THE “SAWTOOTH ED“ ORB WEB OF EUSTALA SP. 
(ARANEAE, ARANEIDAE) WITH A DISCUSSION OF 
ONTOGENETIC CHANGES IN SPIDERS’ 
WEB-BUILDING BEHAVIOR* 
By 
William G. Eberhard 
Smithsonian Tropical Research Institute 
and 
Escuela de Biologia, Universidad de Costa Rica 
Ciudad Universitaria, Costa Rica 
Convergence has been common in the evolution of orb-weaving 
spiders’ webs, and structures such as stabilimenta, reduced orbs, 
spring lines, meshes at the side of the orb, and asymmetrically 
extended “ladder webs” have arisen independently in a number of 
different groups. This paper describes brief observations of Eustala 
sp. that demonstrate still another apparent convergence that is per- 
haps more surprising than some of the others since the adaptive 
advantage of the convergent design is at first glance unclear. 
Ontogenetic changes in web design are also documented for Eu- 
stala sp.; younger individuals make more generalized or less derived 
webs than those of adults. Data on ontogenetic changes in the web 
forms of other spider species are summarized, and it is shown that 
this is a common ontogenetic pattern in spiders. The possible signif- 
icance of this evolutionary pattern is discussed. 
Observations were made in February 1984 in Parque Nacional 
Corcovado on the Osa Peninsula of Costa Rica in second growth 
along a small stream near the Sirena station. Webs of six large 
nymphs and adult females and of two very young nymphs were 
observed. The construction of three of these webs was observed 
between 7 and 1 1 PM. 
Spiders were kindly identified by H. W. Levi. They apparently 
represent an undescribed species; voucher specimens (#2345, 
SAl-127b, and SAE-106) have been deposited in the Museum of 
Comparative Zoology in Cambridge, Mass. 02138, U.S.A. 
* Manuscript received by the editor November 16, I9H4 
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