The mouth is situated anterior to the base of the hypopharynx and immediately behind the base of the clypeus. The mouth leads into the oesophagus, a narrow muscular tube, which extends upward in the anterior part of the head, then turns to pass posteriorly beneath the brain but above the tentorium and leaves the head by the occipital foramen. The oesophagus swells out into the very large crop (Fig. 46, C) which fills the whole thorax and the anterior part of the abdomen. It is used for storage of food. It is capable of considerable distention, and when empty or partially full its wall is thrown into folds. The surface of the cuticular lining of the crop contains many ridges and folds which increase in intensity near the posterior end (Fig. 49). Sections of the crop show slight indentations on the dorsal surface and much stronger ones on the ventral surface (Fig. 47). The stronger indentations have serrated margins forming small teeth. The wall of the crop consists of a thick chitinous intima (Figs. 47, 48, Ci) composed of two layers, an inner heavily staining layer (Fig. 53, Il) consisting of numerous fine indentations and chitinous setae, and a deeper, thicker stratified layer (Figs. 53, 54, Ol). Macropathus differs from Stenopelmatus by having indentations in the inner layer of the intima, while that of Stenopelmatus is smooth. The stratified layer is irregular in thickness. Below this is the chitin secreting epithelium (Fig. 54, Cse) resting on a basement membrane with the cell walls not clearly defined. Outside the epithelium is a thick layer of striated longitudinal muscle (Figs. 47, 48. Lm) and surrounding this is striated circular muscle (Figs. 47, 48. Cm). At its posterior end the crop narrows and its chitinous intima increases in thickness and is thrown into six evenly distributed folds, which form the junction of the crop with the gizzard (Fig. 49). Within the gizzard these folds become raised into prominent ridges each bearing 27 large primary teeth (Fig. 49, It), making a total of one hundred and sixty-two. On either side of each row of primary teeth is a row of smaller secondary teeth (Fig. 49, 2T) making twelve rows in all, each row bearing twenty-one teeth. These are the “i lobi a spazzoli” of Berlese (1909) or “barbated lobes” of Du Porte (1918). At the base of the fold, between each pair of adjacent rows of secondary teeth, is a long, dark, heavily chitinous ridge (Fig. 49, Cr) which bears no teeth Thus the full complement of proventricular teeth is four hundred and fourteen, of which one hundred and sixty-two are primary teeth and two hundred and fifty-two barbated lobes. This differs from Stenopelmatus fuscus, where there are two hundred and sixteen proventricular teeth, one hundred and thirty-eight being primary teeth and seventy-eight barbated lobes; and from Hemideina thoracica, where Maskell records one hundred and twenty primary teeth, but did not count barbated lobes. Internally the gizzard is constricted near its anterior end, the teeth on each ridge in this region being large, flattened apically and covered with yellow microtrichia (Fig. 52, Mt). The clefts between them are very shallow. On each ridge, anterior to the constriction, are seven teeth which grow progressively smaller towards the crop They are rounded, slightly cup-shaped on the posterior margin and covered with stiff posteriorly directed microtrichia The most anterior “teeth” project into the crop and are mere projections of tissue tipped with chitin and bristles. In the region anterior to the constriction there are no barbated lobes or long chitinous ridges. Posterior to the constriction the teeth are large, triangular and yellow-brown in colour, with their apices posteriorly directed and sharply pointed with

dark brown chitin. These teeth become progressively smaller as they approach the mesenteron, the posterior two teeth being smoothly rounded with no chitinous points and the clefts between them very shallow. Both the chitinous ridge and the barbated lobes run from the eighth to the twenty-seventh primary teeth. The six folds pass posteriorly from the gizzard into the lumen of the mesenteron so that the posterior end of each forms one of the six flaps of the cardiac valve (Figs. 59, 60). Three of the flaps are large (Figs. 49, 59, Llg), curving inwards to fit together, and form the main or primary section of the valve. Alternating with them are the other three shorter flaps (Fig. 59, Slg) which do not project as far into the mesenteron, but form a secondary basal section to the valve. In both fresh and preserved material they all appear a clear white. The valve acts as a filter, only allowing such food as is properly ground by the teeth to enter the mesenteron The structure of the valve in Macropathus differs from that described for Hemideina and Stenopelmatus. In all three insects there are six flaps, but in Stenopelmatus the two lateral flaps are longer than the other four, while in Hemideina there are two pairs of lateral folds and a dorsal and a ventral fold, each bearing a flap-valve.

Text-fig. 12. Gizzard Fig. 49—Gizzard cut open to show teeth, and convolutions at junction of crop and gizzard
Cr. chitinous ridges, Cwc convoluted wall of crop; Gw, gizzard wall, Llg, large lips of gizzard, 1T, primary teeth, 2T, barbated lobes.

The gizzard is composed of the same layers as the crop (Fig. 51) The chitinous layer is very thin between the teeth but thickens considerably over the teeth ridges, especially over the barbated lobes and the chitinous ridges Most of the surface of the chitin is produced into chitinous microtrichia. A thin layer of chitin-secreting epithelium (Figs. 51, 52, Cse) lies beneath the chitin Beneath the epithelium is a layer of connective tissue in which is embedded longitudinal muscle (Figs. 51, 52, Lm) The primary teeth have three pairs of lateral projections (Fig. 52, Lp) and the connective tissue and longitudinal muscle pass into each projection as well as into the apex of the tooth. They also line the barbated lobes and chitinous ridges. Lining each primary tooth, next to the longitudinal muscle, is a layer of circular muscle (Figs. 51, 52, Cm). The rest of the tooth cavity is hollow (Fig. 52, Cc). The circular muscle does

not project into the barbated lobes (Fig. 51, St) or into the chitinous ridges (Fig. 51, Cr). Surrounding the teeth ridges is a thick layer of unstriated circular muscle and outside this a conspicuous layer of peritoneum (Fig. 51, Pm). The histological structure of Macropathus differs from Stenopelmatus and Hemideina in the possession of a large tooth cavity, longitudinal muscle being present in barbated lobes and chitinous ridges, and the circular muscle not being striated.

Mesenteron. The mid gut consists of the intestine proper and two forwardly directed projections from it, the mesenteric caecae (Figs. 59, 60, Mc), which completely enclose the gizzard, one being dorsal to it, and the other ventral The epithelial layer of each caecum has several deep folds in it. which serve to increase the secretory surface. The histological structure of intestine and caeca is similar (Figs. 56, 57, 58)

The whole of the mesenteron (Figs. 56, 57, 58) is covered with a thin layer of peritoneum. There are three muscle layers—a thin outer layer of scattered smooth longitudinal muscle fibres (Lm) which are more numerous in the caeca than in the intestine proper, a middle thin layer of smooth circular muscle (Cm), which differs from Hemideina where it is striated, and an inner layer of smooth longitudinal muscle This inner layer of longitudinal muscle is not present in Hemideina but does occur in Stenopelmatus Stenopelmatus differs from both Hemideina and Macropathus in having all the muscles of the mid gut striated. Within the muscular layer is a layer of connective tissue (Ct) which, on its inner side, penetrates a little way between the epithelial cells forming thick basal walls or nests (“nidi”) (Cn).

The epithelium of the mid gut forms the greater part of the gut wall Within each of the nests formed by the connective tissue is a mass of cells, the lower ones of which are continually dividing to form new cells The nuclei are very crowded and flattened in the centre of the nest On each side of them, continuing up the nest walls, are the attenuated bases of the mature epithelial cells (Mec). As the cells are pushed outward and upward from the centre, they elongate and the nuclei become first round, then much elongated The outermost cells are fully mature and functional and should bear the striated hem which forms the lining of the intestine This striated hem, however, has never been observed in Macropathus filifer. The outermost cells of each nest eventually collapse during secretion and are lost, and the younger cells next to them are forced outward to fill the gaps Perfectly formed “nidi” are common in the caeca, but not so common in the intestine proper.

Several peritrophic membranes (Ptm) may be seen between the epithelium and the food contents Davis (1927) says, “The peritrophic membrane is an envelope of secreted substance that encloses the food material in the intestine of some insects It is elastic and non-cellular, so that it is not a true membrane”. There has been much discussion as to the origin of the peritrophic membrane Van Gehuchten (1890), working with Ptychoptera larvae, thought it to be a product of special glandular cells at the anterior edge of the mesenteron and Cuenot (1898), Vignon (1899) and Bordas (1905) agree with this Thus the peritrophic membrane would be a single continuous chitinous tube, being continuously formed at the posterior end of the stomodaeum and passing out of the anus surrounding the faeces. Imms (1925) says, “The results of recent research indicate that the peritrophic membrane is continuously secreted by a band of