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50 best two being the development of resistance to pesticides and the formation of melanic forms of certain moths in areas of industrial air pollution. Of greater interest in the present context are the long-term climatic changes that have taken place over millions of years, for it is these that have controlled the evolution of insects both directly and indirectly through their influence on the evolution of other organisms, especially plants.

Although life began at least 2.5 billion years ago, it was not until the Upper Ordovician/ Lower Silurian periods (about 425-450 million years ago) that the first terrestrial organisms appeared, an event probably correlated with the formation of an ozone layer in the atmosphere, which reduced the amount of ultraviolet radiation reaching the earth's surface (Berkner and Marshall, 1965). The earliest terrestrial organisms were simple low-growing land plants that reproduced by means of spores. They were soon followed by mandibulate arthropods (scorpions, myriapods, and apterygotes) that presumably fed on the plants, their decaying remains, their spores, or on other small animals (Peck and Munroe, 1999). In these early plants, spores were produced on short side branches of upright stems. An important evolutionary development was the concentration of the sporangia (spore-producing structures) into a terminal spike. Whether these spikes were particularly attractive as food for insects and whether, therefore, they may have been important in the evolution of flight is a matter for speculation (Smart and Hughes, 1973).

During the Devonian and Lower Carboniferous periods, a wide radiation of plants occurred. Especially significant was the development of swamp forests that contained, for example, tree lycopods, calamites, and primitive gymnosperms. The evolution of treelike form, though in part related to the struggle for light, may also have enabled the plants to protect (temporarily) their reproductive structures against spore-feeding insects and other animals. In contrast to the humid or even wet conditions on the forest floor, the air several meters above the ground was probably relatively dry. Thus, the evolution of trees with terminal sporangia may have been an important stimulant to the evolution of a waterproof cuticle, spiracular closing mechanisms, and, eventually, flight (Kevan et al., 1975).

The trees, together with the ground flora, would provide a wide range of food material. As noted earlier, winged insects appeared in the Lower Carboniferous. Though some of these were mandibulate and fed on soft parts of plants or litter on the forest floor, the great majority (>80%), notably the paleodictyopteroids and ancestral hemipteroids, had mouthparts in the form of a proboscis, which has led to the suggestion that these insects were adapted to feeding on either free-standing liquids or plant sap. Smart and Hughes (1973) believed, however, that the proboscis might have been used as a probe for extracting pollen and spores from the plants' reproductive structures. They argued that not until the Upper Carboniferous did plants evolve with phloem close enough to the stem surface that it was accessible to Hemiptera. Yet other insects such as the Protodonata and, later, Odonata, were predators, feeding on the sucking forms as well as early Ephemeroptera.

Possibly as a result of an "arms race" between predator and prey, some members of all these groups became very large (Peck and Munroe, 1999). Another view suggests that the evolution of large size (gigantism) was a result of competition between these insects and the earliest terrestrial vertebrates, the Amphibia. Certainly large size would be favored by the year-round, uniform growing conditions (Ross, 1965). A third factor related to the evolution of Late Paleozoic gigantism was the increase in oxygen concentration in the atmosphere that occurred at this time, with values possibly as high as 35%. This would improve the ability of the insects' tracheal system to supply tissues with oxygen (Chapter 15, Section 3.1), which is a major constraint to increased body size (Dudley, 1998).

In addition to the forest ecosystem, there were presumably other ecosystems, for ex- 51

ample, the edges of swamps and higher ground, which had their complement of insects.

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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