visual fields associated web reduction di opell ware (1 risultati)

When visual fields of the primitive orb-weaver, Waitkera waitkerensis, are reconstructed using measurements taken from intact lenses and cross and longitudinal sections of the prosoma, they show that this species has complete visual surveillance, but that none of the visual fields of its eight eyes overlap. The more advanced orb-weaver, Uloborus glomosus, also has eight eyes, but each eye has a greater visual angle, giving this species a complex pattern of overlapping visual fields. Uloborids that spin reduced webs are characterized by reduction or loss of the four anterior eyes and other carapace modifications necessary for them to effectively monitor and manipulate their reduced webs. The eyes of these uloborids have greater visual angles than those of orb-weavers, resulting primarily from perimetric expansion of their retinal hemispheres. Additionally, the axes of their visual fields are more ventrally directed due to greater dorsal than ventral retinal expansion and to ventral redirection of the entire eye. Consequently, even though the anterior lateral eyes of the triangle-weaver Hyptiotes cavatus lack retinae, the species' six functional eyes permit complete visual surveillance and exhibit visual overlap. The single-line-weaver, Miagrammopes animotus, has lost its four anterior eyes, and with them much of the anterior vision and all of the visual overlap found in the other species. However, changes similar to those of H. cavatus permit this species to retain most if its dorsal and ventral visual surveillance. Thus, ocular changes act in consort to maintain relatively complete visual surveillance in the face of eye loss and other major carapace modifications necessary for the operation of reduced webs. 14 pp., 4 figs, 3 tabs, gr. 8.