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.
The mouths of all creatures are constructed upon purely mechanical principles; and in few classes of the animal world have we more beautiful illustrations of design and contrivance than in that before us: jaws armed with strong and penetrating hooks for seizing and securing active and struggling prey - sharp and powerful shears for clipping and dividing the softer parts of vegetables - saws, files, and augers for excavating and boring the harder parts of plants - lancets for piercing the skin of living animals - siphons and sucking-tubes for imbibing fluid nutriment - all these, in a thousand forms, are met with in the insect world, and thus provide them with the means of obtaining food adapted to their habits, and even of constructing for themselves edifices of inimitable workmanship.
Fig. 154. Muscles of the leg of a Cockchafer.
The mouths of insects may be divided into two great classes: - those which are adapted for biting, forming what is called a perfect or mandibulate mouth; and those which are so constructed as only to be employed in sucking, constituting the suctorial, haustellate mouth. It is in the former of these divisions that all the parts composing the oral apparatus are most completely developed; we shall therefore commence by describing the different pieces of which a perfect mouth consists, viz. an upper and an under lip, and four horny jaws. We select the mouth of the Dragon-fly (fig. 155, a) as an example. The upper lip (labrum, b) is a somewhat convex corneous plate, placed transversely across the upper margin of the cavity wherein the jaws are lodged, so that, when the mouth is shut, it folds down to meet the under lip (labium), and these two pieces more or less completely conceal the proper jaws, which are lodged between them.
Fig. 155. Parts of the mouth of a Dragon-fly.
(819). The upper pair of jaws (mandibuloe) are two hard and powerful hooks (fig. 155, c), placed immediately beneath the upper lip, and so articulated with the cheeks that they move horizontally, opening and shutting like the blades of a pair of scissors. Their concave edge is armed with strong denticulations of various kinds, sometimes furnished with cutting edges that, like sharp shears, will clip and divide the hardest animal and vegetable substances; sometimes they form sharp and pointed fangs, adapted to seize and pierce their victims; and not unfre-quently they constitute a series of grinding surfaces, so disposed (like the molar teeth of quadrupeds) as to triturate and bruise the materials used as food. The variety of uses to which these mandibles can be turned is indeed amazing. In the carnivorous Beetles, their hooked points, more formidable than the teeth of the tiger, penetrate with ease the mailed covering of their stoutest congeners; and in the Dragon-fly they are scarcely less formidable weapons of destruction. In the Locust tribes these organs are equally efficient agents in cutting and masticating leaves and vegetable matters adapted to their appetites; while in the Wasps and Bees they form the instruments with which these insects build their admirable edifices, and, to use the words of a popular author, supply the place of trowels, spades, pickaxes, saws, scissors, and knives, as the necessity of the case may require.
(820). Beneath the mandibles is situated another pair of jaws, of similar construction, but generally smaller and less powerful; these are called the maxillae (fig. 155, f).
(821). The lower lip, or labium (fig. 155, e), which closes the mouth inferiorly, consists of two distinct portions, usually described as separate organs, - the chin (mentum), that really forms the inferior border of the mouth; and a membranous or somewhat fleshy organ, reposing upon the chin internally, and called the tongue (lingua) of the insect (d).
(822). All these parts enter into the composition of the perfect mouth of an insect, and, from the numerous varieties that occur in their shape and proportions, they become important -guides to the entomologist in the determination and distribution of species. For more minute details concerning them, the reader is necessarily referred to authors who have devoted their attention specially to this subject; we must not, however, omit to mention certain appendages or auxiliary instruments inserted upon the maxillae and the labium, usually named the palpi, or feelers, and most probably constituting special organs of touch, adapted to facilitate the apprehension and to examine the nature of the food. The maxillary feelers (palpi maxillares) are attached to the external margin of the maxillae by the intervention of a small scale and very pliant hinge, and consist of several (sometimes six) distinct but extremely minute pieces articulated with each other. The labial feelers (palpi labiales) are inserted into the labium close to the tongue, or occasionally upon the chin (mentum) itself.
The joints in the labial palpi are generally fewer than in the maxillary, but in other respects their structure and office appear to be the same.
(823). In the suctorial orders of insects we have the mouth adapted to the imbibition of fluid nutriment, and consequently constructed upon very opposite principles; yet, notwithstanding the apparent want of resemblance, it has been satisfactorily demonstrated by Savigny* that the parts composing a suctorial mouth are fundamentally the same as those met with in the mouths of mandibulate insects, but transformed in such a manner as to form a totally different apparatus.
(824). According to the distinguished authors of the 'Introduction to Entomology1,' there are five kinds of imperfect mouth adapted to suction, each of which will require a separate notice.