xii INTRODUCTION 



of the inhabitants avoid the excessive mid-day heat and drought by 

 their nocturnal behaviour. (Conversely, arctic animals are nearly 

 always diurnal in habit.) The more primitive groups such as scor- 

 pions, whip-scorpions, spiders of the families Liphistiidae, Thera- 

 phosidae, Dictynidae, Dysderidae and so on, have probably be- 

 come secondarily adapted to nocturnal habits as a result of com- 

 petition with more efficient species. At the same time, however, 

 many of them are large and somewhat vulnerable and may need to 

 escape the attentions of potential predators in this way. 



10 20 30 



Temperalure 

 40 50 



60 



20 40 60 80 100 120 140 160 



Saturation deficiency of atmosphere mm Hg 



Fig. 1. Rate of water-loss in dry air at different temperatures, 

 and corresponding saturation deficiencies, from a woodlouse 

 (Porcellio), millipede {Oxidus), spider (Lycosa) and insect (P/'erw 

 larva). In the woodlouse and millipede the rate of water-loss is 

 proportional to the saturation deficiency of the atmosphere, but 

 in the spider and insect it is negligible below about 30° C, the 

 critical temperature at which their epicuticular wax-layers be- 

 come porous. Rate of water-loss is expressed in milligrams per 

 square centimetre of surface area per hour. (After Cloudsley- 

 Thompson, 1955.) 



On the other hand, liquid water can also be very dangerous to 

 small animals that may become water-logged, or trapped by sur- 

 face tension. Their environment must be neither too wet nor too 

 dry. This is especially true of species lacking a waterproofing wax- 

 layer. Because they are susceptible to desiccation, they are also 

 susceptible to water uptake by osmosis: consequently they tend to 



