The specific heat of the cuticle and the morphological differences between the brown and yellow cuticles of hornets.

The morphology and thermophysical properties of stripes of brown and yellow cuticle collected from specimens of the Oriental hornet, Vespa orientalis (Hymenoptera, Vespinae), were analysed and compared by scanning and transmission electron microscopy. In the brown cuticle the colour stems from incrustation of the pigment melanin. In the yellow cuticle the yellow coloured pigment is concentrated in pockets between two layers of cuticle and its colour shows through the transparent cuticle. Both brown and yellow cuticles are composed of more than 30 layers. The upper layers are about 1-4 microm thick and as we proceed inwards, they become thinner so that the deepest ones that can still be discerned are thinner than the upper-most layer by one or two orders of magnitude (i.e., x 1/10 - x 1/100). The brown and yellow cuticles are each about 20 microm thick, but owing to the yellow pigment inserted in pockets of the yellow cuticle the total thickness of the latter is about 45 microm. The morphology of the yellow granules that contain the yellow pigment is shown and discussed. Below 20 degrees C and above 40 degrees C the specific heat (Cp) readings for both types of cuticle coincide, their values lying between approximately 1.55 and 1.60 J kg(-1), but in the temperature range 20-40 degrees C, the specific heat of the yellow strips displays a sharply non-monotonic behaviour, attaining a value of Cp = 1.8 J / Kg at 30 degrees, which contrasts with the relatively smooth curve shown by the brown strips. This anomaly (hump) does not occur following preheating of the samples to 70 degrees C. The shape of the repeated (annealed) 'yellow' curve is basically the same as that of the brown curve. The pockets of yellow pigment and their thermal contribution can be evaluated quantitatively by integration and correspond to an enthalpy of H = 3.5 J g(-1). The difference between the two types of hornet cuticle and the significance of this difference is discussed.