Ultrastructure and cluster formation in ovaries of bark lice,Peripsocus phaeopterus (Stephens) and Stenopsocus stigmaticus (Imhof and Labram) (Insecta : Psocoptera)

Ultrastructure and previtellogenic growth of ovaries of Peripsocus phaeopterus (Stephens) and Stenopsocus stigmaticus (Imhof and Labram) (Insecta : Psocoptera) are described. The germ cell cluster formation was analyzed in an ovariole of a nymph using ultrathin serial sectioning. Fifteen germ cell clusters were found; 13 contained 4 cystocytes each, while 2 clusters, situated in the very tip, were composed of 2 cystocytes each. A fully developed cluster rises by 2 synchronized mitotic divisions, each followed by incomplete cytokinesis. Microtubules derived from the preceding mitoses form a transient midbody within the intercellular bridge. Later on, a fusome fills each bridge, while at fusomal rims parallel oriented microtubules are tightly associated. Some of these microtubules stretch to cell membranes nearby. The fusomes fuse into a polyfusome and a rosette is thus formed by which all intercellular bridges are drawn together. All cystocytes enter the prophase of meiosis up to pachynema. One of the 2 inner cells continues meiosis and develops as an oocyte, whereas all others transform into nurse cells. After rosette formation, the polyfusome-associated microtubules vanish and some time later, when the nurse cell-oocyte differentiation becomes apparent, the polyfusome itself becomes destroyed. The intercellular bridge, joining the first nurse cell with the 3rd moves away from the other 2. During previtellogenesis, 5 phases can be distinguished, 2 of which are interpreted as logarithmical growth phases with different slopes. The whole set of characters elaborated here for the polytrophic meroistic ovary of psocopterans is fully consistent with the characters of polytrophic meroistic ovaries of Holometabola, indicating a monophyletic origin.