36 AMERICAN SPIDERS 



related to some extent with the danger of destruction to which it 

 is subjected. When the mother spider remains with her eggs until 

 the young hatch, the need for a tough sac is not so great. Similarly, 

 a sac hidden away in the depths of a burrow or surrounded by 

 barriers of dry web or viscid strands is usually not strongly made. 

 The situation in which the sac is placed and the length of time it 

 must remain there before the young desert it are the important con- 

 siderations. Probably in response to such stimuli, spiders have devel- 

 oped different means of achieving a normal hatching of progeny 

 under varied circumstances. 



Most spiders are provided with a set of glands especially used 

 for the building of egg sacs. Known as cylindrical glands because 

 of their form, they feed their products through spigots on the out- 

 side of the posterior spinnerets. The silk spun from these glands is 

 frequently different in color from the dry silk, and from that pro- 

 duced by other glands. In addition, the silk of the egg sac is differ- 

 ent in its physical properties, being less elastic and not as strong as 

 the dragline silk. It is apparently never viscid. The outer, varnished 

 layers of some sacs suggest that the outer envelope is different in 

 origin from the silk of most of the sac, or differs at least in the 

 manner of being carded and applied as a layer. 



The egg sac is generally a spherical or lenticular object, resem- 

 bling a little ball, a biscuit, or a flat disc. The manner in which 

 these sacs are produced illustrates the fact that even in realizing such 

 commonplace structures, the spider must give considerable time and 

 exercise great instinctive ingenuity. Take for example the small 

 wolf spiders, whose sac-making can be conveniently observed. Or- 

 dinarily, Pardosa spins a light scaffolding of lines attached to adja- 

 cent objects, and between them lays down a flat sheet of silk. This 

 sheet usually takes the form of a circular disc approximating in 

 diameter the length of the female. It is a closely woven fabric made 

 by brushing the hind spinnerets from side to side and rotating the 

 abdomen and body. The finished base may be nearly flat, but fre- 

 quently it is a shallow basin, a veritable cradle for the eggs. 



The actual deposition of the yellowish eggs requires only a 

 minute or two. The gravid female stands over the sheet and ex- 

 trudes through the oviduct a viscid fluid that forms a pool on the 

 silk into which the eggs, singly or in small groups, are laid. The vis- 

 cosity of the fluid is such that the egg mass largely retains its 

 globular shape. In this fluid are sperms from the seminal recepta- 

 cles. The female next spins, over the mass, a somewhat smaller 



