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.
(825). The first is met with among the Hemiptera, and is formed to perforate the stalks and buds of vegetables, in order to procure the juices which they contain; or, in some bugs, it is employed to puncture the integument of living animals for a similar purpose. This kind of mouth is exhibited in fig. 156. First, there is a long jointed sheath (d), which is in fact the lower lip (labium) considerably elongated, and composed of three or four parts articulated together; secondly there is a small conical scale covering the base of the sheath last mentioned, and representing the upper lip; and between these are four slender and rigid bristles or lancets (scalpella) (c), that, when not in use, are lodged in a groove upon the upper surface of the sheath, so as to be concealed from view. These lancets are, in reality, only the mandibles and maxillae strangely altered in their form and excessively lengthened, so as not merely to become efficient piercing instruments, but so disposed as to form by their union a suctorial tube, through which animal or vegetable fluids may be imbibed. This kind of mouth, when not employed, is usually laid under the thorax, between the legs, in which position it is easily seen in most Hemiptera. In some families, as, for example, in the Plant-lice (Aphides), it is of extraordinary length: thus, in the aphis of the oak it is three times as long as the whole body of the insect, projecting posteriorly like a tail; and in the fir-aphis it is still longer.
(826). The second kind of mouth is that met with among the Diptera; and from its construction in some tribes, we may well understand how they are enabled to become so seriously annoying. The Gnat and the Mosquito furnish sufficiently well-known examples of the formidable apparatus in question, which in the Horse-fly (Tabanus) seems to attain its maximum of development. The oral organs of the Diptera are composed of a sheath or proboscis, that represents the lower lip of the mandibulate insects: it is sometimes coriaceous or horny in its texture; in other cases, as in the common Flesh-fly, soft and muscular, and folds up when at rest in such a manner as to form two angles, representing the letter Z. At the base of this sheath or proboscis there is a small upper lip, between which and the sheath are lodged the setae, knives, or lancets, which form such terrible instruments for cutting or piercing the skin of their victims. These cutting parts vary in number from one to five: when they are all present, the upper pair (cultelli, or knives) represent the mandibles of a perfect mouth, the two lower ones (scalpella, the lancets) are the maxillae, the fifth or middle piece (glossa-rkttn) is the tongue; and between them all is the oral opening.
The strength of the above piercing instruments varies greatly: in the Gnat they are finer than a hair, very sharp, and barbed occasionally on one side; while in the Horse-fly they are flat, like the blades of a lancet or penknife; occasionally they are so constructed as to form a tube by their union, through which the liquid aliment is sucked up and conveyed into the stomach.
Fig. 156. Mouth of Notonecta.
* Memoires sur les Animaux sans Vertebres. 8vo. Paris, 1816. 1 Kirby and Spence, vol. iii. p. 463.
Fig. 157*. Head of a Flea.
* Head of the Flea, as represented by the Solar microscope in Canada balsam; dedicated by permission to the President and Members of the Entomological Society, by W. Lens Aldous.
(827). The mouth of the Flea, although described by Kirby and Spence as forming a distinct type of structure, differs very little from that of the Diptera described above, as will be at once evident on inspecting the figure in the preceding page, reduced from a beautiful drawing by Mr. W. Lens Aldous.
(828). In this insect the piercing organs are two sharp and razorlike instruments (fig. 157, d d), placed on each side of the elongated tongue (e), and enclosed in a sheath (c c), probably formed by pieces representing the mandibles of mandibulate insects. Two palpi or feelers (a a) and a pair of triangular plates (b b) complete this remarkable apparatus.
(829). Another kind of mouth adapted to suction, and which seems to differ more widely from the perfect form than any we have as yet examined, is that which we meet with in Moths and Butterflies. This singular organ is adapted to pump up the nectareous juices from the cups of flowers, and is necessarily of considerable length, in order to enable the insect to reach the recesses wherein the honeyed stores are lodged. When unfolded, the apparatus in question represents a long double whip-lash (fig. 158, a, b, c, d); and if carefully examined under the microscope, each division is found to be made up of innumerable rings connected together, and moved by a double layer of spiral muscular fibres, that wind in opposite directions around its walls. When not in use, the proboscis is coiled up and lodged beneath the head; but when uncurled, its structure is readily examined. Each of the two long filaments composing this trunk (which, in fact, are the representatives of the maocillae excessively lengthened) is then seen to be tubular; and, when they are placed in contact, it is found that their edges lock together by means of minute teeth, so as to form a central canal leading to the orifice of the mouth. It is through this central tube, formed by the union of the two lengthened maxillae, that fluids are imbibed. Burmeister, however, asserts that the cavities contained in each division likewise communicate with the commencement of the oesophagus, so that the Lepidoptera have, as it were, two mouths, or rather two separate methods of imbibing nourishment - one through the common canal formed by the junction of the whip-like jaws, the other through the cavities of the filiform maxillae themselves; such an arrangement, however, which would be quite anomalous, may reasonably be doubted.