Diapause and polyphenism of life-history of Lagria hirta

This paper is focused on diapause and polyphenism of development of Lagria hirta L. and tries to unravel its mechanism of life-history adaptation. Lag ria hirta, distributed widely in Europe, has a strictly univoltine life cyc le. The results showed that larval diapause and moulting polymorphism were the deciding factors that made L. hirta maintain its univoltinism and keep a flexible relation between seasonal changes and life-history phases. In th e laboratory, larvae of this species were not able to pupate if kept at con stant temperatures of 5 degrees, 10 degrees, 15 degrees, 20 degrees, 25 deg rees or 30 degrees C combined with a photoperiod of either LD (L16:D8) or S D (L8:L16). Pupation only occurred if larvae were reared at 15 degrees-25 d egrees C when intervened by a three-month chilling at 5 degrees C in stages L-3, L-4, L-5 or L-6. A chilling treatment was shown to be obligatory for the termination of its larval diapause and had an accelerating and synchron izing effect on larval development. Larval diapause of L. hirta was charact erized by no pupation and more moulting in advanced instars, longer duratio n of each single stage, and moulting desynchronization. Larval development was found to be variable with respect to the total number of instars: most larvae underwent a total of seven or eight moults; some larvae might even m oult once or twice more, but they seldom pupated. It seemed that the choice for the 7-instar or the 8-instar development did not directly relate to an y of the external conditions, such as temperature, photoperiod, and stages with chilling treatment. This polyphenism was observed in the same group un der identical conditions and even in a single egg clutch. In L. hirta, over wintering in different stages of L-3-L-6, and choosing the 7- or 8-instar p athway of development are two features that increase the plasticity and fle xibility in coordinating its life cycle with seasonal change, that varies u npredictably from year to year.