Nitrogenous Excretion And Salt And Water Balance

hemolymph osmotic pressure more or less constant. Like freshwater species, they produce dilute urine and actively resorb salts through the rectal wall. Mosquitoes from these habitats may also take up ions via the anal papillae. However, when the external osmotic pressure rises above that of the hemolymph, leading to loss of water from the body by osmosis, saltwater and brackish-water species employ different strategies.

Saltwater mosquitoes, on which most experimental work has been done, counteract the loss of water by drinking the external medium; for example, A. taeniorhynchus larvae ingest 240% of their body weight per day (Bradley, 1987). The water taken in, and the ions it contains, are then absorbed across the midgut wall, a process that may serve to concentrate food in the midgut prior to digestion (Phillips et al., 1978). Thus, the problem for these insects is to rid themselves of the excess ions that enter the hemolymph. As in insects from other habitats, the Malpighian tubules produce a potassium-rich fluid isosmotic with the hemolymph, and their main function appears to be the secretion of sulfate. Formation of hyperosmotic urine occurs as a result of active secretion of ions across the wall of the rectum, which in these species is divided into anterior and posterior segments (Figure 18.9). The posterior segment appears to be the more important, secreting sodium, chloride, magnesium, and potassium ions into the rectal lumen. The role of the anterior segment is less clear, though exchange of bicarbonate for chloride ions occurs here, as may the resorption of selected inorganic and organic solutes (especially in larvae in dilute media). As a result of these activities, saltwater mosquitoes are able to strongly regulate their hemolymph osmotic pressure and ionic content over a wide range of external concentrations (Figure 18.8).

In contrast to saltwater forms, brackish-water insects become osmoconformers in external media more concentrated than their hemolymph; that is, their hemolymph osmotic pressure increases approximately parallel with that of the surrounding medium (Figure 18.8). Unlike the situation in freshwater species, however, where osmoconformation is largely because of increases in hemolymph inorganic ion levels (mainly those of sodium

FIGURE 18.9. Excretory system of saltwater mosquito larvae. Known pathways of active ion transport are shown. AR, anterior rectal segment; MG, midgut; MT, Malpighian tubule; PR, posterior rectal segment. [After T. J. Bradley, 1987, Physiology of osmoregulation in mosquitoes, Annu. Rev. Entomol. 32:439-462. Reproduced, with permission, from the Annual Review of Entomology, Volume 32 (§1987 by Annual Reviews, Inc.]

FIGURE 18.9. Excretory system of saltwater mosquito larvae. Known pathways of active ion transport are shown. AR, anterior rectal segment; MG, midgut; MT, Malpighian tubule; PR, posterior rectal segment. [After T. J. Bradley, 1987, Physiology of osmoregulation in mosquitoes, Annu. Rev. Entomol. 32:439-462. Reproduced, with permission, from the Annual Review of Entomology, Volume 32 (§1987 by Annual Reviews, Inc.]

and chloride), in brackish-water species the hemolymph inorganic ion levels remain more or less constant and the increased osmotic pressure is the result of the addition of organic components. In Culex tarsalis larvae acclimated to 600 mOsm seawater, for example, the concentrations of amino acids (especially proline and serine) and trehalose were several fold greater than those of insects acclimated to 50 mOsm seawater (Bradley, 1987). The use of these molecules to prevent osmotic dehydration is interesting in view of their somewhat parallel role as cryoprotectants in cold-hardy insects (Chapter 22, Section 2.4.1).

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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