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Psyche 
[Vol. 93 
benefits of correct background selection in both immature and adult 
insects (Hazel & West 1979, Erichsen et al. 1980, West & Hazel 
1982, Sims & Shapiro 1983a, b). This study investigates background 
selection by spiders of the genus Miagrammopes. 
Members of the tropical genus Miagrammopes spin reduced cap- 
ture webs consisting of a horizontal thread which may either be 
sticky or non-sticky and may have one or several vertical or diago- 
nal sticky (cribellar) capture threads extending from it (Lubin et al. 
1978, Opell 1984). These spiders monitor their reduced webs from 
one of the attachment points, where their postures and body form 
make them cryptic. Brown species that spin their webs among twigs 
and vines resemble thorns or broken twig bases, whereas green spe- 
cies that spin webs on moss-covered vegetation resemble extending 
moss phyllidia (Fig. 1). After subduing and wrapping prey, these 
spiders return to one of their web’s attachment points and resume 
their typical cryptic posture while feeding. 
Most individuals of the green Miagrammopes species I observed 
in Costa Rica monitored their webs from moss-covered twigs. This 
occurred despite the fact that some webs were also anchored to bare 
twigs. In order to test the hypothesis that members of this species 
select moss-covered twigs over bare twigs as web-monitoring sites, I 
conducted a series of choice experiments. 
Materials and Methods 
The species used in this study is an undescribed member of the 
Miagrammopes aspinatus species group (Opell 1984). Voucher spec- 
imens are deposited in Harvard University’s Museum of Compara- 
tive Zoology. Spiders used in this study were collected from stands 
of abandoned cacao (Theobroma cacao) at the Organization for 
Tropical Studies’ La Selva research station located near the town of 
Puerto Viejo de Sarapiqui, Heredia Province, Costa Rica. Prior to 
their release onto experimental frames, these spiders were kept for 
two to four days in small, cotton stoppered glass vials. During 
this time, their carapace and leg lengths were measured with a 
micrometer-equipped dissecting microscope. Because adult males 
do not spin capture webs, only immatures and adult females were 
used in this study. 
Experimental frames (Fig. 2) were constructed of 2 mm diameter 
hardwood applicator sticks glued together with epoxy and bound 
