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How does a Fly Cling to The Under Surface of a Glass Sheet?
1 Department of Zoology, The University, Downing Street, Cambridge CB2 3EJ, UK
The climbing of flies on glass is a problem that still lacks an agreed solution. West (1862), who introduced the term ‘tenent hairs’ for the trumpet-like structures on the pulvilli, regarded these as suckers which ensured adhesion by atmospheric pressure. But it is generally agreed that they are too small for that. Dahl (1885), studying adhesive organs in insects in general, favoured adhesion by ‘capillarattraction’ but he considered that other forms of cohesion and adhesion were operating as well. Rombouts (1884) opted for surface tension. In the most recent paper, Walker, Yule & Ratcliffe (1985) give a new description of the adhesive organs in the blowfly Calliphora, and conclude from calculations that surface tension of the lipid secretion under the tenent hairs provides an adequate adhesive force.
The pad of setae at the lower extremity of the tibia of the first two pairs of legs, which is present in the adult of the blood-sucking bug Rhodnius and is used in climbing on a smooth surface (Gillett, 1932), has a totally different structure from that of the adhesive organ of the fly. In making a combined investigation (Gillett & Wigglesworth, 1932) we believed at first that surface tension in the lipid secretion between the oblique endings of the hairs and the surface would prove to be the adhesive force. But after experiments on insects and models, helped by discussions with N. K. Adam, we decided that direct intermolecular attraction, generated when the lipid film breaks down during sliding (i.e. seizure or partial seizure), was probably the more important force.
The tibial organ is present only on the first two pairs of legs of the adult Rhodnius, but these large (2 cm) insects can climb a steeply sloping sheet of glass. When the glass is vertical they slip down very slowly. In the experiments of Edwards & Tarkanian (1970), some Rhodnius adults held on to glass at angles greater than vertical, up to a mean of 109°. Beyond that they fell off. These are characteristic properties of the frictional forces dependent on molecular attraction, which do not resist separation by forces acting in the axis normal to the surface unless the areas in contact are molecularly smooth (Bowden, 1957). Edwards & Tarkanian (1970), in repeating the experiments on Rhodnius, accepted our interpretation - with the proviso that meniscus (surface tension) forces will, of course, be operating at the same time.
Key words: adhesive organs, molecular attraction, Calliphora, Rhodnius
Accepted on January 16, 1987
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