Ileum. The ileo-caecal groove (Fig. 64, Icg) is very distinct and forms the junction between mesenteron and proctodaeum. The ileum is short and its inner surface is thrown into six irregular folds (Fig. 64, Fi) which project into the lumen. The wall of the ileum (Fig. 61) consists of a thin chitinous intima (Ci) beneath which is a layer of columnar epithelium (Ce) with oval nuclei. Connecting the epithelium to the muscle layers is a thin band of connective tissue (Ct). These three layers are thrown into six irregularly-shaped longitudinal folds of greater or less size, which become prominent near the posterior end, and are capable of being extended. Between these are six longitudinal bands of
Text-Fig., 15 Ts Hind Gut Fig. 61—Ts Ileum Fig. 62—Junction colon and rectum—wall cut open and reflected to show constriction Fig. 63—Enlargement of T.S. rectum showing “gland organ” Fig. 64—Junction mid and hind gut—internal showing ileo-caecal groove Fig. 65—Ts Malpighian tubules Fig. 66—T.S rectum showing ends of two rectum glands, “gland organ and cuboidal epithelium linking two glands
Bb, blush border, Bm, basement membrane. C. colon, Cbe. cubordal epithelium. Ct. columnai epithelium. Ci, chitinous intima, Cjcr. construction at Junction of colon and rectum. Cm. circular muscle, Cs, [ unclear: ] strands. Ct, connective tissue Fi, folds ot ileum. Go “gland organ”, Icg ileo-caecal groove, Lm, longitudinal muscle, M, mesenteron, Mt Malpighian tubules, Pm, peritoneal membrane, R, rectum.
columnar epithelium. There is a narrow band of striated circular muscle (Cm) and surrounding this are six bands of striated longitudinal muscle (Lm), which extend the full length of the hind gut. According to most authorities there is an inner layer of longitudinal muscle, but no trace of this has been seen in M. filifer.
Colon. Fig. 72. The colon is long and bent back upon itself. It consists of the same layers as the ileum. In the colon the six folds of the ileum are continued. Here they are large and of approximately equal size Between these folds are six longitudinal bands of columnar epithelium (Ce). There is a very thin layer of striated circular muscle (Cm) and outside this the six bundles of longitudinal muscle (Lm) which correspond to the longitudinal bands of columnar epithelium. There is no internal longitudinal muscle.
Rectum. There is a marked constriction at the junction of colon and rectum (Fig. 62, Cjcr). At the posterior end of the colon the epithelial folds become elongated and the epithelial layers between become narrower (Fig. 68). The striated circular muscle increases considerably in thickness so that it is capable of closing the aperture between colon and rectum At the termination of the colon the hind gut widens abruptly into the rectum. The chitinous intima (Ci) is thin and, unlike Hemideina, of an even thickness throughout the rectum (Fig. 67). The epithelium is of two kinds, that of the rectal glands proper and that between them. The latter is much reduced, consisting of six narrow, corrugated strips of thin cuboidal epithelium (Figs. 66, 67, Cbe) with small round nuclei. The presence of this cuboidal epithelium agrees with the statements of Maskell (1927) on Hemideina and Davis (1927) on Stenopelmatus, but differs from the results of Miall and Denny (1886) on Periplaneta and Cameron (1912) on Bacillus rossi where no epithelium is present beneath the corrugated intima Miall and Denny state that in these areas the chitinous lining blends with the basement membrane, while Cameron says that between each two bands is a non-epitheliated interspace, where the chitinous intima becomes corrugated and is closely applied to the basement membrane. There are six rectal glands (Fig. 67, Rg) which are continuations of the six longitudinal bands of columnar epithelium (Ce) of the colon The cell walls are distinct and the nuclei are oval The inner surface of the epithelium and the intima are thrown into ridges and folds. These rectal glands are similar to those of Hemideina and Stenopelmatus. According to Maskell they have no glandular structure but “agree with the description by Chun of rectal glands in Locusta viridissima (quoted by Packard, 1903) Minot has stated that Chun's description is applicable to the Acridiidae he has investigated, and he states that the rectal folds ‘do not offer the least appearance of glandular structure’” Between the rectal gland and the cuboidal epithelium is a structure similar to that described by Davis (1927) as a “gland organ” (Figs. 63, 66, Go). There is a distinct gap between the two epithelia which contains from four to seven small, lens-shaped cells (Go), consisting of a large nucleus surrounded by a thin layer of cytoplasm. On the side towards the lumen, cytoplasmic strands (Fig. 63, Cs) pass out from each cell and attach themselves singly to the chitinous intima (Ci). No thin plate of non-staining chitin for the attachment of these strands such as Davis describes has been observed. Cytoplasmic strands also leave the other side of each cell, and these strands unite to form a cord which passes into the connective tissue This is markedly different from Stenopelmatus where each cell gives off only one thread Here the threads are twined into a cord passing towards the lumen. Davis says,
Text-Fig. 16 Ts Hind Gut (Scale in mm) — Fig. 67—Ts rectum Fig. 68—Ts junction colon and rectum Fig., 69—Ts posterior end of rectum Fig. 70—Ts. mid anal region Fig. 71—Ts posterior anal region. Fig. 72—Ts colon Fig. 73—Ts fat body C, cytoplasm, Cbe, cuboidal epithelium. Ce columnar epithelium Ci [ unclear: ] . Cm, circular muscle, Ct connective tissue. Gua granules of [ unclear: ] , Lm longitudinal muscle; Og, oil globule, Rg, rectal gland, Vc, vacuoles
“I am unable to explain the function of this peculiar structure … These organs may function as mucus-glands (although they do not resemble mucus-tissue in other animals), or they may draw off the excess water from the faeces. The latter would be a logical theory since the insect is an inhabitant of semi-arid country”. As Macropathus lives under conditions of high humidity, and yet has a “gland” similar to that found in Stenopelmatus, it seems far more probable that the function of this “gland” is for secretion of mucus to assist in the expelling of faeces. The epithelium is joined to a thin layer of striated circular muscle (Cm), about three fibres thick, by well developed connective tissue The six bands of longitudinal muscle (Fig. 67, Lm) are present on the outer surface. The rectum narrows at its posterior end and passes into the anus. At their junction the rectal glands terminate abruptly and cuboidal epithelium (Fig. 69, Cbe) is found in the anal region. The connective tissue (Ct), epithelium (Cbe) and intima (Ci) are thrown into folds which gradually increase in size and number towards the anal aperture (Figs. 70, 71). The striated circular muscle (Cm) is greatly thickened, but no longitudinal muscle occurs.