Introduction

Introduction

Purpose: Ultraviolet (UV) light has dangerous effects on living organisms.

Problem: An increasing amount of ultraviolet light is passing into the atmosphere as a result of pollutants, which are causing the ozone layer to deteriorate.

Rationale: What are the effects of ultraviolet light on tiger swallowtails (Pterourus glaucus)?

Two groups of butterfly pupae, in their chrysalis stage, were exposed to UV light for different amounts of time; and one group, the control, had no UV light exposure. The independent variable was the amount of time that they were exposed to UV light. The dependent variables were the wing span, body length, and the amount of black in the wings of the adult butterflies. It was hypothesized that if butterflies are exposed to ultraviolet light for longer periods of time, then the wing span and body length would decrease, while the amount of black in their wings would increase.

Ozone is the layer of Earth's atmosphere that shields the Earth from harmful ultraviolet radiation. Ultraviolet light is invisible to humans, but contains a high radiation energy level. The problem now facing ecologists is that the ozone layer is slowly disappearing, letting an increasing amount of ultraviolet radiation make its way to the Earth's surface. Ozone holes are formed when pollutants react with the ozone in the stratosphere and cause the concentration of ozone to decrease. There has recently been a large increase in the use of these ozone consuming substances, causing the ozone layer to deteriorate faster and allowing more ultraviolet light to reach the Earth (Freedman, 1995).

Ultraviolet light is light with wavelengths between 10nm and 400nm. Ultraviolet light is divided into two groups, longwave (UVC) and shortwave (UVB). Longwave UV light has wavelengths between 300nm and 400nm, while shortwave UV light has wavelengths under 300nm. Shortwave UV light can kill bacteria, irritate human eyes, and cause a strong sunburn effect on skin (Wain, 1965). Shortwave UV light is harmful to humans and all living things because it has the power to change DNA. The energy in UV light, which is absorbed by DNA, can actually break bonds in DNA; and if left unrepaired, the broken bonds can lead to genetic damage or cancers (NASA, 1997). Unintercepted radiation could cause cataracts, corneal damage, and suppression of the immune system in humans. The ecological effects could be also very damaging, leading to less productivity, possibly related to a degradation of pigments in animals (Freedman, 1995).

It has been shown that environmental factors, such as ultraviolet light, can cause wing coloration changes in butterflies. Brower (1994) tested the DNA of a South American butterfly (Heliconius erato) whose wing patterns seemed to be changing within the species. He found that the butterflies' DNA had evolved and that the butterflies could be separated into two groups according to their DNA patterns. He found that the mutations in DNA, and resulting wing pattern changes, depended on the butterflies' geographical location, meaning that the mutation was most likely caused by a changed factor in their environment. (Brower, 1994). UV light is a possible environmental factor that could have caused the change. The UV dosage for animals is determined by their size and sensitivity. The recommended dosage for a Citrus jambhiri tree is 8 hours per day (McCloud and Berenbaum, 1993). The recommended dosage for Drosophila (fruit flies) is 3 to 4 seconds (Cambridge et al., 1997). This study, on butterfly pupae that were somewhere between trees and fruit flies in size and sensitivity, used 1 minute and 5 minutes for exposure times.

Experiments done by H. Tyler et al. (1994) found that swallowtail butterflies exposed to ultraviolet radiation were either killed or experienced unspecified cell damage. McCloud and Berenbaum (1993) found that Lepidoptera: Noctudidae (Trichoplusia ni) caterpillars which were reared on plants exposed to increased UVB light developed more slowly into larvae. In finches, ultraviolet light is known to cause pigment darkening of the skin (Black,1997). Research on butterflies has demonstrated that external factors can result in aberrations in the color and wing patterns of butterflies (Scriber and Evans, 1988; Clarke et al 1989). UV light is known to have a diverse affect on different types of animals because it has the power to mutate DNA. In tiger swallowtails a DNA mutation could change the amount of black in wing patterns or the size of the butterfly. The purpose of this project was to find out what changes would occur in the wing spans, body lengths, and wing patterns of tiger swallowtails after they have been exposed to ultraviolet light.

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