Fig. 297.—Transverse section through an abdominal segment of larva of Megalopyge crispata, showing the relations of the digestive canal to the other organs: int, hind-intestine, with its mucous or epithelial layer (ep), and ml its outer or muscular layer; ng, ventral ganglion; ht, heart; mp, urinary tubes; f, fat-body; sc, thickened portion of the hypodermis (hy) containing the setigenous cells; m, muscles; m′, a pair of retractor muscles inserted near the base of the lateral glandular process (lgp); cut, cuticula; l, legs. Also compare Figs. 142–144 and 234.
Fig. 298.
The alimentary or digestive canal of insects is a more or less
straight tube, which connects the mouth and anus, the latter invariably
situated in the last segment of the body, under the last
tergite or suranal plate. It lies directly over the ventral nervous
cord and under the dorsal vessel, passing through the middle of the
body (Fig. 297). It is loosely held in place by delicate retractor
muscles (retractores ventriculi, found by Lyonet in the larvæ of
Lepidoptera, and occurring in those of Diptera), but is principally
supported by exceedingly numerous branches of the main tracheæ.
Fig. 298.—Internal anatomy of Melanoplus femur-rubrum: at, antenna and nerve leading
to it from the “brain” or supraœsophageal ganglion (sp); oc, ocelli, anterior and vertical ones,
with ocellar nerves leading to them from the brain; œ, œsophagus; m, mouth; lb, labium or under
lip; if, infraœsophageal ganglion, sending three pairs of nerves to the mandibles, maxillæ, and
labium respectively (not clearly shown in the engraving); sm, sympathetic or vagus nerve, starting
from a ganglion resting above the œsophagus, and connecting with another ganglion (sg) near the
hinder end of the crop; sal, salivary glands (the termination of the salivary duct not clearly
shown by the engraver); nv, nervous cord and ganglia; ov, ovary; ur, origin of urinary tubes;
ovt, oviduct; sb, sebaceous gland; bc, bursa copulatrix; ovt, site of opening of the oviduct (the
left oviduct cut away); 1–10, abdominal segments. The other organs labelled in full.—Drawn from
his original dissections by Mr. Edward Burgess.
Fig. 299.—Digestive canal of Anabrus: m, mouth: œ, œsophagus; sm, the sympathetic nerve passing along the crop; t, tongue; fg, frontal ganglion; br, brain, the nervous cord passing backward from it; sr, salivary reservoir; sg, salivary gland; pv, proventriculus; ur, origin of urinary tubes; sb, sebaceous gland; 1–10, the ten abdominal segments.—Burgess del.
It is in the higher adult insects differentiated into the mouth and pharynx, the œsophagus or gullet, supplementary to which is the crop (ingluvies) or “sucking stomach” of Lepidoptera, Diptera, and Hymenoptera; the proventriculus or gizzard; the ventriculus, “chyle-stomach,” or, more properly, mid-intestine, and the hind-intestine, which is divided into the ileum, or short intestine, the long intestine, often slender and coiled, with the colon and the rectum. Morphologically, however, the digestive or enteric canal is divided into three primary divisions, which are indicated in the embryo insect; i.e., the fore-intestine (stomodæum of the embryo), mid-intestine or “chyle-stomach,” and hind-intestine or proctodæum (Fig. 300). The three primary regions, with their differentiations, may be tabulated thus:—
| Fore-intestine (Stomodæum). | Mouth and pharynx. |
| Pumping apparatus of Hemiptera, Lepidoptera, and Diptera. | |
| Œsophagus. | |
| Crop or ingluvies, food reservoir, or “sucking stomach.” | |
| Proventriculus. | |
| Mid-intestine (Mesenteron). | Mid-intestine, “chylific stomach,” or ventriculus (with cœcal glands). |
| Hind-intestine (Proctodæum). | Ileum, or short intestine (with the urinary tubes). |
| Long intestine. | |
| Colon. | |
| Rectum (with rectal glands). | |
| Anus (with anal glands). |
Fig. 300.—The three primary divisions of the alimentary canal of an embryonic orthopterous insect: br, brain; sbg, subœsophageal ganglion; ng, nervous cord; st, stomodæum; pr, proctodæum; mv, malphigian tubes; mesen, mid-intestine; ht, heart; md, mandibles; mx, mx′, 1st and 2d maxillæ.—After Ayers, with some changes.
The appendages of the alimentary canal are: (1) the salivary and poison glands, which arise from the stomodæum in embryonic life; (2) while to the chylific stomach a single pair of cœcal appendages (Orthoptera and larval Diptera, e.g. Sciara), or many cœca may be appended; (3) the urinary tubes, also the rectal glands and the paired anal glands. In a Hemipter (Pyrrhocoris apterus) appendages arise from the intestine in front of the origin of the urinary tubes. In certain insects a single cœcal appendage (Nepa, Dyticus, Silpha, Necrophorus, and the Lepidoptera) arises from the proctodæum.