SPIDERS 177 



the young climb on their mother's back where they remain for 

 several days. In the Pisauridae the mother spider carries her cocoon 

 under her sternum: when the young are ready to emerge she fixes 

 it to the end of a branch and mounts guard over it, while the female 

 Pholcus phalangioides glues her few eggs together lightly and carries 

 the mass in her chelicerae. Some of the most elaborate cocoons are 

 spun by web-building spiders: the sac may hang from the web or 

 may be fixed to the vegetation nearby. 



The use of silk to protect the eggs was probably evolved early in 

 the history of spiders and many refinements have since been added. 

 The covering is often toughened and thickened with several layers 

 of silk and may be plastered with layers of mud or embellished 

 with bits of wood, leaves and other debris which render it in- 

 conspicuous. Some spiders spin a series of egg-sacs which are left 

 singly here and there, thus minimising the risk of the whole brood 

 being parasitised. 



The number of eggs laid by diflFerent species varies enormously. 

 Theraphosa blondi, the largest of all spiders, is said to lay as many 

 as 3,000 and the larger orb-weavers and Pisauridae may produce 

 more than 2,000. While the tiny Oonops pulcher lays only two, the 

 majority of spiders probably lay about 100 eggs, and those produc- 

 ing more than one cocoon usually lay fewer eggs in each. The 

 larger species tend to lay more eggs than the smaller species. Not 

 only is there an obvious physical relationship, but there may be an 

 ecological connection between size and egg number, for big spiders 

 usually take longer to reach maturity than the smaller kinds. 



The number of eggs laid is clearly related to the size of the 

 mother spider where different species of the same family are con- 

 cerned. Petersen (1950), in a discussion of the significance of this 

 relationship in the evolution of size in various species of Lycosa, 

 concludes that since larger spiders lay more eggs and the size of the 

 mother is to a certain extent inherited, there must be a selection 

 towards larger size which in nature is probably balanced by 

 efficient counter selection. A similar relationship has also been 

 shown to exist in Amaurobius ferox and A. similis by Cloudsley- 

 Thompson (1956). In ^. ferox, about three weeks elapse in the 

 summer after tgg laying before the young begin to leave their 

 cocoon and the mother remains in the breeding nest until they 



M S.S.C.M. 



