Morphological and cytoskeletal characterization of hemocytes in stick insects (Phasmatodea)

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Abstract
The cell content of the hemolymph from the two species of stick insects Bacillus rossius and Carausius morosus was characterized in morphology and cytoskeletal distribution by flow cytometric analysis, phase‐contrast microscopy, and scanning electron microscopy. In in vitro cultured hemocytes, the cytoplasmic distribution of F‐actin microfilaments was revealed with Rhodamine‐labelled phalloidin, and that of microtubules by indirect im‐munofluorescence with a monoclonal anti α‐tubulin. The results show that both stick insects possess peculiar hemocyte sub‐populations, as detected by flow cytometry, composed of plasmatocytic and granulocytic cells. Carausius morosus has a hemocyte subtype, the spherulocyte, that appears to be lacking in Bacillus rossius. The results of cytoskeletal analysis show that the plasmatocytes of both species, smeared on glass coverslips and compared with cultured mammalian epithelial control cells, possess a regular cytoskeletal network, with F‐actin microfilaments organized in stress‐fibres, and microtubules spanning the cytoplasm. In rounded, non‐plasmatocytic cells, the F‐actin stress‐fibres were not detected, and the microtubules were arranged in a fashion resembling that present in human lymphocytes. These data may reflect different functional and/or immunological specializations among stick insect hemocyte subpopulations, and suggest similarities in cytoskeletal organization and cell isotypes between insect and vertebrate blood cells.