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Journal of Experimental Biology 33,82-106 (1956)
Published by Company of Biologists 1956

The Epicuticular Wax Layers of the Pupa of Tenebrio Molitor L.

M. W. HOLDGATE ¹ and M. SEAL ²

¹ Department of Zoology, University of Cambridge
² Research Laboratory for the Physics and Chemistry of Surfaces, Department of Physical Chemistry, University of Cambridge

1. A study has been made of the structure of the outer epicuticle of the pupa of Tenebrio molitor (Coleoptera), particular attention being paid to a surface wax layer.

2. In this study two electron optical techniques, reflexion electron microscopy and electron diffraction, have been used.

3. At moulting, the Tenebrio pupa has a wax layer exposed on its surface. The nature of this wax has been investigated by electron diffraction. Most of the wax melts at about 40° C, and there is no indication of a ‘transition’ below this temperature.

4. A cement layer is secreted soon after moulting. Later, a surface wax layer, made up of numerous fine filaments, appears. These filaments melt at about 80° C, and are removed by wax solvents. Material obtained from chloroform washings of mature pupae shows a wide range of melting-points, but an investigation by electron diffraction provides no evidence for a ‘transition’.

5. The validity of the concept of a ‘critical temperature’ in the water-loss curves is discussed. The experimental data can in every case be fitted by smooth and continuous curves. It is, therefore, suggested that any physical changes (e.g. melting) which may occur in the epicuticular waxes, take place over a range of temperatures rather than at a definite ‘critical temperature’.

6. The water-loss curves for Tenebrio pupae change shape as the outer epicuticle develops. Mature pupae show good waterproofing to a higher temperature than do young pupae. This change in the curves is associated with the secretion of the cement layer.

7. Parallel studies have been carried out on Rhodnius nymphs and similar results obtained.

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