The midgut extends from the proventriculus to the point of entry of the Malphigian tubes. It consists of an anterior thoracic narrow portion and a larger abdominal posterior portion, the so called stomach: this latter becomes greatly distended after feeding. The convexity of the anterior portion lies dorsalward, so that the ventral is shorter. While Christophers states that the cecal tubes or appendages found in many insects in the anterior portion of the midgut are absent, Schaudinn says in Anopheles a considerable number are always present, but that in Culex (pipiens) they are only slightly represented.
The epithelium consists of a single layer of large cells, columnar in the fasting insect, but flattened when the stomach is distended. Their most striking feature is the clear, striated border which is characteristic of the midgut epithelium, for it is not found elsewhere. In the distended organ this appearance is wanting. The cells are granular and stain deeply with hematoxylin. The granules are collected" mainly toward the free border of the cell, and are especially well seen in the anterior portion of the midgut. Besides the granules, there occur a number of vacuoles or drops which also are best developed at the free border of the cells. The cells are evidently glandular in character; they have, besides, a deeply staining nucleus, centrally situated. The muscular layer of the gut is less well developed than in the midgut. It consists of an open meshwork of fibers running longitudinally and circularly, forming, especially in the posterior portion, a regular arrangement of rhomboidal meshes. The muscle consists of long, fusiform, striated fibers. It is embedded in a certain amount of connective tissue containing also elastic fibers. Besides these layers we find, on the outside of the gut, numerous tracheal cells or large fusiform cells in which the tracheae end. The midgut ends in a marked constriction, formed by a development of muscle fiber forming a distinct pylorus.
This commences at a point which corresponds externally to the junction of the sixth and seventh abdominal segments, and is the point at which the five Malpighian tubes enter. It is divided into three portions: (a) The ileum, a dilated portion in which the muscular layers are strongly developed, while the epithelium is flat; this passes insensibly into (b) the colon, which is the longest and thinnest portion of the hind gut. It passes dorsalward and then descends centrally again, forming a broad, open curve. It then narrows, finally dilating into (c) the rectum.
The whole of the hind gut is covered with a chitinous cuticle. This is most strongly developed in the first part, viz., the ileum. From the muscular layer proceed ingrowths between the epithelial cells, producing thickenings of the chitinous cuticle, giving the gut a characteristic thread like appearance. Into the rectum project about six so called rectal glands. They are really conic outgrowths of the epithelium.
The Malpighian tubes opening at the junction of the midgut and hind gut are five in number. They pass forward in loops above their origin, their blind ends being attached in the neighborhood of the rectum by tracheal branches. They are composed of granular cells, with a large, deeply staining nucleus. The cells are alveolar in structure, the alveoli being arranged at right angles to the lumen of the tube. During the resting stage they contain fluid, while during secretory activity they contain spheric granules which are passed out into the lumen of the tube. The lumen is a fine, spirally arranged tube. The spiral arrangement is produced by the alternate bulging of the cells of the Malpighian tube, so that at one level practically only one cell is seen cut across, the others being flattened and displaced, the lumen being also displaced by the bulging, to reappear at the other side when the cell of the opposite side bulges.
These are intrathoracic structures lying close up against the prosternum. They consist of six tubular acini, three lying on each side, and are just visible to the naked eye in dissections, being about 1 mm. in length. The acini lie one above the other in the long axis of the body. The upper and middle acini at their distal ends almost touch the proventriculus, while the lower acini lie close to the conjoined thoracic ganglia. The ventral diverticulum of the esophagus lies between the acini on either side. The acini themselves lie embedded in portions of the fat body. Each acinus is traversed by a duct. These three ducts at the base of the acini widen, to form a common duct. This duct passes upward to the neck and meets its fellow from the other side beneath the sub esophageal ganglion. They then unite to form a common duct, which passes forward and enters the salivary receptacle which lies at the base of the labium. This is a hollow, cone shaped structure, with its apex pointing forward and downward. It is chitinous in structure, though the wall is thin and transparent on the dorsal surface and its basal wall is still more membranous. This receptacle is acted upon by muscles on either side arising from the ventral surface of the lower chitinous plate of the pharynx. By their contraction the cavity is dilated and saliva can flow in during relaxation, the walls of the receptacle or pump probably resuming their original condition. The apex of the receptacle passes upward and enters into the hypopharynx by a V-shaped opening, at the point of origin of the latter from the ventral chitinous wall of the pharynx.
The acini, though usually three in number in most genera, are five in number in Psorophora. They may become bifid at their extremities, and it is not uncommon to find a small accessory acinus given off from the basal portion. Each gland is composed of a single layer of cells lying on a basement membrane; the cells are bounded internally by the wall of the intraglandular cluct. The course of this duct is somewhat different in Culex and Anopheles; while in the latter there is a dilatation near the base of the duct, in Culex this is not so, but it remains of the same caliber throughout until it opens into the extraglandular portion. The extra acinar duct is a thick walled tube bearing a spiral chitinous thread in its internal layer. It has a superficial resemblance to a trachea, and has been mistaken for this structure. The main ducts leaving the acini are lined with an apparently homogeneous but nucleated intima. The character of the glandular epithelium varies in the different acini, both in Culex and Anopheles; they have been termed by Christophers the granular and the clear type respectively.