Cuticle ultrastructure with respect to the lipid waterproofing barrier

Lipids (waxes) deposited on or incorporated in the epicuticle provide an important waterproofing barrier in most terrestrial insects and arachnids. Although the chemical nature of these lipids is well documented, ultra‐structural evidence for their association with specific cuticular layers and sublayers is less conclusive. Difficulties here include relating layers observed with the electron microscope with layers seen under light microscopy, plus the fact that much lipid is removed in normal preparative procedures for electron microscopy. Moreover, epicuticular layers which presumably are lipid‐rich are often resistant to organic solvents. The latter suggests that if lipid is present, much of it is bound, possibly to protein. Preliminary studies using new techniques such as myrcene partitioning and freeze‐fracture microscopy which improve lipid rétention and visualization have yielded some promising results. Scanning electron microscopy of surface waxes, in contrast, has revealed complex lipid accumulations which can be altered or removed by appropriate organic solvents. Surface waxes no doubt supplement lipids associated with epicuticular layers in restricting water loss, with the effectiveness of the former being dependent upon their thickness, physical structure and arrangement, and their chemical composition. Pore and wax canals are likely the major routes via which lipids are transported from sites of synthesis to the epicuticle, but the exact mechanisms involved must still be resolved.