(7) “Eine Linguatula aus der Mesenterialdrüse des Schafes und Dromedars als zweites ungesche. Stadium von Pent. taenioides,” Notiz. u. Tagsber. u. d. Geb. d. Nat. u. Heilk. Jena, 1862, v, pp. 127, 128, Colin.
B. INSECTA (Hexapoda).
Three separate regions can always be distinguished in the body of insects, namely, the head, thorax and abdomen. The HEAD is a roundish unsegmented capsule and possesses four pairs of appendages. The first pair are the various shaped feelers (antennæ), which are placed on the superior surface of the head next to the eyes; then more ventrally placed a pair of upper jaws (mandibles) without palpi and without articulations; they are powerful masticatory organs.[359] The first pair of lower jaws (maxillæ) are jointed and bear a palpus (palpus maxillaris); the second pair of maxillæ are soldered together and form the lower lip (labium), and likewise carry a palpus labialis on each side. The upper lip (labrum), as well as the other parts (which, however, are only appendages), belong to the mouth, which is really formed of a number of closely united pieces. The mouth parts are modified according to the functions required of them. Coleoptera, Neuroptera, and Orthoptera have biting or masticatory mouth parts which conform with the scheme described above. In the licking mouth parts of the Hymenoptera the maxillæ and under lip are considerably elongated, while the mandibles retain their form and are used for triturating the food; in the Lepidoptera nearly all the mouth parts are shortened except the maxillæ, which form a long and sometimes spirally rolled suctorial proboscis; the Diptera and Rhynchota have piercing and sucking mouth parts. The mandibles and maxillæ are metamorphosed into needle-like structures, while the suctorial apparatus is formed by the labrum.
The thorax consists of three segments, which are frequently united; ventrally it carries three pairs of legs, which consist of a definite number of articulated pieces joined together. Their form also changes according to their function, so that legs for running, walking, digging, swimming, jumping, and preying are seen. A pair of wings are respectively attached to the last and last but one thoracic rings, and these may be traced back, not to metamorphosed appendages, but to tracheal branchia. They are composed of chitinous membranes supported by branched structures (veins or ribs). Their size and formation vary; they are seldom of equal size and form (Neuroptera); often the posterior wings are larger than the anterior wings, the former then only serving as protective coverings for the latter (Coleoptera), or the anterior wings are larger (Lepidoptera), or the posterior wings are shortened or are entirely absent (Diptera); and finally there are insects in which both pairs of wings are lacking.[360]
The abdomen retains its segmentation, but, with the exception of a few groups related to the primitive forms of insects, has no appendages in the imago condition; the abdomen usually consists of ten segments, on the last of which the anus is situated.
We need only observe the following characters in considering the anatomy of insects:—
The EPIDERMIS consists of the chitinous cuticle, which is separate from the cellular layer beneath (hypodermis); the various appendages are supported by the chitinous layer.
The INTESTINAL CANAL usually consists of the anterior, median and terminal intestine, and as a rule passes straight through the body; salivary glands discharge into the anterior part, and, in some cases, yield a stiffening secretion which serves for spinning webs; numerous or scanty hepatic tubes are appended to the median intestine, while on the border between the median and terminal intestine open four to six long tubes (vasa malpighiana), which act as urinary organs. Finally the end portion of the intestine carries various glands (anal and rectal glands, etc.).
The CENTRAL NERVOUS SYSTEM agrees in structure with that of the Annelids, but is more highly developed. The pharyngeal ring surrounds the front part of the intestine; the sensory nerves originate from its SUPERIOR PHARYNGEAL GANGLIA and are the seat of the higher psychical functions; the INFERIOR PHARYNGEAL GANGLIA govern the mouth parts, and in addition appear to regulate the movements (cerebellum) of the vertebrates.