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.
In this mouth, therefore, all the parts, except the maxilla), would seem at first sight to be wanting; they may nevertheless be detected upon a very careful examination, and rudiments of the upper lip, of the mandibles, of the lower lip, as well as of the labial and maxillary palpi, be distinctly demonstrated.
Fig. 158. Mouth of a Butterfly.
(830). The last kind of mouth to which we shall advert is that met with in the Louse tribe (Pediculi); but, from the extreme minuteness of the parts composing it, the details of its structure are only imperfectly known. It seems to consist of a slender external tube, wherein a sharp sucker, armed with barbs adapted to fix it securely during the act of sucking, is lodged; when feeding, the barbed piercer is denuded and plunged into the skin, where it is retained until a sufficient supply of nourishment has been obtained.
(831). Inviting as the subject is, we are compelled, by the strictly general character of our investigations, to abstain from entering upon further details concerning the mouths of perfect insects, and consequently to omit noticing innumerable secondary modifications in the mechanical structure of the oral organs of these little animals. When we turn our attention to the consideration of their internal viscera, connected with the preparation and digestion of so many different materials, we may well expect to find equal variety of conformation; and, in fact, the course, dimensions, and relative proportions of the alimentary canal will be seen to be different, to a greater or less extent, in almost every species. Considered as a whole, the internal digestive apparatus of insects must be regarded as a delicate membranous tube, in which the digestion of the substances used as food is accomplished partly by mechanical and partly by chemical agency. For the former purpose, gizzard-like muscular cavities are not unfrequently provided; and to fulfil the second, various fluids are poured into the canal in different parts of its course.
The arrangement of the cavities and the nature of the secreting vessels will, however, be modified in conformity with the necessities of the case, and certain parts will be found to exist, or to be deficient, as circumstances may require; it would be absurd, therefore, to attempt to describe particular examples; our observations must be of general application, and such as will enable the reader to assign its proper function to any organ which may present itself to his notice. The first part of the digestive apparatus is disposed in the same manner in all insects, and is a slender canal arising from the mouth and passing straight through the thorax into the cavity of the abdomen; this portion represents the oesophagus (fig. 159, a a; fig. 160, o.) The stomach and intestine succeed to this; and if the body of the insect be very thin, their course also passes nearly in a direct line to the tail. But in those families which have the abdomen thick and largely developed, especially if herbivorous, the intestine becomes much elongated and winds upon itself in various convolutions; nevertheless, however tortuous the canal may be, its windings are never sustained by any mesentery or peritoneal investment: the air-tubes (that, as we shall afterwards see, permeate the body in all directions) form a sufficient bond of connexion, and one which is better adapted to the wants of these animals.
(832). We must now examine more minutely the different portions of which the alimentary canal may consist, premising at the same time that the structures mentioned do not invariably exist together, as sometimes one part and sometimes another may be entirely wanting, or only found in a very rudimentary condition. They are, the crop, the gizzard, the stomach, the small intestine, and the large intestine.
(833). The crop, or sucking-stomach, as it is called by some writers, is only met with in Hymenoptera, Lepidoptera, and Diptera - insects which have no gizzard*. In Bees, Wasps, and other Hymenoptera, it is a simple bladder-like distention of the oesophagus (fig. 159, b); in Butterflies and Moths it forms a distinct bag, that opens into the side of the gullet (fig. 160, v v); while in the Diptera it is a detached vesicle, appended to the oesophagus by the intervention of a long thin duct. This organ, which in Bees is usually called the honey-bladder, is regarded by Burmeister (who founds the opinion upon the result of experiments made by Treviranus upon living insects) as being not merely a receptacle for food, resembling the craw of birds, as Ramdohr1 and Meckel consider it, but as being a sucking instrument for imbibing liquids, by becoming distended, as he expresses it, and thus, by the rarefaction of the air contained within it, facilitating the rise of the fluids in the proboscis and oesophagus. It must, however, be confessed that there is something very anomalous in the idea of a delicate bag having the power of distending itself: its muscular walls might indeed contract; but that a thin sacculus should forcibly expand itself would be a fact new to physiology.
Fig. 159. Alimentary canal of the Honey Bee, Apis mellifica. a a, oesophagus; b, the crop or sucking-stomach; c, d d, the stomach proper; e e small intestine; f, large intestine; g, anal orifice; h h, biliary vessels; i i, auxiliary glands.
* Burmeister, op. cit. p. 125. Treviranus, Vermischte Schriften.
1 Ramdohr, Ueber die Verdauungswerkzeuge der Insekten. Halle, 1811.
(834). The gizzard is found in insects which possess mandibles and live upon solid animal or vegetable substances. It is a small round cavity with very strong muscular parietes, situated just above the stomach properly so called, and, like the gizzard of granivorous birds, is employed for the comminution of the food preparatory to its introduction into the digestive stomach. In order to effect this, it is lined internally with a dense cuticular membrane, and occasionally studded with hard plates of horn, or strong hooked teeth, adapted to crush or tear in pieces whatever is submitted to their action.