494 type of PM, whether or not a PM is produced, and the manner in which it is produced, may vary between life stages (Lehane, 1997).

The PM is formed in two principal ways. In Type I PM delamination of successive concentric lamellae occurs along the midgut (in Odonata, Ephemeroptera, Phasmida, some Orthoptera, some Coleoptera, and larval Lepidoptera). The Type II PM forms by secretion from a special zone of cells (cardia) at the anterior end of the midgut (in Diptera, Dermaptera, Isoptera, Embioptera, and some Lepidoptera). In this method the esophageal invagination presses firmly against the anterior wall of the midgut so that the originally viscous secretion of the PM-producing cells, as it hardens, is squeezed to form the tubular membrane. In Dic-tyoptera, other Orthoptera and Lepidoptera, Hymenoptera, and Neuroptera, a combination of both methods seems to be used. In mosquitoes, larvae produce a Type II PM, whereas the adults have a Type I PM.

The PM is made up of a meshwork of microfibrils between which is a thin proteinaceous film. The microfibrils have a constant 60° orientation to each other in Type I PM, thought to result from their secretion by the hexagonally close-packed microvilli of the epithelial cells. In Type II PM the orientation of the microfibrils is random. The PM is permeable to the products of digestion and to certain digestive enzymes released from the epithelial cells (Section 4.2.1). However, it is not permeable to other large molecules, such as undigested proteins and polysaccharides, indicating that the PM has a distinct polarity and is not merely an ultrafilter (Richards and Richards, 1977; Lehane, 1997).

The midgut is usually not differentiated into structurally distinct regions apart from the development, at the anterior end, of a varied number of blindly ending ceca, which serve to increase the surface area available for enzyme secretion and absorption of digested material. In many Heteroptera, however, the midgut is divided into three or four easily visible regions. In the chinch bug (Blissus leucopterus) four such regions occur (Figure 16.5). The anterior region is large and saclike, and serves as a storage region (no crop is present). The second region serves as a valve to regulate the flow of material into the third region where digestion

FIGURE 16.5. Alimentary canal of chinch bug (Blissus leucopterus) showing regional differentiation of midgut. [After H. Glasgow, 1914, The gastric caeca and the caecal bacteria of the Heteroptera, Biol. Bull. 26:101-170.]

FIGURE 16.6. Alimentary canal of cercopid (Cercopoidea) showing filter chamber arrangement. [From R. E. Snod-grass, Principles of Insect Morphology. Copyright 1935 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Company.]

probably occurs. Ten fingerlike ceca filled with bacteria are attached to the fourth region, which may be absorptive in function. The role of the bacteria is not known.

In many homopterans, which feed on plant sap, the midgut is modified both morphologically and anatomically so that excess water present in the food can be removed, thus preventing dilution of the hemolymph. Though details vary among different groups of ho-mopterans, the anterior end of the midgut (or, in some species, the posterior part of the esophagus) is brought into close contact with the posterior region of the midgut (or anterior hindgut), and the region of contact becomes enclosed within a sac called the "filter chamber" (Figure 16.6). Such an arrangement facilitates rapid movement of water by osmosis from the lumen of the anterior midgut across the wall of the posterior midgut and possibly also the Malpighian tubules. Thus, relatively little of the original water in the food actually passes along the full length of the midgut.

The lack of morphological differentiation within the midgut of most species is reflected in its uniform histology. Throughout its length, the mature cells lining the lumen are identical and serve to produce digestive enzymes, to absorb the products of digestion, and in some insects secrete the Type IPM. Replacement of degenerate cells occurs with the maturation and differentiation of regenerative cells found singly or in groups (nidi) near the base of the epithelium (Figure 16.4). Numerous peptide hormone-containing cells also occur in the midgut, which may play a role in modulating midgut contraction (Lange and Orchard, 1998).

In some species histological differentiation is found. For example, specialization of certain anterior cells for Type I PM production was noted earlier. In addition, differentiation into digestive and absorptive regions occurs in some species. In tsetse flies the cells of the anterior midgut are small and are concerned with absorption of water from the ingested blood. They produce no enzymes and digestion does not begin until food reaches the middle region where the cells are large, rich in ribonucleic acid, and produce enzymes. In the posterior midgut the cells are smaller, closely packed, and probably concerned with absorption of digested food. In some species different regions of the midgut are apparently adapted to the absorption of particular food materials. In Aedes larvae the anterior midgut is concerned

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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