The Organs of Adhesion in the Aphid Megoura Viciae

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Abstract
The anatomy of the adhesive organs is described in Megoura viciae Buckton. The claws serve as grappling hooks which are locked in position by the retractor unguis tendon. The pulvilli, pliable cuticular sacs, are everted from the tibiotarsal articulation. They bear a surface adhesive, a non-volatile oil, which is secreted through the cuticle. The pulvilli are everted by blood pressure and are withdrawn inside the tibia by contraction of the tibial muscles. The adhesion of these organs to various types of surface was tested by covering over the pulvilli or by claw amputation. Pulling forces were applied by progressively loading the aphid with weights. The pulvilli perform best on relatively smooth surfaces. Aphids are dislodged from the underside of polished glass by a mass of approx. 9–14mg (a pulling force of 8.8-13.7×10−5N). The claws cannot engage a completely smooth surface but are highly effective on rough ones, withstanding a load of approx. 57 mg on dry filter paper. Adhesion of the pulvilli to waxy surfaces is only slightly less than on glass but is greatly reduced on surfaces with still lower free energies, such as Teflon. On a glass surface the pulvilli sometimes leave ‘footprints’ consisting of oily droplets of variable size. Nevertheless, in many instances such deposits are small or absent, indicating that the bond sometimes breaks between the adhesive and the substratum. In a selected series of footprints with large deposits, their volume was found to be sufficient to form a layer at least 17.7x−9m in thickness over the area of pulvillar contact. This is consistent with the view that surface tension forces would adequately account for adhesion, an additional limiting factor being the physical properties (wettability) of the substratum. Viscous forces would impede the very rapid (less than 0.02 s) removal of the pulvilli, were it not for the retraction mechanism which peels off the pulvillus from the distal edge, like a piece of adhesive tape. The ultrastructure of the pulvillar gland is described. The epidermal cells discharge into a cavity which is confluent with a further reservoir within the spongy endocuticle, itself a meshwork of dissociated cuticular microfibrils. The points of exit of the secretion appear to be the epicuticular filaments. The product of the gland, which can be collected by pricking the pulvilli, is water soluble and proteinaceous, differing entirely from the hydrophobic lipid-soluble end-product on the pulvillar surface. We presume that the lipoprotein precursor is dissociated enzymatically at the level of the inner epicuticle.

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