When the eye is exposed to light, it is bleached of the pigment in the cells of the photoreceptors. Upon entering a dark room, dark adaptation must take place for the eye to become adjusted to the change in light intensity. This is similar experiencing the darkness of a movie theater when coming inside on a sunny afternoon. When the eyes dilate accordingly to the low level of light, objects in the environment slowly become more visible. When plotting a dark adaptation curve, it usually shows a decreased sensitivity level with time in the darkness. Conversely, light adaptation refers to the time it takes the eye to constrict and adapt to the intensity of light. The eye must quickly adapt to the background illumination to be able to distinguish objects. (http://webvision.med.utah.edu/light_dark.html)

Cockroaches normally are oval shaped with long spiny legs and filamentous antennae. Some have wings and are capable of flight. Reddish-brown in appearance, these 2 inch invertebrates can spoil food, damage walls and clothing, and produce and unpleasant odor that can be allergenic in some humans. The American cockroach lives in subtropical to tropical climates among human homes. They feed upon the crumbs and trash of humans, and reproduce within the walls. Primarily a nocturnal insect, the cockroach will scatter when exposed to light. Roaches are commonly found in dark, moist areas such as the bathroom, sewers, drains, and soiled laundry where they feed on decaying matter. Adults usually scatter when disturbed, but rarely rely on flight for escaping. (http://ohioline.osu.edu/hyg-fact/2000/2096.html)

Adult crickets are approximately an inch in length, and their yellowish-brown bodies are encircled by 3 dark bands near the head. The antennae exceed the length of the body and aid in the detection of physical structures. As observed in our collection, one of the females carried a sack of eggs protruding from her abdomen. Crickets are usually found outdoors when it is warm and will migrate indoors during cold temperatures. They prefer living near human trash and can feed upon multiple fabrics including cotton. Adults swarm towards bright lights, but are nocturnal insects. Male crickets use their chirping to attract potential mates. Since the females rely on auditory processing for distinguishing sex among the species, it can be inferred that the vision system plays little role in conspecific recognition. (http://ohioline.osu.edu/hyg-fact/2000/2066.html)

Compound eyes are made up of many ommatidia that work as an individual visual receptor. Pigment cells and visual cells arrange themselves in a radial pattern and provide information on light intensity. As the light passes through the lens of each compound eye, it is filtered through a crystalline cone and passed onto the cell. Pigment cells bend the light so only light entering parallel to the axis can reach the visual cell to trigger an action potential. These potentials provide the brain with an individual pixel of an image. The more pixels, the clearer the image. The compound eye is for detecting motion, and can cause a flickering effect in the visual field that will alert the insect to movement. (http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CompoundEye.html)

Simple eyes, or ocelli, are small eyes sensitive to light. There are two primary types of ocelli; Dorsal and Lateral. Dorsal ocelli are not independent organs and only occur in species that have a compound eye. These eyes look like small swellings on the dorsal or facial regions of the head. Unlike compound eyes, several light sensitive rods share one corneal lens in the ocelli. These eyes do not form an image, rather they convey information about the presence of light and its intensity. They respond quickly to changes in light intensity, and are therefore of interest in the study of light adaptation. Some physiologists report that the ocelli forms as a light indicator for the compound eye of the species, allowing the pair to function as one unit. Lateral ocelli are primarily found in larvae and other invertebrate species, and they detect the presence of light in much the same fashion as the dorsal ocelli. This eye however is the sole visual sensory source functioning without a compound eye. (http://www.cals.ncsu.edu/course/ent425/tutorial/photo.html)

Besides ridding the world of one less cockroach and cricket, we are attempting to discover which insect adapts to darkness faster. By measuring groups of cells in the eye, we can discern how each insect is adapting to the change in light. Through our electrophysiological processes we hope to gain knowledge of how the eyes of insects work and how they experience light and dark adaptation. It is the goal of our experiment to visualize a relationship between light stimulation and groups of neural activity within the eye while plotting the dark and light adaptation curves of our models.

To conduct this experiment we purchased a group of crickets from an animal supplier, and lured and caught a similar sized group of cockroaches. They were housed in a clear cage with plentiful food and moist cotton. The only source of light was from overhead usage in the room, however this may be substantially more photoreceptors than the roaches previously experienced.

Procedure

After setting up all equipment, we captured a cricket or a roach in a tube containing dry ice. We anesthetized the insects for several seconds until all body movement ceased. The insect was then transferred onto a wooden block and fastened securely from head to toe with tacky wax. When the insect was bound, it was placed under a video microscope. We placed a small reference electrode into either the head of the cricket or the antennae of the roach. Our recording electrode was placed into the extracellular space of the compound eye. These electrodes were connected to a dated amplifier which was attached to an oscilloscope. Upon commencing our test, we dark adapted the insect long enough to get a maximal response. When we reached this level, we calibrated our amplifier to estimate size of the B-wave. We calibrated to reduce noise in our current, and waited with the insect in the dark. A one minute bleaching was performed on the insect’s eye through an optic cable. The light was then turned off, and .25 second bursts of light were periodically delivered to the eye. Upon each flash, we measured the amplitude of the B-wave and logged the time that had passed since the onset of darkness. After reaching the maximum response, we applied various optical density filters and gave another burst of light through each to record each maximum response. The data was then logged and transferred to Excel to plot our results.

As indicated in Fig 1, both the cricket and the roach were subjected to periodic blasts of light until the maximal response was reached. The curves in each graph represent the time and mm reading of each blast of light. The cricket had a slightly lower maximal response when fully adapted. Fig. 2 reflects the linear regression we logged by inserting filters. The points represent the maximal response for each filter administered to the insect. This information allowed us to construct Fig. 3 which compares the two dark adaptation rates. As represented on the graph, each point shows the intensity of light needed to get the maximal response during a period of darkness. The cricket takes more light to get a response immediately following lights out, but dark adapts two times faster than the roach.

As our results show, there is a brief intersection immediately following the onset of darkness. This data suggests that the cricket takes a brighter burst of light to get the maximal response than the cockroach. What does this mean in a natural setting? As reported by Ohio State University, cockroaches are nocturnal insects that scatter when exposed to bright light. If the eye responds too slowly to the introduction of bright light, it may result in the difference between escape and potential threats. This disadvantage would certainly impact the species ability to live until reproduction, therefore from an evolutionary stand point, the roach must be able detect light at a faster rate than its adversary. As for the cricket, although it is a nocturnal insect, it is attracted to bright light and therefore would have little need to adapt an eye capable of detecting light as quickly as the roach. The cricket, although possibly a foe to many other species, has little worry of scattering away when the light is quickly intensified.

The two species used in this experiment were not chosen intentionally. Due to complications, it was decided to compare the data of the two species after receiving both insects on different lab testing days. Cockroaches provide an excellent eye for examining dark adaptation, but their nocturnal environment consists of too much darkness for the receptors to be useful in lighted environments. The roaches used to conduct this experiment were lured from the sewage pipes of our research building’s basement, and the light they were exposed to prior to testing was far more photons then they had previously experienced. Therefore, it should not be surprising that the roach does not adapt to darkness after a thorough bleaching. In contrast, the crickets were purchased from a local pet supplier where artificial light and sunlight basked the insects freely. This exposure would affect the eye’s development, thus it would be more readily adaptive in changes in light intensity.

To improve this experiment in the future it would not be unreasonable to capture or purchase newborn roaches so that they may be light reared during their development. If the eye is exposed to light more often than the previous selection, nature might affect it enough to adapt better to changes in light intensity. Conversely, one could also dark rear a group of crickets to potentially give them the same developmental environment. If nature has a 50% relation to the outcome of genes, then it is feasible to affect the way the eye in the cricket responds to light if it is taken out of its natural setting early in development. This would not only help to balance the differences in the two species, it would also show the evidence for the “nature and nurture” theory.

In conclusion, this experiment could benefit from disrupting the natural life cycle of each insect to better compare the dark adaptation between them. However, it may be more practical to test two species that have more similarities than the cricket and roach. Although both of these insects are branded as nocturnal, it is clear through this experiment that nocturnal does not mean both have the same dark adapting abilities. Investigating two insects that have closer environmental settings could produce a better comparison than just comparing the two insects available for our lab.

Hartline, H. K. and Mcdonald, P. Robb. 1947. Light and Dark Adaptation of Single Photoreceptor Elements in the Eye of Limulus. Journal of Cellular and Comparative Psychology. University of Pennsylvania, Philadelphia.

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Kimball, J. The Compound Eye. April 1, 2006. http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CompoundEye.html

Kolb, Helga. Light and Dark Adaptation. April 1 2006. http://webvision.med.utah.edu/light_dark.html

Lyon, William F. American Cockroach. April 1, 2006.

http://ohioline.osu.edu/hyg-fact/2000/2096.html

Lyon, William F. Crickets. April 1, 2006.

http://ohioline.osu.edu/hyg-fact/2000/2066.html