ISSN:
1432-1351
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Medicine
Notes:
Summary 1. The impulse-response was used to measure the dynamics of the photoresponse of 8 species of insects from 6 orders in both light- and dark-adapted states. 2. The impulse-responses of all cells were well fitted by the two-parameter log-normal curve. 3. In the dark-adapted state, the time-to-peak of the response varies from 38 ms in the drone-fly to 55 ms in the locust. Though interspecies variation is small, the house-flyMusca (41 ms) is significantly faster than the locust. In the light-adapted state, there are highly significant variations in the time-to-peak between species. The order is: housefly (12.0 ms), drone-fly (16.5 ms), dragonfly (17.5 ms), mantid (18.1 ms), locust (21.9 ms) and cricket (22.1 ms). This variation in speed correlates with flight behavior. 4. There are significant, though small, differences in the shape of the dark-adapted impulseresponse, with that of the cockroach more symmetrical and the dragonfly more skew than the others. The impulse-response of the fly in the lightadapted state is more symmetrical than that of the other species and results in an even higher frequency response. 5. Despite these differences in shape, it is concluded that all species have a similar transduction mechanism. Interspecies differences in time-scale can, at first approximation, be accounted for by the change of a single time-constant. 6. The insects' impulse-responses were compared to those of verbrates by using the cascade models of Fuortes and Hodgkin (1964) and Baylor et al. (1974). A large number of stages were required (between 10 and 50) and a greater than 50% variation in the number of stages was needed in order to fit response from different cells within a single species. Furthermore, the basic assumption of Fuortes and Hodgkin (1964) that the timecourse is causally linked to the gain does not hold in the insect. We conclude that no first-order system of chemical cascades can sensibly predict either the time-course of the photoresponse in insects, or the effects of light adaptation and hence that the insect transduction mechanism is fundamentally different to that of vertebrates. Finally, we find that a model using two first order poles, two underdamped second order poles and a pure time delay (French 1980a, b) provides as good a fit to the frequency response as does the log-normal model.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01350224
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