The ability of the two-spotted spider mite, Tetranychus urticac. Koch, to detect and avoid various wavelengths was examined. The mites were given a choice between two regions: an area on a leaf exposed to light of defined spectral composition or an area protected from certain selected wavelength components. The mites showed little avoidance of visible, UV-A or UV-C wavelengths. In contrast, the animals strongly avoided the UV-B spectral region, indicating that this arthropod can discriminate and respond to the presence of UV-B wavelengths. The adult mites did not appear to be killed by the amount of UV-B they could receive on a sunny summer day in Kentucky (1.e. 20SU). However, the egg laying capacity of the females was reduced in a linear fashion as dose increased. Thus, avoidance of high UV-B regions on a leaf may contribute to the survival of the species in nature.

Three species of protozoans and two crustaceans were irradiated with simulated solar ultraviolet radiation to investigate their ability to detect and avoid UV-B (32C280 nm). Horizontal and vertical movements to sheltered areas by these organisms suggest that UV-B is an important environmental factor. UV-B survival curves were determined which indicated the resistance for each organism studied. The tolerance correlated well with the positioning behavior (i.e. sensitive organisms avoided exposure by moving into sheltered areas whereas the more resistant organisms showed a less pronounced avoidance).

The extreme variation in biological effectiveness of the various components of solar ultraviolet radiation (solar UV) which reaches the earth's surface, especially photons of wavelengths between 295 and 330 nm, makes the dosimetry of solar UV a complex and, as yet, unresolved problem. A proper weighting of the various components of solar UV would permit expression of expsoure as a single parameter (dose). Weighting could compensate for the variations in composition of solar UV which might occur during exposure or the differences in sources of UV radiations; weighting would permit comparison of exposures at various locations on the earth and extrapolation of laboratory observations to field situations where wavelength composition might be rather different. Various radiation-sensitive microorganisms have been proposed as biological dosimeters. Biological dosimeters automatically weight the subcomponents of solar UV differently than a purely physical irradiance meter. We have examined the available evidence regarding the weighting which repair-defective mutants provide in comparison with response of a number of wild-type organisms and would caution investigators that, for broad-band UV sources, especially those with significant biological actions through the range of 300-330 nm, repair-sensitive mutants may improperly weight the components, leading to errors of dosimetry and thus to possible errors of interpretation of results of solar UV exposure of wild-type organisms Repair-defective microorganisms have been proposed as potential biological dosimeters suitable for monitoring UV radiations (Harm, 1969;Billen and Green, 1975;Tyrrell, 1978). Biological dosimeters have many advantages for the purposes of evaluating the ecological actions of solar UV radiation. As noted by Tyrrell (1978) they may constitute a sensitive, temperatureindependent, convenient and relatively inexpensive form of integrating dosimeter. In addition, biological dosimetry systems are intended to accomplish another function which is essential and not obvious. Different wavelength components of broad-beam UV sources (such as sunlight) will vary greatly in their biological effectiveness. Biological dosimeters will automatically weight the incident UV components in relation to the effectiveness of the different wavelengths. The measured result (survival of the exposed microorganisms in most cases) will depend on the summation of the incident irradiance rimes the biological effectiveness of the various incident photons. For instance, the mutant system proposed by Tyrrell (1978) would weight a 254-nm photon 1 million times greater than a 365-nm photon (see Fig. 1). Before employing biological dosimeters based on repairdefective mutants for the evaluation of the ecological action of broad-band UV sources such as sunlight on repair-competent organisms, one should consider the appropriateness of the weighting factor for the wavelengths in question.There is reason to believe that there may be significant qualitative differences in the injurious action of ...

There is an increasing likelihood that the solar UV-B radiation (lambda = 280-320 nm) reaching the earth's surface will increase due to depletion of the stratospheric ozone layer. It is recognized that many organisms are insufficiently resistant to solar UV-B to withstand full summer sunlight and thus mechanisms which facilitate avoidance of solar UV-B exposure may have significance for the survival of sensitive species. There are many alternative pathways which would lead to avoidance of solar UV-B. We have investigated the dynamics of biological reactions to stimulated solar UV-B radiation in two small arthropods, the two-spotted spider mite Tetranychus urticae Koch and the aquatic copepod Cyclops serrulatus. Observations of positioning and rate of movement were made; a mathematical formalism was developed which assisted in interpretation of the observations. Our observations suggest that, although avoidance would mitigate increased solar UV-B effects, even organisms which specifically reduce their UV-B exposure would encounter additional stress if ozone depletion does occur.

Four differently pigmented species of planarians were examined for UV-B (i 7 320 280nm) tolerance and ability to detect and avoid CV-B-exposed areas. I t was found that lighter-pigmented species are more UV sensitive and that movement out of the UV-B is directly related to UV sensitivity. Three species showed positioning preference corresponding to their pigmentation; the darker the speclcs, the less avoidance of UV-B. The white (unpigmented) species showed exceptional and very sensitive behavior. IhTRODUCTlOh