1985] 
Eberhard — Orb web of Eustala 
113 
spin webs (R. Jackson, pers. comm.)- This case must be considered 
tentative, since the suspected ancestral nature of web spinning (e.g. 
Jackson and Blest 1982) in Salticidae is not yet firmly established. 
The designs of the webs of other orb weavers ( Araneus , Neo- 
scona, Argiope, Leucauge ) also change as the spiders mature (Mayer 
1953, Witt and Baum 1960, Reed et al 1969, Risch 1977, Robinson 
and Robinson 1978, Maroto unpub.); although in these cases prob- 
able ancestral web forms have not been determined, younger indi- 
viduals of Araneus, Neoscona and Argiope conform to expectations 
as they build more similar webs than do adults (Risch 1977) (in 
Argiope the sex of young spiderlings also affects the web design! — 
Robinson and Robinson 1978). 
There are ontogenetic changes in the webs of several other species 
of spiders, but they differ from the cases just cited. Very young 
Zygiella x-notata can build only incomplete orbs or tangles prior to 
the age at which they normally emerge from the egg sac (Peters 
1969). These changes in behavior differ from the examples above in 
that they probably stem at least in part from incomplete develop- 
ment of silk glands and from the completion of embryonic cytodif- 
ferentiation processes (Peters 1969). Young Uloborus, Philoponella, 
Zosis, and other uloborids spin “orbs” in which sticky cribellar silk 
is replaced by mats of very fine radial and non-radial lines (Szlep 
1961, Eberhard 1977), but again in this case the spiderlings also lack 
the morphological structures necessary to produce the types of silk 
used by older individuals. 
There are also groups (e.g. Miagrammopes, Hyptiotes, Ogulnius) 
in which young spiders spin the same derived web form as the adults 
(Lubin et al. 1979, Opell 1982, pers. obs.). There is, however, appar- 
ently only one clear case of more derived webs being used by 
younger spiders: this is Mastophora dizzydeani, a species whose 
ancestors made orbs but in which young spiders generally spin no 
trapping web whatsoever and adults usually make a remnant web in 
the form of a sticky globule. There is, however, a probable selective 
advantage in greater web reduction in smaller individuals, since 
surface to volume relations may make the use of globules especially 
difficult for smaller spiders (see Eberhard 1980). 
Spiders thus show the same trend in their behavior that is well 
known in the morphology of many animal groups: adult characters 
tend to be more derived than those of the young. Why this should 
