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The Maintenance of Annual Life-Cycles in two Species of Tipulidae (Diptera); A Field Study Relating Development, Temperature and Altitude

J. C. Coulson, J. C. Horobin, Jennifer Butterfield and G. R. J. Smith
Journal of Animal Ecology
Vol. 45, No. 1 (Feb., 1976), pp. 215-233
DOI: 10.2307/3776
Stable URL: http://www.jstor.org/stable/3776
Page Count: 19
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
The Maintenance of Annual Life-Cycles in two Species of Tipulidae (Diptera); A Field Study Relating Development, Temperature and Altitude
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Abstract

(1) The study was designed to investigate the development of Molophilus ater and Tipula subnodicornis (Tipulidae) at a series of altitude sites on the Moor House National Nature Reserve. (2) Detailed records of the soil temperature were made at eight altitude sites, from 370 m to 847 m a.s.l. These showed that there was a decrease in the mean annual temperature of 1 degrees C for 207 m rise in altitude, and that the temperature gradient varied seasonally being most pronounced in autumn and least in winter and spring. (3) In general, higher altitude sites were colder throughout the year and there were marked differences in the temperature-sums, equivalent to those produced by large differences in latitude. (4) The date of adult emergence of both species is delayed at higher altitudes (seven days for Molophilus ater and eleven days for Tipula subnodicornis between the 520 m and the 820 m sites) but not to the degree expected from the differences in annual temperature sum. (5) Laboratory experiments have shown that the delay in the emergence of the two species at higher altitudes, could be accounted for purely by the lower temperatures influencing the length of the pre-pupal and pupal period, both of which have been shown to be markedly temperature-dependent. (6) This conclusion is supported, in the case of Molophilus ater, by field experiments in which larvae were transferred from higher to lower altitude sites. In all cases the time of emergence assumed that of the host site, even when larvae were transferred just before pupation. (7) Larvae taken from any of the altitude sites in midwinter pupate at the same mean date when put at a constant temperature of 10 degrees C in the laboratory. They are therefore, despite the differences in temperature regimes in the field, at the same stage of development. (8) Methods whereby larvae living under different temperature regimes could reach the same stage of development at the same time (in this case in winter) are considered. (9) No evidence was obtained of larvae pupating at markedly different sizes. (10) A diapause in the overwintering larvae (final stadium) may help to synchronize the life-cycle but, at least in M. ater, the main reason for larvae at the different altitude sites being at the same state of development by midwinter is that larval growth is independent of temperature. (11) It is suggested that such an independence of temperature during larval growth is a major adaptation to life under montane conditions, where there are considerable variations in altitude and mean temperature.

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