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Psyche 
[Vol. 92 
on or very near the backbone of the ridge, not those growing on the 
slopes. Moreover, the frequency with which males claim a tree is 
primarily a function of (1) its altitude on the ridge backbone, with 
trees closer to the peak favored over trees farther down the ridge, 
and (2) the width of the tree, with large bulky trees preferred over 
smaller ones. 
These results suggest that visual conspicuousness is an important 
attribute of a preferred territorial perch site for male wasps. They 
appear to be drawn to large targets clearly outlined against the sky 
rather than small trees or shrubs whose forms are obscured by a 
hillside. In order to test this hypothesis we designed experiments in 
which male tarantula hawks were offered two artificial landmarks in 
sequence. This gave the wasps a chance to choose between perch 
sites that differed in their height, size, or contrast. If visual conspi- 
cuousness is a key property of a perch site, then male tarantula 
hawks should inspect and perch on taller, larger, darker targets 
more often than lower, smaller, and paler landmarks. Our report 
discusses the response of male wasps in these choice experiments. 
Materials and Methods 
The experiments were carried out near Usery State Park, Mesa 
AZ (see Alcock 1979 for a description of the study site) and at South 
Mountain Park, Phoenix AZ from late March to mid-May 1981 
and 1982. Both areas have vegetation typical of the Sonoran Desert 
of central Arizona. 
The experimental landmarks consisted of a pole composed of 
aluminum tubing supported by a base of metal slats and tubes; the 
entire structure was painted flat black mottled with tan (Fig. 1). At 
the top of the pole targets of various sorts were mounted upon 
which male wasps might perch. Totally artificial perch materials 
were used to eliminate any effects of familiarity with perch 
substrates on male choices. The height of the pole could be adjusted 
by adding or subtracting units of tubing. Each experimental trial 
consisted of two trials with first one artificial landmark offered to a 
male after which the landmark was altered and presented again for 
the same period (either 12 or 15 min, depending on the experiment). 
The experimental apparatus was erected between 2.5 and 3.0 m 
from a natural perch site occupied by a territorial male. This wasp 
