INTRODUCTION xi 



mechanisms for the conservation of water and the maintenance of 

 a constant internal medium must be evolved. That these problems 

 are by no means easily overcome is shown by the fact that several 

 invertebrate phyla are almost entirely marine while others as yet 

 possess few terrestrial species. Indeed, the Arthropoda have been 

 unusually successful in exploiting the terrestrial habitat and by 

 adaptive radiation have succeeded in establishing themselves in 

 nearly all the habitable corners of the earth. Each aspect of their 

 adaptation to life on land affects and is affected by other aspects. 

 For example, it might appear a fairly simple matter for an organism 

 to eliminate water-loss by the evolution of an integument com- 

 pletely impervious to water- vapour; but such an integument would 

 also be impervious to oxygen and carbon dioxide. A respiratory 

 mechanism has therefore had to be evolved which permits gaseous 

 exchange without excessive water-loss. If the integument is rigid 

 and provides support, then growth becomes impossible except by 

 moulting and this limits size. The physiology of nutrition and ex- 

 cretion too are closely concerned with water conservation and 

 superimposed upon this basic physiological requirement are the in- 

 numerable concomitants of behaviour and ecology. 



There are two obvious ways in which small animals can escape 

 desiccation on dry land. One is to avoid dry places and to remain 

 most, if not all of the time, in a humid environment; the other, to 

 evolve an impervious integument. Both methods have been ex- 

 ploited by the Arthropoda and each has its drawbacks and ad- 

 vantages. Indeed, on the basis of this character the terrestrial mem- 

 bers of the phylum can be divided roughly into two main ecological 

 groups: the first includes woodlice, centipedes, millipedes and their 

 allies which lose water rapidly in dry air; the second, the Arachnids 

 and insects which are covered with a layer of wax that renders 

 them comparatively independent of moist surroundings. 



In the following chapters the significance of this generalisation 

 will become apparent. Forms lacking an epicuticular wax-layer 

 are almost entirely nocturnal in habit, and can wander abroad only 

 after nightfall when the temperature drops and the relative humidity 

 of the air rises. In contrast, most insects, spiders and most other 

 Arachnida are potentially diurnal except perhaps in deserts and 

 other regions with rigorous climates where, anyway, the majority^ 



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vV 





