Malpighian Tubules Rectum

The Malpighian tubules and rectum, functioning as a unit, form the major excretory system in most insects. Details of the rectum are given in Chapter 16, Section 3.4, and only the structure of the tubules is described here.

The blindly ending tubules, which usually lie freely in the hemocoel, open into the alimentary canal at the junction of the midgut and hindgut (Figure 18.1A). Typically they enter the gut individually but may fuse first to form a common sac or ureter that leads into the gut. Their number varies from two to several hundred and does not appear to be

FIGURE 18.1. (A) Excretory system of Rhodnius. Only one Malpighian tubule is drawn in full; (B) junction of proximal and distal segments of a Malpighian tubule of Rhodnius. Part of the tubule has been cut away to show the cellular differentiation; (C, D) sections of the wall of the distal and proximal segments, respectively, of a tubule; and (E) tip of Malpighian tubule of Apis to show tracheoles and spiral muscles. [A, B, E, after V. B. Wigglesworth, 1965, The Principles of Insect Physiology, 6th ed., Methuen and Co. By permission of the author. C, D, from V. B. Wigglesworth and M. M. Saltpeter, 1962, Histology of the Malpighian tubules in Rhodnius prolixus Stal. (Hemiptera), J. Insect Physiol. 8:299-307. By permission of Pergamon Press Ltd.]

FIGURE 18.1. (A) Excretory system of Rhodnius. Only one Malpighian tubule is drawn in full; (B) junction of proximal and distal segments of a Malpighian tubule of Rhodnius. Part of the tubule has been cut away to show the cellular differentiation; (C, D) sections of the wall of the distal and proximal segments, respectively, of a tubule; and (E) tip of Malpighian tubule of Apis to show tracheoles and spiral muscles. [A, B, E, after V. B. Wigglesworth, 1965, The Principles of Insect Physiology, 6th ed., Methuen and Co. By permission of the author. C, D, from V. B. Wigglesworth and M. M. Saltpeter, 1962, Histology of the Malpighian tubules in Rhodnius prolixus Stal. (Hemiptera), J. Insect Physiol. 8:299-307. By permission of Pergamon Press Ltd.]

closely related to either the phylogenetic position or the excretory problems of an insect. Malpighian tubules are absent in Collembola, some Diplura, and aphids; in other Diplura, Protura, and Strepsiptera there are papillae at the junction of the midgut and hindgut. With the tubules are associated tracheoles and, usually, muscles (Figure 18.1E). The latter take the form of a continuous sheath, helical strips, or circular bands and are situated outside the basal lamina. They enable the tubules to writhe, which ensures that different parts of the hemolymph are exposed to the tubules and assists in the flow of fluid along the tubules.

A tubule is made up of a single layer of epithelial cells, situated on the inner side of a basal lamina (Figure 18.1B-D). In many species where the tubules have only a secretory function (Section 3.2) the histology of the tubules is constant throughout their length and basically resembles that of the distal part of the tubule of Rhodnius (Figure 18.1C). The inner (apical) surface of the cells takes the form of a brush border (microvilli). The outer (basal) surface is also extensively folded. Both of these features are typical of cells involved in the transport of materials and serve to increase enormously the surface area across which transport can occur. Numerous mitochondria occur, especially adjacent to or within the folded areas, to supply the energy requirements for active transport of certain ions across the tubule wall. In many species various types of intracellular crystals occur which are presumed to represent a form of storage excretion (Section 3.3). Adjacent cells are closely apposed near their apical and basal margins, though not necessarily elsewhere.

In some insects (e.g., Rhodnius), two distinct zones can be seen in the Malpighian tubule (Figure 18.1C, D). In the distal (secretory) zone the cells possess large numbers of closely packed microvilli, but very few infoldings of the basal surface. Mitochondria are located near or within the microvilli. In the proximal (absorptive) part of the tubule the cells possess fewer microvilli, yet show more extensive invagination of the basal surface. The mitochondria are correspondingly more evenly distributed. In the flies Dacus and Drosophila, where pairs of Malpighian tubules unite to form a ureter prior to joining the gut, the ultrastructure of the ureter resembles that of the proximal part of the Rhodnius tubule, suggesting that the ureter may be a site of resorption of materials from the urine.

Yet other species have even more complex Malpighian tubules in which up to four distinct regions may be distinguished on histological or ultrastructural grounds. On the basis of the structural features of their cells, these regions have been designated as secretory or absorptive, though it must be emphasized that physiological evidence for these proposed functions is largely lacking. For a survey of insects whose tubules show regional differentiation and a discussion of tubule function in such species, see Jarial and Scudder (1970).

A cryptonephridial arrangement of Malpighian tubules is found in larvae and adults of many Coleoptera, some larval Hymenoptera and Neuroptera, and nearly all larval Lepidoptera (Figure 18.2). Here the distal portion of the Malpighian tubules is closely apposed to the surface of the rectum and enclosed within a perinephric membrane. The system is particularly well developed in insects living in very dry habitats, and in such species its function is to improve water resorption from the material in the rectum (Section 4.1).

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Beekeeping for Beginners

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