Two samples from early developmental stages were fixed in 3% glutaraldehyde for 24 h. Mandibles were washed three times in 0.1 M cacodylate buffer and post-fixed in 1% OsO4 solution for 1 h. Samples were embedded in epoxy resin Durcupan. Semithin sections were stained with Toluidine Blue to evaluate general morphology. Ultra-thin sections were placed on formvar-coated nickel grids and contrasted with lead citrate and uranyl acetate. More details were published previously elsewhere17.