SPIDERS 157 



number of species inhabit desert regions, others are aquatic or 

 semi-marine. Indeed, there are few parts of the world where 

 spiders do not thrive. A number of generalisations can, however, 

 be made. Some spiders such as the Mygalomorpha, Salticidae and 

 Sparassidae flourish in the tropics and diminish progressively to- 

 wards the cooler regions; the Gnaphosidae, Dysderidae, Ageleni- 

 dae and various cribellate families thrive most successfully in the 

 warmer temperate or sub-tropical areas, while* others, including 

 the Lycosidae and particularly the Linyphiidae, reach their great- 

 est dominance in the arctic or sub-arctic regions. Even in such a 

 small area as that covered by the British Isles there are marked 

 differences between the spiders found in the north and south, 

 differences which demonstrate the same trend as that outlined 

 above. Scotland lacks Mygalomorpha, Oxyopidae, Pholcidae, 

 Scytodidae, Sparassidae etc., whilst the Linyphiidae and Salticidae 

 represent 50% and 4% of its fauna respectively, compared with 

 42% and 6% for England (Bristowe, 1939).* 



Some plants harbour more spiders than others. For example, the 

 fauna of bracken, bluebells, mint, laurel, willow and beech is far 

 less rich than that of grass, heather, gorse, holly, yew, conifers and 

 oak. It may be that the scents of certain plants are avoided, but the 

 density of the foliage is also an important factor as it affects the 

 humidity of the environment. Again, the abundance of insect life . 

 living on, or coming to different plants also affects the number of 

 spiders that can live there. Bristowe (1939)* has considered in 

 detail the spider fauna of sandhills, marshes, mountains, caves, 

 mines and cellars, houses, towns, sewage works, ants' and termites' 

 nests, beehives, birds' and mammals' nests, and other spiders' 

 webs. 



A number of authors have stressed the importance of moisture 

 on the distribution of spiders, and Nemenz (1954) has discussed its 

 physiological significance. The conflict between the incompatible 

 requirements of respiratory exchange and the prevention of water- 

 loss has recently been demonstrated by the writer by a comparison 

 of two common British species, Amaurobius ferox and A. similis. 

 both of which have a cuticular wax-layer with a critical temperature 

 at about 30° C, above which they quickly lose water by evaporation 

 in dry air. At lower temperatures, however, the rate of water-loss 



