The morphology of the head sclerites of final instar larvae of aphid parasitoids prepared from cast larval skins and excised heads of whole larvae are compared, using both optical and scanning electron microscopy. Remarkable differences in the apparent shape of head sclerites, particularly mandibles, were found. Some of these differences are probably due to a consistent dorso-ventral flattening of the head during moulting, observed in cast skin preparations, whereas excised heads tend to flatten antero-posteriorly. The importance of these findings for understanding the three-dimensional shape of final instar mandibles and for the study of larval morphology and taxonomy of Hymenoptera is discussed. none