and abdomen; but in some cases, as in Hydrophilus piceus and Acrida viridissima, the first ganglion is in the head[48]; in others, as in the louse, the water-scorpion, and the grub of the rhinoceros-beetle, they are confined to the trunk, their functions in the abdomen being supplied by numerous radiating nerves[49]; in others again, as in the scorpion, they are all abdominal. The ganglions vary also in their situation with respect to each other. Thus in some, as in the larva of the Chamæleon-fly (Stratyomis Chamæleon), they are so near as to appear like a string of beads[50]; in that of the ant-lion (Myrmeleon) the two ganglions of the trunk are separated by an interval from those of the abdomen, which are so contiguous as to resemble the rattle of the rattle-snake[51]. In others the internodes are longer, and the ganglions occur at nearly equal intervals, as in the larva of the Ephemeræ[52]; but in the majority they are unequal in length: thus in the scorpion the three first ganglions are the most distant[53]; in the hive-bee the third and fourth[54]; and in the spider the last[55].

2. The ganglions also in different species, and often in the same insect in its different states, vary in their number. Thus in the grub of the rhinoceros-beetle the whole spinal marrow appears like a single ganglion divided only by transverse furrows[56]; in the water-scorpion there are two[57]; in the louse there are three[58]; in the rhinoceros-beetle there are four