An Electron Diffraction Study of the Crystalline Structure of the Lipids in the Pupal Exuviae of Calliphora Erythrocephala

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Abstract
A comparison is made between the known crystalline structures of long-chain fatty acids, paraffins, and the free and bound lipids in the epicuticle of Calliphora pupal exuviae. Artificial collodion-wax membranes give electron diffraction patterns which are similar to those of the insect cuticle, indicating that the free, or thermo-labile waxes in the epicuticle consist of a three-dimensional arrangement of orthorhombic micro-crystals oriented perpendicular to the cuticle surface, but otherwise distributed at random. The thermo-labile waxes of the epicuticle undergo irreversible disorientation when the pupal skin is exposed to high beam currents, and similar changes occur with the artificial collodion-fatty acid membranes. This is correlated with the mechanism of membrane formation from a solution of fatty acid and collodion in amyl acetate. When a drop of solution is allowed to spread on a water surface, a lens is formed and a monolayer of fatty acid adsorbed at the amyl acetate-water interface. This monolayer acts as a basis on which an alinement of crystals occurs in the bulk framework of the lens as the solvent evaporates. The crystals are immobilized in the membrane of collodion which is simultaneously deposited. When the dry membrane is heated no monolayer is present to act as an organizing factor in crystal growth, which occurs at random in the bulk framework of the collodion membrane. Similar random crystallization occurs in collodion-paraffin membranes, where the waxy component is non-polar, and does not form a monolayer at the solvent-water interface during membrane formation. Polymorphic crystalline modifications are observed after the collodion-fatty acid membranes and the Calliphora pupal skin are exposed to a more intense beam. Apart from a reduction in the degree of preferred orientation of the thermo-labile waxy components, there is a transition from an orthorhombic to a hexagonal type of lateral close crystalline packing. This is correlated with the irreversible increase in permeability of the cuticle to water at high temperatures. After the thermo-labile lipids are removed from the pupal skin by chloroform, the residual lipids give a diffraction pattern which does not fade when the cuticle is heated. The pattern suggests that the bound lipids can form orthorhombic crystalline aggregates in the cuticulin layer, and that the thermo-stability is due to association between the lipids and the structural proteins of this layer. The organized structure of the bound lipids is destroyed when the pupal skin is in contact with strong alkali before the membrane itself is disrupted. The pupal skin is homogeneous when examined with the electron microscope at a screen magnification of 20,000 times. The structural relationships between the free and bound hpids of the epicuticle are discussed.

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