(929). The silk-secreting apparatus of such genera as possess the means of spinning a silken thread is peculiar to the larvae; and, after the commencement of the pupa state, no traces of its previous existence are to be detected.

(930). But while the above-mentioned organs disappear, others become developed; and the perfect insect is found to possess viscera for which a skilful anatomist might seek in vain in the earlier stages of its existence. The generative system appears, at first, to be absolutely wanting in the larva; but Herold*, after much patient investigation, succeeded in detecting the undeveloped rudiments of the future sexual organs both of the male and female. It is during the maturation of the pupa that these important parts expand; and before the disclosure of the imago they are found to have attained their complete proportions, so as to be ready to perform their functions as soon as the expansion of the wings endows the insect with means of locomotion sufficiently perfect to ensure the due dispersion of the species.

(931). It is in the nervous system, however, that the most interesting phenomena are observable; and in the lessons afforded by watching the correspondence between the state of the animal during the several phases of its existence and the development of the nervous ganglia, the physiologist cannot fail to recognize those great and general principles upon which our arrangement of the animal creation is based. In the worm-like larva the ganglia are numerous, but of small dimensions - too feeble to be capable of animating powerful limbs, or of appreciating impressions from the organs of the higher senses; the animal is, in fact, precisely in the condition of an Annelidan, which it would seem to represent. External limbs are therefore absolutely wanting in many larvae; in others they are represented by short and stunted appendages; and even in the most perfect, or hexapod larvae, they are feeble instruments in comparison with those of the mature imago. The senses exhibit equal imperfection; and eyes are either entirely wanting, or are mere ocelli - simple specks, exhibiting the lowest possible organization of a visual apparatus. But as the growth of the larva goes on, a change in the arrangement of the nervous system is perpetually in progress.

The series of nervous cords connecting the different pairs of ventral ganglia in the larva (fig. 182, a) become flexuous as the insect attains the pupa state; the whole chain becomes shorter; the brain, or encephalic ganglion, increases in its proportionate dimensions; and, moreover, several ganglia, originally distinct, coalesce, and form larger and more powerful masses (fig. 182, b.) This coalescence of the ganglia, which takes place more especially in the thoracic region, is evidently a preparation for the concentration of greater power and activity in this part of the body; and although in inactive chrysalides this change is not as yet visible by its effects, in the active forms even the pupa is distinguished from the larva by a considerable increase of vigour and energy in its movements. In the imago the concentration of the nervous centres is carried to that extent which is adapted to the necessities of the mature state: their number is still further reduced (fig. 182, c); their size, in the thorax especially, considerably increased; and the brain, now arrived at its maximum of development, is furnished with the wonderful apparatus of eyes, and other instruments of the senses, which heretofore would have been absolutely useless, but now, with the expansion of the brain, have become suited to the more exalted faculties of the insect.

* Entwickelungsgeschichte der Schmetterhnge. 1815, 4to.

Changes which the nervous system undergoes during the progress of Insect metamorphosis.

Fig. 182. Changes which the nervous system undergoes during the progress of Insect metamorphosis. A, Nervous system of the Larva. B, that of the Pupa. C, that of the Imago or perfect insect.

(932). Many insects are capable of producing audible sounds; and sometimes the noises they make are exceedingly shrill, and may be heard at some distance. Such sounds originate from various causes in different tribes, and it is not always easy to detect the mode of their production. In many Beetles they are caused by rubbing different parts of their dense integument against each other; and the chirping of several Orthoptera seems to have a similar origin: the acute note that these insects utter is apparently produced by friction, the edges of their hard pergamentaceous wings being either scraped against each other, or against the long and serrated edges of their thighs. The buzzing and humming noises heard during the flight of many genera result from the forcible expulsion of the air as it streams through the respiratory spiracles, whose orifices Burmeister imagines are furnished with vibratory laminae, to the rapid movements of which the noise may be due. In the genera Qryllus and Cicada among the Orthoptera, however, there is a peculiar apparatus specially provided for the production of the loud chirping to which such insects give utterance.

Upon the first segment of the abdomen, covered by a broad moveable plate (fig. 183, a), there is a large aperture, wherein a tense plicated membrane is observable. This membrane is acted upon internally by certain muscles able to throw it into rapid vibration, and thus give rise to the sound in question.

(933). One other point connected with this interesting class of animals requires brief notice. Many insects are endowed with the faculty of emitting phosphorescent light, which is in some species exceedingly brilliant. The Elateridae among Beetles are pre-eminently luminous; and in them the light seems to be principally given out by two oval spaces upon the thorax, which in the dead insect are of a greenish hue: during life, some species (Elater noctilucus) are so strongly phosphorescent as to enable a person to read a book by passing the animal over the lines. The Lampyrides emit a light of great brilliancy; and in Italy, during the summer nights, the groves, illuminated by their incessant scintillations, exhibit a scene equally strange and beautiful. The females of Lampyris noctiluca have two large yellowish-white luminous plates upon the ventral surface of the sixth and seventh abdominal rings, and, besides these, two minute organs of a similar description on the eighth or caudal segment. The latter only (and these of a smaller size and greyish transparent hue) are present in the males.