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.
(916). Having thus become acquainted with the various conditions under which insects arrive at maturity, and the principal forms that they exhibit during the different stages of the metamorphosis, the reader will be prepared to investigate more minutely the changes in progress during the process, and the gradual development of the organs which successively make their appearance. On examining the viscera of a Caterpillar, they are found scarcely at all to resemble those of the Butterfly or Moth, into which a larva of this description is ultimately matured. The jaws (fig. 180, b b), widely different, both in structure and office, from the proboscis which represents them in the perfect insect (fig. 160), are strong and horny shears adapted to cut the leaves of vegetables and other coarse materials used as food; the oesophagus (fig. 178, g h) is strong, muscular, and capacious; and the stomach h i), in capacity corresponding with the extraordinary voracity exhibited by the larva, passes insensibly into a wide intestine (I m), the line of separation being only indicated by the entrance of the biliary vessels (k) that wind in numerous convolutions around the posterior half of the alimentary canal. It is sufficient to contrast this arrangement of the digestive organs with what we have already described in the Butterfly (fig. 160), to appreciate the amazing dissimilarity: it would be difficult, indeed, to imagine, did not anatomy convince us of the fact, that the digestive apparatus of the imago, with its slender oesophagus, dilated crop, short sacculated stomach, long and convoluted small intestine, and capacious colon, was derived from a gradual modification of such viscera as those we have just been considering. The salivary glands of the Caterpillar (fig. 178, q, r) are large cylindrical caeca; and their ducts (p) pour into the mouth an abundance of saliva proportioned to the coarse nature of the materials used as food.
Fig. 178. Viscera of a Caterpillar: g h, oesophagus; h i, stomach; k, hepatic vessels; I m, intestine; q,r, salivary glands; p, salivary duct; a b c, longitudinal tracheal trunks; d,eeee, air-tubes distributed to the viscera; fff, the epiploon or fat-mass; v xy, silk-secretors; z, their excretory ducts, terminating in t, the spinneret or fusulus.
(917). The sides of the body are traversed by the wide longitudinal tracheae (a b c), that communicate on the one hand with the lateral spiracles, and on the other give off at regular intervals the air-tubes (d,eee e), which ramify most minutely over all the viscera, and convey the atmospheric air throughout the entire system.
(918). Besides the above organs, there are other viscera, which, although of considerable importance to the Caterpillar, would be utterly useless to the imago, and consequently are more or less completely wanting in the mature state.
(919). The whole body of the larva is filled with a peculiar fatty tissue (fig. 178,fff), called by entomologists the rete, epiploon, or fat-mass. This material, found in great abundance in mature and well-fed larvae, consists of an oily or greasy substance enveloped in a most delicate cellulosity, and seems to correspond to the fat of higher animals, like which it is indubitably a product of digestion, and a repository of superabundant nourishment, stored up, no doubt, for the sustenance of the animal during its helpless condition in the dormant or pupa state - serving, like the fat of hibernating quadrupeds, for food during the confinement of the imago.
(920). But the most remarkable peculiarity of the larvae under consideration is the presence of an apparatus employed for producing a tenacious thread of extreme delicacy, appropriated by different species to various purposes. In many cases (fig. 148), it is made subservient to locomotion; and by its assistance, as by a rope, the larva can suspend itself from any object, or let itself down from one branch to another in search of food. The most important uses, however, to which this thread is applied are connected with the concealment and protection of the quiescent and defenceless pupa; either furnishing the means of suspending the chrysalis in a place of safety* (fig. 179), or, as is the case Fig. 179 (in the next page) illustrates the different steps attending the process. The larva (a), having spun some loose silk, and fixed it upon the under side of a leaf or other suitable object, suspends itself therefrom by its hind-legs. The skin of the caterpillar then gradually splits down the back (b, c), and is slowly pushed upwards towards the tail of the chrysalis. The with the Silkworm (fig. 175), supplying the material with which the caterpillar encases itself preparatory to throwing off the last skin of the larva. The thread of the last-named insect, the Silkworm, is of great tenacity; and, notwithstanding its fineness, may be wound off from the cocoon in a continuous thread, forming the important article of commerce, silk.
* For a most amusing account of the manner in which some chrysalides manage, without any external limbs, to suspend themselves by the tail in a position of security, the reader is referred to Kirby and Spence, vol. iii. p. 207.
(921). Nothing can be more simple than the apparatus provided in caterpillars for the production of this valuable commodity: - Placed on each side of the intestine are two long and tortuous secreting caeca (fig. 178, vxy), that separate from the surrounding juices of the body a tenacious viscid fluid, which is liquid silk. The viscid secretion thus formed is in the Silkworm of a golden-yellow colour, and is conveyed by the excretory ducts of the secerning organs (z) to the labium or under-lip, where the ducts terminate at the base of a tubular instrument, the fusulus or spinneret, through which the silk is drawn (fig. 180, c.) The fusulus of the Silkworm, represented in the annexed figure upon an enlarged scale, is a simple nipple-shaped prominence, perforated at its extremity, and surrounded by four rudimentary palpi. When about to spin, the larva, by placing the extremity of its spinneret in contact with some neigh-bouring object, allows a minute drop of the glutinous secretion to exude from its extremity, which, of course, adheres to the surface upon which it is placed: the head of the Silkworm being then slowly withdrawn, the fluid silk is drawn out in a delicate thread through the aperture of the spinneret, its thickness being regulated by the size of the orifice, and, immediately hardening by the evaporation of its fluid parts, forms a filament of silk, which can be prolonged at the pleasure of the animal until the contents of its silk-reservoirs are completely exhausted. (922.) Such is the structure of the larva of a Lepidopterous insect; and the arrangement of its internal viscera, when arrived at maturity, has been already described. We have yet, however, to mention the series of phenomena observable during the progress of its growth, and the mode of its expansion from the minute size that it exhibits on leaving the egg to the full dimensions which it ultimately acquires.