Sensitive Periods: Hugo De Vries, Jacques Loeb, and the Porthesia Larvae

By Uma Ramani

I first heard the story of Hugo De Vries and the butterfly larvae in the context of the sensitive periods while in training – I still have the notes I furiously scribbled down at the time. In the decades since, I have heard the story repeated in other training courses. A few years ago, I set out on a quest to find the primary source for this reference – not Dr. Montessori’s writings, but the original work by De Vries. With so many scientific publications having digitized their archives, I was sure that I would be able to find the original paper. I was surprised, however, that I could find only Montessori literature that made any reference to Hugo De Vries, the sensitive periods and the Porthesia butterfly. 

A broader search for Hugo De Vries revealed that he was Professor of Botany at the University of Amsterdam, and that his work focused on mutation. In his experiments with poppies, De Vries found that there was a specific period of time during which the seedlings were extremely susceptible to environmental conditions. He referred to this as the “sensitive period.”

“In the young flower-bud of the pistilloid poppy there must evidently be some moment in which it is definitely decided whether the young stamens will grow out normally or become metamorphosed into secondary pistils. From this moment no further change of external conditions is able to produce a corresponding change in the degree of the anomaly. The individual strength of the whole plant may still be affected in a more or less manifest degree, but the number of converted stamens of the flower has been definitely fixed. The sensitive period has terminated…” Hugo De Vries, Species and Varieties: Their Origin by Mutation.  Lectures delivered at the University of California, 1904

All the publications by Hugo De Vries that I could trace were related to plant studies and there was no reference to his work with butterfly larvae. Recently his biographer, Erik Zevenhuizen confirmed that Hugo De Vries was a botanist. In an email to Heidi Phillippart, Trainer in Training (A to I), who followed up with the quest, he wrote,

Dear Heidi Philippart,

Your message was forwarded to me by Reinout Havinga from the Amsterdam Hortus Botanicus.

Your question is indeed quite strange, at least it is to me. For my biography of Hugo de Vries I have thoroughly studied his plant physiological research. Not being a biologist myself I had to read and re-read (sometimes several times) everything he wrote about it in order to understand what he was doing. So I can assure you that De Vries never used caterpillars in his research, nor any other animal. The same goes for his genetic research. De Vries was every inch a botanist, in fact I got the impression that he was not even remotely interested in zoology. So your question surprised me, the more so as I have never read or heard anything about De Vries working with caterpillars. I hope this answer will contribute to busting this myth!

Best wishes,

Erik Zevenhuizen

Puzzling indeed! I went back to check Dr. Montessori’s writings. In the Secret of Childhood, she writes,

“And this was the next discovery, the discovery of the sensitive periods…

It was the Dutch scientist Hugo de Vries, who discovered the existence of sensitive periods in animal life, but we ourselves, in our schools and by observing the life of children in their families, were the first to discover the sensitive periods of infancy, and to respond to them from the standpoint of education." Montessori, The Secret of Childhood

But Hugo De Vries did not work with animals… So, who did the work with the butterfly larvae?


Introducing Professor Jacques Loeb M.D. Professor of Physiology, University of Chicago. Studying animal physiology and focused on the functioning of the brain, Professor Loeb conducted a series of experiments on different animals, among them the Porthesia larvae, during the last decade of the 19th century. In his book, Comparative Physiology of the brain and comparative psychology – G.P. Putnam’s Sons, New York, 1900 Page 188-189, he writes:

“The larvae of Porthesia chrysorrhoea creep out of the eggs in the autumn and winter in colonies in a nest on trees or shrubs. The warm spring sun drives them out of the nest and they crawl up on the branches of the tree or shrub to the tip, where they find their first food. After having eaten the tips, they crawl about until they find new buds or leaves, which in the meantime have come out in great numbers. It is evident that the instinct of the caterpillars to crawl upwards, so soon as they awake from the winter’s sleep, saves their lives. Were they not guided by such an instinct, those that crawl downwards would die of starvation…

“I have found that the young caterpillars of Porthesia are oriented by the light. Until they have taken food they are positively heliotropic. This positive heliotropism leads them to the tips of the branches where they find their food…

“We have seen, however, that these same caterpillars leave the tips of the branches as soon as they have eaten and crawl downward. Why does the light not hold them on the highest point permanently? My experiments showed that these caterpillars are only positively heliotropic as long as they remain unfed’ after having eaten they lose their positive heliotropism.”

Professors Loeb and De Vries met in California in 1904 and they remained friends. Did they discuss their work and find the connections between their fields of experimentation? 

A further reading from The Secret of Childhood can be interpreted differently in the light of the above:


“In biology these periods were first studied by De Vries, and are particularly apparent in living creatures that reach their adult state through metamorphoses, as in the case of insects. We may take, for instance, the butterfly caterpillar…

“It is a strange fact that when the caterpillar has passed through its first stage and is full grown, it can eat other food, and then loses its sensibility to light. This has been proved in scientific laboratories where there are neither trees nor leaves but only the caterpillar and the light. The caterpillar will wriggle rapidly towards any ray of light that comes through a hole in the dark box in which it has been enclosed for the experiment. After a certain period, rays of light leave it completely indifferent.”  Montessori, The Secret of Childhood

In this paragraph, Dr. Montessori does not directly attribute the work with the butterfly larvae to Prof. De Vries. But in all these years of reading this text, I have always jumped to the conclusion that it was De Vries who had worked with the Porthesia larvae. 

So many of Dr. Montessori’s writings are transcribed from her lectures – all of them have been translated from the original Italian. It is not surprising that some things are lost in translation. Where did Dr. Montessori encounter the work of De Vries and Loeb? And how did she make the connection to children? The secrets may lie in the archives in Amsterdam…

This discovery about the sensitive periods does not change our practice or have any direct implication for our work. But we can begin to give credit to Professor Loeb and continue to marvel at Dr. Montessori’s ability to make inter-disciplinary connections. And it is a reminder that we must continue her legacy of scientific inquiry.

Uma Ramani is the Director of Primary Training at the Montessori Institute of North Texas. She has over 30 years of experience in Montessori education. Uma shares her passion for Montessori as a coach, consultant, AMI examiner, and presenter across the United States and in India.

Posted on November 10, 2015 and filed under From MNW Staff, Resources.