48 11 and 18). Arthropods are segmented animals and therefore have been able to exploit the advantages of tagmosis to the full extent. Directly related to this is the adaptability of the basic jointed limb, a feature used fully by different groups of arthropods (see Figure 1.6 and descriptions of segmental appendages in Chapter 3).

As all arthropods possess these advantageous features, the obvious question to ask is "Why have insects been especially successful?" or, put differently, "What features do insects have that other arthropods do not?" Answering this question will provide only a partial response for two reasons. First, as was stressed above, and is discussed more fully in Section 4.2, the environmental changes that take place are also very important in determining success. Consider, for example, the Crustacea. Compared to other invertebrate groups they must be regarded as successful (at least 40,000 extant species have been described), yet in comparison to the Insecta they come a very distant second. Although this is related partly to their different features, it must also reflect the different habitats in which they evolved. As a predominantly marine group, crustaceans evolved under relatively stable environmental conditions. Further, it is likely that when they were evolving the number of niches available to them would be quite limited because most were occupied by already established groups. Insects, on the other hand, evolved in a terrestrial environment subject to great changes in physical conditions. They were one of the earliest animal groups to "venture on land" and, therefore, had a vast number of niches available to them in this new adaptive zone. Second, the success of the Insecta as a whole is primarily related to the extraordinarily large number of species in a handful of orders, namely, the Coleoptera, Lepidoptera, Diptera, and Hymenoptera. Thus, the question ultimately becomes "What is it about these groups that allowed them to become so species-rich?" The answer to this is considered below.

Most insects, modern and fossil, are small animals. A few early forms achieved a large size but became extinct presumably because of climatic changes and their inability to compete successfully with other groups. Small size confers several advantages on an animal. It facilitates dispersal, it enables the animal to hide from potential predators, and it allows the animal to make use of food materials that are available in only very small amounts. The great disadvantage of small size in terrestrial organisms is the potentially high rate of water loss from the body. In insects this has been successfully overcome through the development of an impermeable exoskeleton.

The ability to fly was perhaps the single most important evolutionary development in insects. With this asset the possibilities for escape from predators and for dispersal were greatly enhanced. It led to colonization of new habitats, geographic isolation of populations, and, ultimately, formation of new species. Wide dispersal is particularly important for those species whose food and breeding sites are scattered and in limited supply.

Reproductive capacity and life history are two related factors that have contributed to the success of insects. Production of large numbers of eggs, combined with a short life history, means a greater amount of genetic variation can occur and be tested out rapidly within a population. This has two consequences. First, rapid adaptation to changes in environmental conditions will occur. This is best exemplified by the development of resistance to pesticides (see Chapter 24, Section 4.2). Second, there will be rapid attainment of genetic incompatibility between isolated populations and formation of new species. For example, the approximately 10,000 species of native Hawaiian insects are thought to have evolved from about 100 immigrant species. The evolution of a pupal stage between the larval and adult stages has led to a more specialized (and, in a sense, a more "efficient") life history. In some species this has led to the exploitation of different food sources by the larvae and adults (compare the foliage-feeding caterpillar with the nectar-drinking adult moth).

Further, it enables insects to use food sources that are available for only short periods of 49

time. Eventually, the main function of the larva becomes the accumulation of metabolic reserves, whereas the adult is primarily concerned with reproduction and dispersal (and in some species does not feed). Although the pupa is primarily to allow transformation of the larva to the adult, in many species it has become a stage in which insects can resist unfavorable conditions. This development and the restriction of feeding activity to one phase of the life history have facilitated the expansion of insects into some of the world's most inhospitable habitats.

Four orders of insects have become extremely diverse: Coleoptera (300,000 species) Lepidoptera (200,000), Hymenoptera (130,000), and Diptera (110,000). Clearly, these must have particular features that allowed them to preferentially exploit new niches as these became available through evolutionary time.

For Coleoptera, the features were the development of elytra that protect the hind wings and cover the spiracles to reduce water loss, the "compact" body as a result of housing the coxal segments in cavities, and the increased proportion of the integument that was sclerotized. These allowed this group to occupy enclosed spaces and cryptic habitats such as soil and litter, and to invade arid environments. Within the Coleoptera two groups are especially diverse, the Curculionoidea and the Chrysomeloidea, which collectively total more than 130,000 species (see Chapter 10, Section 5). The ancestor of these groups likely fed on primitive plants such as pteridophytes, cycads and conifers. The species "explosion" that led to the modern curculionoids and chrysomeloids began in the post-Jurassic period and closely paralled the evolution of the angiosperms (Farrell, 1998).

The evolution of the proboscis enabled adult Lepidoptera to easily ingest water, hence avoid desiccation, and to obtain nectar, often stored cryptically by the plants with which they coevolved. Diptera, too, with their specialized mouthparts have been able to exploit particular liquid food sources, notably nectar, juices from decaying materials, and animal tissue fluids. It is generally considered that the radiation of the Lepidoptera and Diptera closely paralleled that of the flowering plants and while this may be correct, Labandeira and Sepkoski (1993) noted that the accelerated radiation of insects began 100 million years before that of the angiosperms, and that the great majority of mouthpart types were in existence by the Middle Jurassic. In Labandeira and Sepkoski's view it may have been the evolution of seed plants in general, and not specifically the angiosperms, that was the driving force behind the explosive evolution of the Insecta.

The Hymenoptera, the great majority of which are small to minute, have "piggybacked" on the success of other insect groups, by becoming parasitoids, especially on larvae or eggs, through the development of the ovipositor as a paralyzing organ. In a further step, many species evolved simple forms of parental care (e.g., by placing the prey in special cells along with their egg), leading eventually to cooperative nest care and true sociality.

Several features of insects have contributed therefore to their success (diversification). It is important to realize that these features have acted in combination to effect success, and, furthermore, little of this success would have been possible except for the changing environmental conditions in which the insects evolved.

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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