Bøger i Developments serien

This book is concerned with two problems: how eusociality, in which one individual forgoes reproduction to enhance the reproduction of a nestmate, could evolve under natural selection, and why it is found only in some insects-termites, ants and some bees and wasps. Although eusociality is apparently confined to insects, it has evolved a number of times in a single order of insects, the Hymenoptera. W. Hamilton's hypothesis, that the unusual haplodiploid mechanism of sex determination in the Hymenoptera singled this order out, still seems to have great explanatory power in the study of social ants. We believe that the direction, indeed confinement, of social altruism to close kin is the mainspring of social life in an ant colony, and the alternative explanatory schemes of, for example, parental manipu­ lation, should rightly be seen to operate within a system based on the selective support of kin. To control the flow of resources within their colony all its members resort to manipulations of their nestmates: parental manipulation of offspring is only one facet of a complex web of manipul­ ation, exploitation and competition for resources within the colony. The political intrigues extend outside the bounds of the colony, to insects and plants which have mutualistic relations with ants. In eusociality some individuals (sterile workers) do not pass their genes to a new generation directly. Instead, they tend the offspring of a close relation (in the simplest case their mother).

In the field of plastics technology, the process of extrusion is widespread and important. It is employed in the compounding and pelletising of plastics materials, in their conversion into products (such as profiles, pipe, hose, sheet, film or bottles) and in the coating of wires, cables, paper, board or foil. A major reason for its use is the screw extruder's ability to melt efficiently and pump continuously large amounts of plastics materials. The understanding of the melting/pumping operation of the extruder and the development of larger and faster-running machines so as to give higher outputs have been given great attention and the results have been widely published. However, the whole manufacturing technology for extruded products has also developed, particularly in recent years. This has occurred not only by the use of modern screw extruders, but also by the incorporation of improved process control systems, the better design of dies and extrudate handling machinery and by the utilisation of improved plastics materials and additives. It is the purpose of this book to present selected topics which contribute to, or exemplify, these developments in extrusion-based processes.