Fig. 601.—Nervous system of the larva of Sphinx ligustri.
Fig. 602.—Nervous system of the pupa of Sphinx ligustri, soon after pupation.—This and Fig. 601, after Newport.
The change in the internal organs.—These were especially, as regards the nervous system, first carefully examined and illustrated by that great English entomotomist, Newport, and those of the reproductive organs by Herold as early as 1815. A glance at the figures (598–604), reproduced from Newport’s article Insecta, will show the changes wrought especially in the digestive and nervous systems of Sphinx and Vanessa, his account of the alterations in the muscles having already been quoted. As the pupal form is much nearer to that of the imago than of the larva, so the digestive canal is seen to be nearly as much differentiated in the pupa as in the imago, though the reservoir (“sucking-stomach”) of the imago is not indicated in the pupa. These changes are such as occur in an insect which is enormously voracious as a larva, and which often, passing through a period of complete inactivity, taking no food at all, finally becomes an insect which needs to suck in only a minimum quantity of water or nectar, and which practically abstains from all food. The head and genital glands also, as well as the urinary vessels, are nearly the same. On the other hand, the salivary glands have undergone, in the imago, a thoroughgoing reduction.
The changes undergone by the nervous system of Sphinx ligustri and Vanessa urticæ have been described by Newport with fulness of detail. An abstract of his observations on Vanessa urticæ, which undergoes its changes in June in 14 days, and in August in eight days, we will now give, in part verbatim, the subject being rendered much clearer by his figures, which are reproduced.
During the last larval stage, certain changes have already taken place in different parts of the cord, which shows that they had been a long time in progress. Besides the lateral approximation of the cords, the first change consists in a union of the 11th and 12th ganglia, the latter one being carried forwards; these two ganglia being entirely separate before the 3d moult.
Two hours after the larva of Vanessa urticæ has suspended itself in order to pupate, the brain is not yet enlarged, but the subœsophageal ganglion is nearly twice its original size and the ganglia behind are nearer together. “A little while before the old larval skin is thrown off there is great excitement throughout the body of the insect.” About half an hour (Fig. 603, 2) before this occurs the alary nerves and the cerebral, 2d, 3d, 4th, and 5th ganglia are slightly enlarged, and the 1st subœsophageal ganglion very considerably. Immediately after the insect has entered the pupa state (Fig. 603, 3), all the ganglia are brought closer together. One hour after (Fig. 603, 4) pupation the cerebral ganglia are found to be more closely united, the 4th and 5th ganglia are nearer, and the distance between the remaining ganglia is also reduced.
Seven hours after pupation there is a greater enlargement of the cerebral ganglia, optic nerves, and ganglia and cords of the future thoracic segments.
At 12 hours (Fig. 603, 5) the 5th pair of ganglia has almost completely coalesced with the cord and the 4th; at 18 hours (Fig. 603, 6) the whole of the ganglia, cords, and nerves have become more enlarged, especially those of the wings, while the 4th and 5th ganglia of the cords have now so completely united as to appear like an irregular elongated mass. At 24 hours (Fig. 604, 7) the 4th and 5th ganglia are completely united, the 5th being larger than the 4th. At 36 hours (Fig. 604, 8) the optic nerves have attained a size almost equal to that of the brain. The 1st subœsophageal ganglion now forms, with the cerebral ones, a complete ring around the œsophagus, the crura having almost disappeared. The 6th ganglion has now disappeared, but the nerves arising from it remain. At 48 hours (Fig. 604, 9) the cord is straight instead of being sinuous, and the 7th ganglion has disappeared, while the thoracic ganglia are greatly enlarged. At the end of 58 hours the 2d and 3d thoracic ganglia have united, and the double ganglion thus formed is only separated from the large thoracic mass composed of the 4th, 5th, and part of the 6th ganglia, by the short but greatly enlarged cords which pass on each side of the central attachment of the muscles. “The optic and antennal nerves have nearly attained their full development, and those numerous and most intricate plexus of nerves in the three thoracic segments of the larva form only a few trunks, which can hardly be recognized as the same structures. The arrangement of the whole nervous system is now nearly as it exists in the perfect insect. The whole of these important changes are thus seen to take place within the first three days after the insect has undergone its metamorphosis; and they precede those of the alimentary canal, generative system, and other organs, which are still very far from being completed, and indeed, as compared with the nervous system, have made but little progress.” (Art. Insecta, pp. 962–965.)