THE LIFE OF THE SPIDER 39 



a stone or under boards. The golden brown balls of Theridiosoma 

 frequently are found hanging to vegetation, suspended by a fine 

 long thread. Very likely such a pendant sac offers difficulties to 

 predators that might destroy it if it were nearer at hand. 



The use of silk coverings to give the eggs a relative security 

 from depredation must have been discovered early in the history of 

 spiders. Even a superficial silken covering would be a deterrent, 

 since many insects cannot penetrate it and might even become en- 

 tangled in the threads. Spiders have, in the course of time, added 

 many refinements to their sacs and thus gained greater protection 

 from predators. The covering has been toughened, thickened, vari- 

 egated with tufted and woolly silks, and, in many cases, several 

 blankets envelop the egg mass. Often the sac is plastered with 

 layers of mud, or embellished with bits of wood, leaves, stones, and 

 other debris, rendering it less conspicuous. Some are glued to stones, 

 tied to twigs, enclosed in folded leaves, or suspended at the end of 

 fine threads. Others sit in the center of the web or lie behind a 

 tangle of threads in a retreat. 



Some spiders have divided the risk by putting their eggs in 

 several baskets. They spin a series of sacs, which hang as a string in 

 the center of their snare or are left singly here and there. It is 

 uncommon to find every sac in a string parasitized, whereas the 

 whole effort of a mother spider may be lost in a single bag. 



In addition to this type of protection, the spider often plays an 

 active role in seeing her eggs through to hatching and babyhood. 

 The crab spiders and many hunting spiders guard the egg sac and 

 strenuously resist effort to pilfer the contents. Wolf spiders drag 

 their sac attached to their spinnerets, and, later, carry the young 

 around on their backs until the spiderlings are able to fend for 

 themselves. The varied efforts made by mother spiders to provide 

 for the welfare of their eggs or young are remarkable and complex, 

 and especially noteworthy because they are largely instinctive 

 activities. 



HATCHING AND EARLY DEVELOPMENT 



Spiders undergo a development within the egg that is compara- 

 ble to that of other arachnids and also of insects. The embryo 

 spider gradually takes form on the outside of the vast sphere of 

 yolk that makes up most of the egg. On the generalized part, which 

 will become the cephalothorax, appear little buds, which gradually 



