Note on a New Organ, and on the Structure of the Hypodermis, in Periplaneta Orientalis
Authors: Minchin Edward A. 
Published: April 2021

The organ which I have to describe, and of which I have been unable to find an account in the latest works on the anatomy of the Cockroach, consists of two pouch-like invaginations of the cuticle lying close on each side of the middle line, between the fifth and sixth terga of the dorsal surface of the abdomen. In the normal condition these are covered by the fifth tergum, but when this is removed they are seen as two shallow pouches lying beneath the transparent intertergal membrane and opening to the exterior by two slit-shaped openings, which face backwards (fig. 1). They are lined by a continuation of the chitinous cuticle, which forms within the pouches numerous stiff, branched, finely-pointed hairs, beneath which, i. e. on the side towards the body-cavity, are numerous glandular epithelial cells. Fig. 2 shows a section longitudinal to the body of the animal passing through one of the pouches. The figs, v and vi are placed above the fifth and sixth terga respectively; d. denotes the upper dark layer, and t. the lower transparent layer of the chitinous cuticle, and h. the hypodermis of at least two layers, and resting on the basement membrane (b.m.). At p., the most posterior point of the fifth tergum, the transparent layer of the cuticle and the uppermost layer of the cells of the hypodermis are continued into the intertergal membrane (i. m.), which again becomes continuous with the tergum immediately behind, the whole forming a continuous chitinous investment. The letter a., figs. 1 and 2, denotes the most anterior extremity of the sixth tergum, to which the longitudinal tergal muscles (m. 1 and m. 2) are attached; r. is a ridge close behind this part, the space between a. and r. being usually overlapped by the hinder part of the fifth tergum. These parts are present between every pair of terga, but between the fifth and sixth are found in addition the peculiar glandular pouches (P., figs. 1 and 2) above mentioned. As may be seen, they are lined by a cuticle continuous with that of the intertergal membrane (c. l.), which is produced into numerous stiff hairs, which bend towards the opening of the pouch. Below this cuticle is a layer of small flattened cells, with somewhat irregularly-shaped nuclei; this layer is evidently continuous with the hypodermic layer beneath the intertergal membrane (figs. 2 and 4, u. l.). These cells are crowded very close together, and their outlines are very hard to make out. Beneath this layer again comes a number of columnar cells with very distinct outlines, forming one layer on the dorsal side, and two or more on the ventral side of the pouch (l. l.). These cells have large rounded nuclei, each with a distinct nucleolus, and their cell contents are very granular. Outside all is a basement membrane continuous with that of the hypodermis. The tergal muscles pass immediately under the pouch, but I have satisfied myself, both by sections and by dissecting carefully, that the pouches have no special muscles. In fig. 3 are shown some of the hairs highly magnified. They are hollow and swollen at the base, and in a chloride of gold preparation processes of the cells of the upper layer (u. l.) can be seen running into them from below; sometimes nuclei are to be seen inside the swollen base. These hairs exactly resemble those found lining the abdominal stigmata, which are branched in exactly the same manner. Their chitinous nature is evident from the fact that if one of the pouches be macerated for a sufficient time in caustic potash (30 per cent.) nothing is left but the hairs and the cuticle on which they stand.