Figure 8

Early air sac (AS) consisting of a single row of cell fragments (F) of similar width. First instar, Centruroides gracilis. The aligned cells release fragments of cytoplasm from their apical surface (apocrine secretion). This results in a narrow channel (~1 μm wide) filled with cell fragments about the same width. The cells (N, nucleus) are commonly tapered toward the primordial channel as fragments are released. The cell fragments are initially enclosed in plasma membrane that gradually becomes thicker and more dense with presumptive cuticle (Figs. 9, 10, 12, 13). Small dense particles (P) at the periphery of the fragments are apparently formed from components inside the fragments and/or secreted from the apical surface of adjacent precursor cells. In addition to the cell fragments, the channel contains some osmiophilic material (probably fluid). The asterisks indicate short lengths of electron-opaque material that apparently results from merocrine secretion and/or enzyme action at the apical surface of the cells. The basal surface of the aligned cells is in contact with the primordial hemolymph channels (H) that are wider in some locations (H'). Scale, 2 μm.