This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(908). The first class comprises all insects of which the larva is a maggot entirely deprived of legs, that, after having changed its skin, or moulted, a certain number of times, becomes, previous to its last change, encased in an oval horny sheath, or pupa-case, whereon not the least trace of the limbs of the mature insect is to be detected. Such pupa3 are absolutely without the power of motion, and are distinguished by the name of coarctate. Examples of this sort of metamorphosis are met with in the common House-flies (Muscidae); and the forms of their larvae and pupae are familiar to every one.
(909). Of the second kind, technically named obtected, the Lepidoptera furnish well-known instances. The changes which occur in the development of the Silkworm, represented in the annexed figure (fig. 175), may readily be witnessed. In such insects the full-grown caterpillar, having enclosed itself in a silken ball, throws off its last skin, and becomes a quiescent pupa; but while in this state, the position of the rudiments of the wings and other appendages of the perfect insect is strongly indicated upon the exterior of the chrysalis (a), though these parts are still closely wrapped up in the external covering.
* Vide Smeathman, Phil. Trans, vol. lxxi. 1781.
Fig. 175. Metamorphoses of the Silkworm: A, Chrysalis.
(910). The third form of metamorphosis, called incomplete, is seen in the Hymenoptera, and in many Coleopterous insects. The maggot, in such tribes as exhibit this kind of change, is in some species a simple worm, deprived of feet or other external organs; in others these parts exist in a very imperfect condition: in the pupa, however, the form of the legs and antennae is perfectly distinct, and even the wings may be seen as rudiments projecting from the thorax. This kind of chrysalis we have seen in the Cockchafer (fig. 149, b), in which the grub (c) possessed feebly-de-vclopod legs; and in the Hive-bee, although the larva (fig. 176, a, c, d, e,f) has no legs or exterior appendages, in the pupa (b) all the limbs of the perfect Bee are recognized with the utmost facility. Yet all these organs are still enclosed in distinct cases (thecae), to each of which names have been applied by entomological writers; and it is only on throwing off the integument which thus imprisons the mature insect that the Bee makes its appearance in a capacity to begin its active and industrious existence in the winged state.
Fig. 176. Metamorphoses of the Hive-bee: a, full-grown larva; b, pupa; c, d, e, f, eggs and young, newly hatched.
(911). Those insects whose larva only differs from the imago in not being possessed of wings (fig. 145, c), Fabricius regarded as undergoing a semicomplete metamorphosis; and when the perfect insect did not acquire wings at all, but precisely resembled the pupa, he called the latter complete.
(912). But there are innumerable examples of metamorphosis which will not conform to any of the above definitions; and in some of them the phenomena exhibited are not a little remarkable. We have already mentioned the changes which the Dragon-fly undergoes (figs. 146,147), and have seen that in this case there is no very striking resemblance between the pupa and the adult creature, but, on the contrary, that very wonderful changes occur during the last stage of the metamorphosis. The pupa lives in water; and, besides six jointed legs adapted to climb the stems of subaquatic plants in search of prey, is possessed of a very peculiar locomotive apparatus whereby it can propel itself through the element which it inhabits. Appended to the posterior extremity of the abdomen we find three or five leaf-like appendages, which the creature continually opens and closes, and at the same time takes in a quantity of water, sufficient to fill the muscular termination of the rectum, which is expanded for the purpose; this water is, at intervals, forcibly expelled, mingled with bubbles of air, and thus effects the propulsion of the animal by a mechanism which human ingenuity has imperfectly attempted to imitate.
(913). But the contrivance above mentioned is also made subservient to respiration; for, from the observations of Cuvier*, it appears that the interior of the rectum exhibits to the naked eye twelve longitudinal lines of black spots arranged in pairs; and these, when examined under the microscope, are found to be composed of little conical tubes, from which branches go off to join the principal longitudinal tracheae that distribute air through the body.
(914). Another remarkable peculiarity is met with in the structure of the mouth of these aquatic larvae; for the oral apparatus here forms an instrument of prehension adapted to seize prey at a distance, and constitutes, in fact, a kind of projectile forceps of a very curious construction. Let the reader contrast the following description with that already given of the oral organs of the Dragon-fly (§ 818), and observe the remarkable difference: - "Conceive," says Kirby and Spence*, "your under lip to be horny instead of fleshy, and to be elongated perpendicularly downwards, so as to wrap over your chin and extend to its bottom; that this elongation is then expanded into a triangular convex plate attached to it by a joint, so as to bend upwards again, and fold over the face as high as the nose, concealing not only the chin and the first-mentioned elongation, but the mouth and part of the cheeks: conceive, moreover, that to the end of this last-mentioned plate are fixed two other convex ones, so broad as to cover the whole nose and temples; that these can open at pleasure, transversely, like a pair of jaws, so as to expose the nose and mouth, and that their inner edges, where they meet, are cut into numerous sharp teeth or spines, or armed with one or more long and sharp claws; - you will then have as accurate an idea as my powers of description can give of the strange conformation of the lip in the larvae in question, which conceals the mouth and face precisely as I have supposed a similar construction of your lip would do yours.