Field Life-histories of Four Species of Calathus (Coleoptera, Carabidae)

Abstract

DURING October 1950–August 1952 populations of four beetles, Calathus erratus Sahlb., C. fuscipes (Gze.), C. mollis (Marsham) and C. melanocephalus (L.), occurring together on coastal dunes in Anglesey, were sampled regularly. Callow imagos of all species were found in late May and early June, this being probably the only period of emergence, synchronous pupation having occurred throughout the populations during April. Excepting C. fuscipes, older imagos, presumably of the previous year's emergence, occurred with the callows. Both males and females of this older fraction contained ripe gametes in what is believed to have been their second summer as adults; examination of the ovaries showed that they had produced eggs at least once previously. During June and July about half the active populations of C. erratus and C. mollis consisted of the older fraction. Maximal activity for the whole adult populations occurred during July and August in C. erratus and C. fuscipes, the numbers of trapped animals then declining to the virtual disappearance of adults of these species in October. Some adults survived the winter to reappear in the following spring. C. mollis and C. melanocephalus adults, on the other hand, had their peak during September and early October; the numbers trapped decreased after this although adults were present throughout the year. In C. erratus and C. fuscipes the egg-laying season extended through July, August and September, the larvæ overwintering in the final (third) instar. Egg-laying in C. mollis and C. melanocephalus occurred during Augusts–February with a peak in the autumn; overwintered adults may have laid eggs in July. In both species larvæ of all three instars were found in winter. As mentioned above, pupation occurred in all four species during April. This synchrony suggests that pupation was triggered by an environmental factor; the fate of any larva not then ready to pupate is unknown.