In larval insects and others (Synaptera, Orthoptera, etc.), in which the digestive canal is simplest, it is scarcely longer than the body, and passes through it as a straight tube.
In the caterpillar, which is a voracious and constant feeder, the digestive canal is a large straight tube, not clearly differentiated into fore-stomach, stomach, and intestine; but in the imago, which only takes a little liquid food, it is slender, delicate, and highly differentiated. In the larva the mid-gut forms the largest part of the canal; in the imago, the intestine becomes very long and coiled into numerous turns; at the same time the food-reservoir (the “sucking stomach”) develops, and the excretory tubes are longer.
a. The digestive canal
Fig. 303.—Interior view of the bottom of the head of Danais archippus, the top having been cut away, showing, in the middle, the pharyngeal sac with its five muscles: the frontal (f.m), dorsal pair (d.m), and the lateral pair (l.m); cl, clypeus; cor, cornea; œ, œsophagus; p.m, one of the large muscles which move the labial palp.—After Burgess.
It will greatly simplify our conception of the anatomy of the digestive canal if we take into account its mode of origin in the embryo, bearing in mind the fact that during the gastrula condition the ectoderm is invaginated at each pole to form the primitive mouth and fore-gut (stomodæum) and hind-gut (proctodæum). The cells of the ectoderm secrete a chitinous lining (intima), which forms the continuation of the outer chitinous crust, and thus the lining of each end of the digestive canal is cast whenever the insect molts; while the mid-intestine (mesenteron), arising independently of the rest of the canal much later in embryonic life from the mesoderm, is not the result of any invagination, being directly derived from the mesoderm, and is not lined with chitin.
The mouth, or oral cavity, and pharynx.—This is the beginning of the alimentary bounded above by the clypeus, and labrum, with the epipharynx, and below by the hypopharynx, or tongue, as well as the labium. Into it pour the secretion of the salivary glands, which passes out through an opening at the base of the tongue or hypopharynx. On each side of the mouth are the mandibles and first maxillæ.
The sucking or pharyngeal pump.—This organ has been observed by Graber in flies and Hemiptera, but the fullest account is that by Burgess, who was the first to discover it in Lepidoptera. In the milk-weed butterfly (Danais archippus) the canal traversing the proboscis opens into a pharynx enclosed in a muscular sac (Figs. 303, 304, and 310).
The pharyngeal sac, says Burgess, serves as a pumping organ to suck the liquid food through the proboscis and to force it backwards into the digestive canal.
