Leeches in Sri Lanka: A Coincidence Shines the Light

Last November, having got drenched from a sudden tropical shower, I was sheltering with others in the party at the entrance to Sinharaja Forest Reserve in Sri Lanka (Ceylon, in old money and to stamp collectors) while eating my packed lunch. Leeches were all around us. I was watching the leeches on the edges of the path through binoculars just after the rain had stopped. I noticed that as the sun started to break through, many of the leeches in deep shade started to loop towards to the bright areas. That the leeches (a fair way from the attraction of human bodies) might be phototropic crossed my mind. A just-so story flitted across the consciousness: animals like wild boar and deer move along defined paths which are more exposed to the sun than the undergrowth, hence it would make sense to hang around light places in the hope of passing trade. While I am singularly attractive to lurking leeches (with ample evidence from Borneo and, now, Sri Lanka) and particularly inept at spotting their attachment quickly enough, my attention was soon taken by having to remove two of them that were taking an armful and a legful, respectively, and the question of leeches and light was put to the back of my mind.

Last week, I was looking up something else entirely. I was reading through J.R. Baker’s Biographical Memoir¹ and suddenly came across this:

It is certainly true that anyone who visits the wilder parts of Ceylon [Sri Lanka] is liable to be set upon by leeches, but not many people would think of sitting down at a table to play a game with them. Baker noticed that the leech is not only chemically attracted to the human body but it is also strongly phototropic. Thus a leech placed upon a table illuminated by a table lamp can be controlled in the direction of its movements by periodically changing the position of one’s seat or of the table lamp.

Baker spent the Oxford summer vacation of 1936 in Ceylon. His great-uncle was Sir Samuel Baker (1821-1893) the famous explorer of Africa, and there remained a family financial interest in a tea plantation at Nuwara Eliya. Baker, the great-uncle, had chartered a ship and taken people, animals, equipment and seed in order to found an agricultural settlement there in 1848; he stayed until 1855, leaving his brother in charge of further development, first of the farm and, later, as tea growing became established, of a tea plantation. We have been to Nuwara Eliya twice: a drive-through in 1968 and to stay in 2013. Apart from resembling a Scottish town in terms of the architectural style of the houses, the fields around Nuwara Eliya are full of typically British crops like leeks (field upon field of them) and beetroot (delicious curried). That’s all down to Baker apparently.

I was even more surprised to find in the references at the end of the Biographical Memoir, that Baker had been based at Sinharaja from the end of July to the beginning of September; his paper, describing what he did and what he saw is entitled, The Sinharaja Rain-Forest, Ceylon². He notes: 

The abundance of land-leeches (Haemadipsa zeylanica, Moquin-Tandon) greatly detracts from the pleasure of studying the Sinharaja Forest. I found it best to wear thin breeches cut like "plus-fours," stockings, boots with the tongues sewn in right up to the top, and puttees. This clothing by no means prevents leeches from penetrating to the skin of one's legs, but it greatly reduces the annoyance.

Baker included a map showing where he had camped (about 5 miles from the lodge where we stayed last year) and where he had walked. His walks had taken him along the same rough track we had walked and where I had sheltered and watched the leeches.

Here is Baker’s map of Sinharaja:

And here is a very short video of the leech in question and its acquisition of my blood:

John Randal Baker (1900-1984) was, and still is, well known for a number of different activities: the breeding season of birds in the tropics and the factors that trigger the onset of reproduction; the introduction of the terms proximate and ultimate causation; his cytological studies and textbook on the subject; his work on chemical contraception; his wars, especially with J. Brontë Gatenby, on the existence of the Golgi apparatus.

I have done a quick search to see what is now known of phototropism in leeches. Unfortunately, some people have interpreted his anecdotal findings of evidence of thermotropism; in other words, that his leeches were moving towards the heat of the lamp, but did not do proper experiments to distinguish between the two. So there we are, a small research project for somebody awaits. I can almost feel the cold light of an led torch being turned on.

1 Willmer, E.N., Brunet, P.C.J. 1985. John Randal Baker. 23 October 1900-8 June 1984. Biographical Memoirs of Fellows of the Royal Society 31, 33-63.

2 Baker, J.R. 1937. The Sinharaja rain-forest, Ceylon. Geographical Journal 89, 539-551.