In this study researchers examined the visual system adaptations and differences between two closely related butterfly species, Caligo memnon and Morpho peleides. The researchers wanted to see if there was an adaptive advantage in the crepuscular insects’ visual performance for increased sensitivity and temporal resolution. Both species of butterfly are found in the rainforests of Central America, are of similar size, and possess afocal apposition eyes. The afocal apposition eye is an intermediate between the superposition and apposition basic eye designs in insects and is best suited for bright light, according to the study. The interesting difference between the two butterfly species is that M. peleides is known to be active only during the day while C. memnon is active at dawn and at times at dusk.

The researchers used only male butterflies in their study to eliminate any implications that the visual adaptations were sex specific and related only to reproductive success. They kept the males in a controlled room where “dark adaptation” was achieved by switching off all lights and “light adaptation” was performed by using a background lighting of 200 cdm^-2 intensity. The researchers indicated penetration of photoreceptor cells within the eyes by a depolarization response to a flashlight using a goniometer to measure the visual axis of the cell. From this the spectral sensitivity, impulse response, and angular sensitivity were recorded following the direction of maximum electrical response generation using the goniometer. They found that the cells with a sensitivity peak around 550nm were the only cells to be used because they were considered as an essential component for the visual pathway for these butterflies to discern contrast and luminance visually.

Researchers were also able to take a magnified image of the eye from the front and center of the butterfly’s visual field, and used this to measure the rhabdom diameter in the frontal visual field from the two eyes in each species. They found that C. memnon had very large facets with a maximum diameter of 48μm while M. peleides rhabdom diameter was more moderate at 34μm. This difference was said to be attributed to the overall larger eye size of C. memnon. This finding relates to material learned in lecture because the enlarged eye of C. memnon is beneficial for increased sensitivity due to its enlarged aperture and increased spatial acuity. Lecture material pointed towards various adaptations used to increase sensitivity and this adaptation agrees with the material presented in class.

Researchers used the angular sensitivity function in order to quantify spatial resolution in the two butterfly species. They stated that in the compound eye, the spatial resolution is dependent upon the focused image quality, rhabdom acceptance function, and sampling density. This study found C. memnon to have broader angular sensitivity functions than M. peleides; the smallest angles were found close to the equator of the visual field at 10º to 20º lateral of anterior in the eye. This was determined by the ratio of the facet diameter D, to the radius of curvature R of the eye.

Finally, researchers found that the integration times for impulse responses in the eyes of the butterflies differed as C. memnon had longer integration times and times to peak than M. peleides in “dark adaptation.” The integration time was used as a measure of the temporal resolution of the eye, because the impulse response is the photoreceptor’s response to a short and dim light flash that causes an electrical response in the eye, similar to a response elicited by a photon. C. memnon was determined to be approximately 4 times as sensitive as M. peleides to the impulse response. This finding added to the increased adaptations found in C. memnon for longer integration time, larger acceptance angles, and enlarged facets as stated earlier. While C. memnon has larger eyes and facets, it has also maintained high temporal and spatial resolution, as demonstrated in this study. Researchers stated that this evolutionary change may have happened early in time to increase sensitivity in crepuscular insects, because the enlarged eye and aperture size increases sensitivity while maintaining high visual acuity.

Researchers concluded that the crepuscular insects visual systems have “evolved adaptations that improved their visual reliability in dim light” settings and that these adaptations have been optimized for specific light intensities within their natural habitats. (Frederickson & Warrant, 2008) I chose this research article because I thought it directly related to adaptations stated in lecture for optimizing sensitivity in low light and improving resolution of the apposition eye design by increasing aperture size and eye radius. This article was interesting because it examined two closely related butterfly species whose eyes have evolved based on the different times of day during which they are most active.