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Psyche 
[Vol. 92 
(above), so the sawtoothed web is probably also derived from a 
typical orb. Thus the two genera have almost incredibly converged 
on the same strange web architecture, and employ many of the same 
behavioral sequences to produce it. 
A possible explanation for this convergence is related to the fact 
that sticky lines are usually only able to absorb about one tenth as 
much momentum as radial lines in completed orbs (Eberhard in 
press). By laying their sticky lines on radial and frame lines, Eustala 
sp. and Polenecia effectively increase the sticky lines’ ability to stop 
moving prey. Such relatively strong sticky lines will most effectively 
intercept prey when they are placed far enough apart that the prob- 
ability of “wasting” more than one line intercepting any given prey is 
reduced (Chacon and Eberhard 1980). Thus the two webs may be 
adaptations to strengthen sticky lines and array them so as to 
increase their probabilities of intercepting prey. 
The interpretation that the web form of older individuals of Eu- 
stala sp. is derived from an orb-web is supported by the fact that 
very young individuals construct typical orbs. A review of the litera- 
ture on ontogenetic changes in spiders’ webs shows that similar 
tendencies for younger individuals to spin less derived webs occur in 
a number of other orb weaving araneids, including Zygiella 
(Petrusewiczowa 1938), Scoloderus (Eberhard 1975), Nephila (Rob- 
inson and Robinson 1973, pers. obs.), Heretmia (Robinson and 
Lubin 1979a), Tetragnatha ( =Eucta ) (Crome 1954), and Pachygna- 
tha (Bristowe 1958). Very young Uloborus conus (Uloboridae) 
sometimes also omit the presumably derived cone structure that is 
consistently present under the orbs of mature females (Y. D. Lubin, 
pers. comm.). Some species of non-orb weaving spiders also have 
less derived juvenile webs and/or behavior. These include the psech- 
rid Fecenia (Robinson and Lubin 1979b), the mygalomorph 
Diplura (younger spiders more often make webs without lines above 
the sheet, and the funnel retreat is more often nearer the center of 
the sheet — A. Deccae, pers. comm.; ancestral spiders presumably 
lived in holes where silk accumulated on the surface of the ground 
more or less symmetrically around the edges of the hole), and (per- 
haps) web-weaving oxyopids of the genus Tapinillus (Griswold 
1983, Mora in press). Still other possible examples are the salticid 
genera Euryattus and Simaetha, in which younger spiders routinely 
spin sheet webs for prey capture while older individuals only seldom 
