Of this nature are the ovipositors of Ichneumons and other insects, and the sting of Bees and Wasps. In the Earwig (Forficula) these caudal appendages form a pair of forceps; whilst in many Insects they are in the form of bristles, by which powerful leaps can be effected, as is seen in the Spring-tails (Podurae). In some insects (as the Mole-cricket and Cockroach), the ninth or tenth abdominal segment carries jointed antenniform appendages, which, though perhaps partially or even primarily generative in function, are certainly organs of sense, being connected with smell or hearing.
The organs about the mouth in Insects are collectively termed the "trophi," or "instrumenta cibaria." Two principal types require consideration - namely, the masticatory and the suctorial - both types being sometimes modified, and occasionally combined.
In the Masticatory Insects, such as the Beetles (fig. 176, A), the trophi consist of the following parts, from before backward: 1. An upper lip, or "labrum," attached below the front of the head; 2. A pair of biting-jaws, or "mandibles;" 3. A pair of chewing-jaws, or "maxillae," provided with one or more pairs of "maxillary palps," or sensory and tactile filaments; 4. A lower lip, or "labium," composed of a second coalescent pair of maxillae, and also bearing a pair of palpi, the "labial palps." The primitive form of the labium - that, namely, of a second pair of maxillae - is more or less perfectly retained by the Orthoptera and some of the Neuroptera.
Fig. 176. - Organs of the mouth of Insects, enlarged ; (A) of a Beetle (Carabics)', (B) of the small Cabbage White Butterfly (Pontia rapae); (C) of the Bed-bug (Cimex lectu-larius), the mandibles and maxillae being displaced to one side. l Labrum; m Mandible ; mx Maxilla ; mp Maxillary palpus; la Labium ; Ip Labial palpus; an Base of one of the antennas. (Fig. B is slightly altered from Westwood.)
The lower or basal portion of the labium is called the "men-turn," or chin, whilst the upper portion is more flexible, and is termed the "ligula." The central portion of the ligula is often developed into a kind of tongue, which is very distinct in some Insects (as in Bees), and is termed the "lingua."
In the typical suctorial mouth, as seen in the Butterflies (fig. 176, B) the following is the arrangement of parts: The labrum and the mandibles are now quite rudimentary; the first pair of maxillae is greatly elongated, each maxilla forming a half-tube. These maxillae adhere together by their inner surfaces, and thus form a spiral "trunk," or "antlia " (inappropriately called the "proboscis"), by which the juices of flowers are sucked up. Each maxilla, besides the half-tube on one side, contains also a tube in its interior; consequently on a transverse section the trunk is found really to consist of three canals, one in the interior of each maxilla, and the third formed between them by their apposition. To the base of the trunk are attached the maxillary palpi, which are extremely small. Behind the trunk is a small labium, composed of the united second pair of maxillae. The "labial palpi" are greatly developed, and form two hairy cushions, between which the trunk is coiled up when not in use.
In the Bee there exists an intermediate condition of parts, the mouth being fitted partly for biting, and partly for suction. The labrum and mandibles are well developed, and retain their usual form. The maxillae and the labium are greatly elongated; the former being apposed to the lengthened tongue in such a manner as to form a tubular trunk, which cannot be rolled up, as in the Butterflies, but is capable of efficient suction. The labial palpi are also greatly elongated.
In the Hemiptera, the "trophi" consist of four lancet-shaped needles, which are the modified mandibles and maxillae, enclosed in a tubular sheath formed by the elongated labium (fig. 176, C). Lastly, in the Diptera - as in the common House-fly - there is an elongated labium, which is channelled on its upper surface for the reception of the mandibles and maxillae, these being modified into bristles or lancets.
The mouth in the Masticating Insects leads by a pharynx and oesophagus into a membranous, usually folded, stomach - the "crop," or "ingluvies" - from which the food is transmitted to a second muscular stomach, called the "gizzard" (fig. 177). The gizzard, or proventriculus, is adapted for crushing the food, often having plates or teeth of chitine developed in its walls, and is succeeded by the true digestive cavity, called the "chylific stomach" (ventriculus chylopoieticus).
From this an intestine of variable length proceeds, its terminal portion, or rectum, opening into a dilatation which is common to the ducts of the generative organs, and is termed the "cloaca." The oesophagus is furnished with salivary glands of varying size and complexity, which open into the cavity of the mouth. Besides the proper salivary glands, the larvae of Insects very usually possess a pair of silk-glands, which discharge their secretion by a single duct, furnished with a spinneret, and developed upon the labium. Rarely (as in Myrmeleo), there are silk-glands opening in the abdominal region. Those silk-glands which open into the mouth are to be regarded as modified salivary glands, and they are almost invariably confined to the larvae. No true liver is present, but the stomach is lined by secreting cells, which appear to exercise an hepatic function. Behind the pyloric aperture of the stomach, with very few exceptions, is a variable number of caecal convoluted tubes (fig. 177, e), which open into the intestine, and are called the "Malpighian tubes." These vessels are now generally regarded as discharging a renal function, and as corresponding with the kidneys of the higher animals. There are no absorbent vessels, and the products of digestion simply transude through the walls of the alimentary canal into the sinuses or irregular cavities which exist between the abdominal organs. The apparatus of digestion does not differ essentially from the above in any of the Insects; but the alimentary canal is, generally speaking, considerably lengthened in the herbivorous species.