The most important features of the surrounding medium that affect the distribution and THE ABIOTIC

abundance of aquatic insects appear to be its temperature, oxygen content, ionic content, ENVIRONMENT

and rate of flow. The influence of temperature on development and activity (through its effect on oxygen content) has already been outlined in Sections 2.1 and 2.2.

The ability of insects to regulate both the total ionic concentration and the level of individual ions in the hemolymph is a major determinant of their distribution. Typical freshwater insects are restricted to waters of low ionic content because, although they are capable of excreting excess water that enters their body osmotically, they have no mechanism for removing excess ions that enter the body when the insect is in a saline medium; that is, they cannot produce a hyperosmotic urine (Chapter 18, Section 4.2). Further, members of some species may be unable to colonize some freshwater habitats because these contain certain ions such as Mg2+ and Ca2+ in too high a concentration.

In contrast, members of many species that normally inhabit saline environments appear to be able to regulate their hemolymph osmotic pressure and ionic content over a wide range of external salt concentrations. In other words, they can produce hyperosmotic urine when it is necessary, in a saline medium, to excrete excess ions, or hypoosmotic urine, when in fresh water, to remove excess water from the body (Chapter 18, Section 4.3). As they are normally found only in saline habitats, it must be assumed that their distribution is governed by other environmental factors.

The insect fauna of an aquatic habitat may be correlated with the speed at which the water is moving. Insects in still or slowly moving water are not prevented from moving, for example, in search of food or to the surface for gaseous exchange. In contrast, rheophilic species (those that live in swiftly moving streams or rivers) have evolved structural, behavioral, and physiological adaptations to survive in these habitats. Among the structural adaptations that may be found in rheophilic insects are flattening or streamlining of the body, and the development of friction discs or hydraulic suckers (Hynes, 1970a,b). Flattening may take on differing significance among species, though ultimately its function is to enable insects to avoid being washed downstream by the current. In members of some species, which live on exposed surfaces, flattening enables them to remain within the boundary layer, a thin layer of almost static water covering the substrate. For members of most species flattening is associated with their cryptic habit, permitting them to live under stones, in cracks, crevices, etc. Streamlining, too, is a modification mainly used by insects to avoid currents by burrowing into the substrate, though members of a few streamlined species, for example, most species of Baetis and Centroptilum (mayflies), do live on exposed surfaces and are able to swim against quite strong currents (Hynes, 1970a,b).

The major physiological adaptation of rheophilic species is related to gas exchange. Because of the danger of being washed downstream, insects in moving water cannot come to the surface to obtain oxygen; they rely on oxygen dissolved in the medium. Through evolution, members of rheophilic species have become adapted to a medium with high oxygen content and conduct most or all gas exchange directly across the body wall. Further, they depend on the water current to renew the oxygen supply at their body surface. As a result, in many species, gills, if present, are reduced, and the ability to ventilate, by flapping the gills or undulating the abdomen, has been lost.

Their relative inability to move because of the current has been paralleled, in many rheophilic insects, by the evolution of devices that enable them to obtain food passively; that is, they depend on the current to bring food (especially microorganisms and detritus)

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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