Effects of heavy metals (Hg2+, cd2+, Pb2+) during the embryonic development of acridid grasshoppers (Insecta, caelifera)
Heavy metals present in the soil get differently accumulated in organisms and show different rates of toxicity at different stages of the life cycle of any organism living there. To see whether such toxicants get accumulated during the embryonic development and/or change the normal developmental processes of organisms exposed to heavy metals, freshly laid eggs and egg pods of two acridids, Aiolopus thalassinus and Eyprepocnemis plorans, were tested against different concentrations of Hg2+, Cd2+, and Pb2+. At increasing Hg2+ concentration in the substrate, an increased egg mortality and reduced nymphal hatching in A. thalassinus were observed, yet no change in the normal duration for embryonic development was noticed. A higher hatching rate of E. plorans nymphs than that of A. thalassinus could possibly be due to the higher tolerance, contributed by larger egg volume and thicker foam around the egg pods of the former. Treated concentrations of Hg2+ could be of sublethal (0.121 &mgr;g Hg2+/g substrate) to lethal (0.605 &mgr;g and more Hg2+/g substrate) doses. The mortality of eggs did not always increase with increasing substrate concentration of Cd2+ and Pb2+, and the hatching rates of both grasshopper species in many cases were even higher than that in the control. Still, lower accumulation factors of Cd2+ and Pb2+ than that of Hg2+ were found coupled with not increased mortality. The tested concentrations of Cd2+ and Pb2+ in the substrates, thus, could be of subacute doses. The embryonic development was found prolonged due to Cd2+ and Pb2+. During parallel egg and egg pod treatments, higher metal concentrations were found in not developed eggs than that in fresh nymphs hatching from the same substrates. This indicated the role of foam around the egg pods as "protective filter" against the toxic substances in the soil.
Devkota
B
Department of Biological Sciences, Kathmandu University, P.O. Box. 6250, Kathmandu, Nepal.
Schmidt
GH