The ultrastructure of haemocytes in trombiculid mites (Acariformes: Trombiculidae)

The ultrastructure of haemocytes during the entire ontogenesis in Hirsutiella zachvatkini (Schluger) and in pre-larvae and unfed larvae in Leptotrombidium orientale (Schluger) was investigated using transmission electron microscopy. Two, fine- structurally distinct types of granular cells, namely granulocytes of types I and II, were observed. The granulocytes of both types are oval to amoeboid and contain numerous specific granules, their most characteristic feature, in addition to other cell organelles. No changes in the structure of the haemocytes during the development of the mites were observed. The granulocytes are most abundant in quiescent stadia in the life cycle and in particular during moulting, when they migrate in large numbers to the exuvial space, develop pseudopodia-like processes on the cell surface, which are the most distinguishing feature of type I granulocytes and, finally, disintegrate and release their granules into the exuvial space. Within the cytoplasm of type II granulocytes, large crystals, presumably of haemocyanin, are accumulated. Prohaemocytes have not been identified as a distinct cell type. The possible functions of the haemocytes and also their structure and differentiation in comparison with other arthropods are discussed.