Friday, 31 March 2017

The butterfly’s tongue and the steel rule

‘Why is that there?’, said my four-year old grandson in an accusatory tone when he spotted a Stanley coiled flexible steel rule on my desk. ‘Ah, I need to take a photograph of it to show how a butterfly’s tongue works’, I replied. A look of utter puzzlement and then of disdain on the boy’s face demonstrated his certain knowledge of having a silly grandfather.

L.E.S. Eastham
My wish to illustrate this post with a steel rule was occasioned by a jolt of memory when I was writing a previous one on BEPS—The Invertebrata by Borradaile, Eastham, Potts and Saunders. I had a vivid recollection of the words being said, but not of the face behind the words, during a lecture at Sheffield on insects that in the old days (i.e. before 1958 when Eastham retired as Professor of Zoology) we would have had to have known how the butterfly proboscis works since he, Eastham, it was who worked it out. However, the lecturer added that the only thing about the mechanism that one had to remember was that it works like a steel rule. The more I thought about who the lecturer was, I have the vague notion that it was the Sheffield born-and-bred—and educated—Fred Segrove (1911-2003).




Having remembered nothing else about the butterfly proboscis, I recently found the original paper by Eastham and Eassa, published in 1955 when Eastham was 62. It is a highly impressive and long paper and involved making serial sections and some physiological studies. It demolished previous theories on how the proboscis worked. Eastham himself provided a short explanation in the later editions of BEPS:

The adults live on the nectar of flowers, and to absorb this a highly specialized proboscis has been formed from the greatly elongated galeae of the maxillae, each being grooved along its inner face and locked to its neighbour…Each half of the proboscis is a tube in itself into which passes blood from the head, and also a trachea and a nerve. Across the cavity of this tube there pass a number of oblique muscles. At rest the proboscis is tightly coiled like a clock spring under the head. When feeding the proboscis is extended and its tip placed in the food source. It is now recognized that the elastic properties of the cuticular wall of the proboscis account for the coiled condition when resting. Extension of the proboscis is brought about by the internal oblique muscle of each galea. These, working in conjunction with a stipital valve controlling the closure of the passage between cephalic and galea haemocoeles, cause the proboscis to develop a dorsal keel along its whole length. The attainment and retention of this new shape depends on the turgidity of the galea tube and the elasticity and flexibility of parts of the cuticular wall. For mechanical reasons it cannot in the keeled position be retained in the coiled state and extension of the proboscis results.In feeding, a complex pharyngeal muscular apparatus causes the fluid food to be sucked into the mouth. The length of the proboscis in many cases corresponds to the depth of the corolla of the flower which the species frequents, and in the Sphingidae (hawk moths) may be greater than that of the body…

In Eastham’s chapter in BEPS there is though no mention of the analogy of a retractable, flexible steel rule, which was used in the original paper and in which a Mr F.W. Adams was thanked for thinking of it:

…a coiled steel rule, when coiled in its case, is flat in section, but when it is drawn out it assumes a curved transverse section, convex on one side and concave on the other, and in this state attains a condition of extension. In this case the rule has to be forced into its case, and bending can only occur when its transverse curvature is flattened out. The condition of rest is one of extension and force has to be applied to coil it up, the rule having been manufactured with these properties.

The difference between a steel rule and a butterfly’s proboscis is, of course, that it is muscular action which produces the curved transverse section for extension and elasticity which returns it to a flat cross section after the muscles relax and thence its recoil.

Small Tortoiseshell with proboscis coiled

Comma feeding with proboscis extended. Eastham & Eassa also explained
how the 'knee-bend' in the proboscis was formed

There is an statement in the paper which sixty-odd years after it was written I find odd:

…As a result of complete analysis of these structures, of observations on the animal during feeding—of numerous operations involving nerve sections and perforations of the haemocoele—a new theory is offered on the proboscis mechanism for which the senior author (L.E.S.E[astham]) alone is responsible.

I can think of several explanations for this statement. Did Eassa disagree is one. Did Eastham think that if the new theory were to be shot down in flames, he was making sure that Eassa could not be blamed? But then the paper was sent to the Royal Society for publication by V.B. Wigglesworth, the name in insect physiology in the 20th Century; he must have been sufficiently impressed. There remains the possibly unworthy thought that Eastham was making sure he got the credit but that goes against Eastham’s reputation in Sheffield amongst those who had worked in his department like E.T.B. Francis, F. Segrove and J.D. Jones as a gentlemanly, paternal figure. And what had Eastham to gain at the age of 62 and three years from retirement? The only possibility is that he was, and perhaps for the last possible time, up for election to the Royal Society. He, like all the other authors of BEPS, was not elected but I do not know—and cannot know until the archives for that period are opened—if he was ever proposed.

But what of Eassa? Who was he, what was he doing in Sheffield and what happened to him?



Youseff Ezeddin Eassa (1914-99) was a famous Egyptian author as well as being Professor of Zoology at the University of Alexandria. There is a website devoted to him and his work. He was a graduate of the University of Cairo and is shown as arriving in Sheffield in 1948 as a Ph.D. student but also as an established playwright and author. However, his Ph.D. thesis (A contribution to the postembryonic development of the head of Pieris brassicae (Linn)) is dated 1949 so he may have travelled to Sheffield earlier than 1948. He must have stayed in Sheffield for some time after 1949 because the Eastham & Eassa paper was sent to the Royal Society in April 1954 and no new address for Eassa is shown.

While in Sheffield he wrote stories and plays that were broadcast on BBC radio. He recalled that his favourite companion at coffee was Hans Krebs, later Sir Hans and then Professor Biochemistry at Sheffield. Eassa was a Fulbright Fellow in Berkeley and Illinois in 1960-61.

Youseff Ezeddin Eassa (from here)

I hope I get the opportunity of explaining to my grandchildren not only how the butterfly feeds, using, of course, the steel rule as an aid, but also of making sure they know that international scientific collaboration was not invented by the EU (as some young academics in Britain clearly believe) and that academics could meet and exchange ideas in university staff (faculty for readers more familiar with the U.S. university system) clubs rather than remain ensiled, as appears to be the case at present, in departmental comfort zones.

Oh, and here is the rule:



Eastham LES, Eassa YEE. 1955. The feeding mechanism of the butterfly Pieris brassicae L. Philosophical Transactions of the Royal Society B 239, 1-43.