Microspectrophotometry of the dioptric apparatus and compound rhabdom of the moth (Manduca sexta) eye

Absorption spectra were obtained by microspectrophotometric (MSP) axial measurements of the compound rhabdom of the night moth Manduca sexta. Difference spectra derived from partial or complete bleaches revealed the evidence of four visual pigments with approximate λ_max at 350, 450, 490, and 530 nm. Upon bleaching with light of the pigment maximum at 21°C, pH 7·4–8·5, each pigment, save the u.v.-sensitive one, formed a photoproduct whose spectral maximum (ca. 370 nm) was indicative of a mixture of free and bound retinal. Rarely, small amounts of an additional photoproduct (λ_max 325–330 nm) formed, which is suggestive of retinol. The u.v.-sensitive pigment, when irradiated with u.v., formed an unknown photoproduct (λ_max 290–300 nm). Bleaching kinetics were of first order. Separate absorption determinations through lens or crystalline cones showed each component of the dioptric apparatus served as a filter effecting a sharp decrease in corneal transmission at 310 nm while being increasingly transparent from near u.v. to red. The survival benefits accruing to a largely nocturnal moth with a presumptive colour vision mechanism are discussed.