Darwin’s work on insectivorous plants began by accident. While on holiday in the summer of 1860, staying with his wife’s relatives in Hartfield, Sussex, he went for long walks on the heathland and became curious about the large number of insects caught by the common sundew (Drosera rotundifolia). He reported to his friend Joseph Dalton Hooker: ‘I amused myself with a few observations on the insect-catching power of Drosera; & I must consult you some time whether my “twaddle” is worth communicating to Linnean Soc.’ He initially collected a dozen plants and found that over half of the leaves had the remnants of dead insects adhering to them. The project was a welcome distraction from his daughter Henrietta’s long-term illness, probably typhoid fever. While caring for Etty, Darwin’s wife Emma wrote to a friend:

‘Charles is too much given to anxiety, as you know, and his various experiments this summer have been a great blessing to him , as he can always interest himself about them. At present he is treating Drosera just like a living creature, and I suppose he hopes to end in proving it to be an animal.’ (Emma Darwin 2: 177)

By the end of August he had ascertained that the sticky tentacles on the upper surface of the Drosera leaf bend over to trap insects. He had been busy performing experiments on the rate of inflection of the leaves and their sensitivity to various household substances. He concluded: ‘The leaves are first rate chemists & can distinguish even an incredibly small quantity of any nitrogenised substance from non=nitrogenised substances’.

 In this song the lyrics are based on Darwin's statements about insectivorous plants in his letters.

In September, Darwin roped in Hooker’s assistant at Kew Gardens, Daniel Oliver, to help him with his observations, initiating a sustained correspondence on insectivorous plants, the two men exchanging over twenty letters in the autumn of 1860 alone. Darwin started by asking Oliver to compare Drosera with Dionaea muscipula (the venus flytrap) and stated that 1/1920^th part of a single grain of ammonia was all that was needed to cause decided movement of a Drosera leaf, adding that his facts were ‘not stated without numerous & carefully repeated experiments’. Oliver observed related plants in the collection at Kew, such as the Australian Drosera, and tried to reproduce Darwin’s results on the reaction of Drosera leaves to various substances. When Oliver got a reaction from gum which Darwin had not, Darwin exclaimed: ‘I am infinitely obliged for your note … For Heaven sake try some pure gum.— You have given me a panic. I tried milk on 9 leaves thin Gelatine on 4 White of egg on 6 Saliva on 8 Urine on 11 Mucus on 4 infusion of meat 1 — 43 leaves.— and all strongly contracted on different plants’. He continued his experiments on an extended trip to Eastborne where his daughter was recuperating and wrote to the American botanist Asa Gray: ‘I have been infinitely amused by working at Drosera: the movements are really curious; & the manner in which the leaves detect certain nitrogenous compounds is marvellous.’

Darwin turned his attention to the mechanism of movement in Drosera and as he was ‘so ignorant of vegetable physiology’, he consulted his former Cambridge teacher John Stevens Henslow. But Henslow knew nothing of curious phenomenon where the ‘elongated cells in the hairs of Drosera are filled, when expanded, with homogeneous pink fluid: after inflection this fluid always separates into colourless fluid & into thick, viscid, dark red fluid.’

By the end of November Darwin wrote to Charles Lyell: ‘I will & must finish my Drosera M.S. which will take me a week, for at this present moment I care more about Drosera than the origin of all the species in the world… Is it not curious that a plant shd be far more sensitive to a touch than any nerve in the human body!’ Although he read a paper on Drosera at a meeting of the Philosophical Club of the Royal Society in February 1861 (Bonney 1919, p. 154), Darwin decided not to publish his results, but told Hooker that he was so interested that he would resume the following summer when Drosera opened up again, exclaiming, ‘By Jove I sometimes think Drosera is a disguised animal!’ His health interfered however, and his research on the inflection of the leaf hairs in response to mechanical and chemical stimuli, and on the molecular process of ‘aggregation’ associated with it, had to be postponed. He announced to Hooker: ‘My health has lately been very bad … It will be a cruel blow to me, as I must give up Drosera, crossing Primulas & many other experiments for this summer.’ In the end he managed a few experiments in May before going to Torquay for the summer and putting off the work for another year.

While on holiday in Bournemouth in September 1862 for the sake of his son Leonard’s and wife’s recovery from scarlet fever, Darwin began experiments with various vegetable substances, which were known to affect the nervous system of animals:  ‘As yet I can make out no sort of rule; but the difference in action is very great. Thus, strychnine produces no effect; belladonna causes movement, as does veratrine; Henbane does not cause movement, but does not in the least check subsequent & immediate action of meat.— Opium on the other hand, does not cause movement … it puts the plant to sleep!’. Reasoning from these experiments, he told Hooker he could not avoid the conclusion ‘Drosera possesses matter at least in some degree analogous in constitution & function to nervous matter.’ Darwin wrote to the surveyor Edward Cresy, who had assisted him with the quantification and sensitivity of his chemical tests,  ‘I intend to give up my beloved Drosera till I have got out a separate volume on Variation; and Heaven knows when that will be.’ He did not return to extensive research on insectivorous plants for 10 years.

Early in 1872, Asa Gray reminded Darwin ‘pray don’t run off on some other track till you have worked out and published about Drosera & Dionæa’. By August and September, Darwin was ordering essential oils and opium, salts of ammonia, chloroform and other chemicals for further tests from his usual chemist William Walmisley Baxter and borrowing different species of Drosera from Kew. After 4 or 5 weeks hard work, he reported back to Gray that he had been tracing the ‘nerves’ of Drosera along the vascular bundles: ‘By a prick with a sharp lancet at a certain point, I can paralyse 1/2 the leaf, so that a stimulus to the other half causes no movement. It is just like dividing the spinal marrow of a Frog.’ Gray piqued the interest of Mary Treat from Vineland, New Jersey with these remarkable observations and Darwin asked her to observe the North American species Drosera filiformis (the threadleaf sundew).

As part of his medical training, Darwin’s son Francis studied histology at the Brown Animal Sanatory Institution, London. In 1873 Darwin wrote to the Institute’s superintendent, John Burdon Sanderson, about acquiring a copy of their Handbook for the physiological laboratory and there began a very fruitful collaboration with members of the institute performing comparative experiments on animals. After Darwin had sent Burdon Sanderson an abstract of his work on Drosera, he embarked on a series of analogous experiments on the muscle and nerve tissue of animals After a visit to Down, Burdon Sanderson was also persuaded to conduct electrical experiments on Dionaea, using an induction coil to stimulate the leaves and measuring the response with a galvanometer, to test whether it had a nerve-like structure. Sanderson was so pleased with his results he excitedly telegraphed Darwin and presented them in paper to British Association for the Advancement of Science.

Another of the Brown Institution’s staff, Thomas Lauder Brunton, assisted Darwin with the digestibility of chondrin and chlorophyll as part of Darwin’s attempts to discover whether the digestive secretions of Drosera were analogous to those found in an animal stomach. The director of the Royal College of Chemistry Edward Frankland supplied pure chemicals for Darwin’s study of digestion and analysed the fluid from Drosera to see if it contained a pepsin-like substance. After many careful experiments, in May 1874 Darwin proudly reported to his cousin William Darwin Fox: ‘One of the chief new points is that it secretes a fluid analogous to gastric juice, for it contains a ferment, closely analogous to pepsine, with an acid, & can thus in a few hours dissolve the hardest cartilage, bone & meat &c. &c.—’ 

Hooker had been studying Nepenthes (the genus of tropical pitcher-plants) in parallel with Darwin’s study of Drosera and Dionaea and in the summer of 1874 they compared the digestive power of their experimental subjects. Hooker boldly claimed of Nepenthes, ‘its apetite for cartilage is simply prodigious— it reduces lumps, as big as your finger nail in 48 hours to lovely jelly’, while Darwin could only reply: ‘Poor Drosera & Dionæa cut quite an insignificant figure, as a cube of cartilage of 1/10 inch is almost beyond their digestive power—‘

Most of Darwin’s experimental work was on Drosera and this was subject of the first twelve chapters of his resulting book. After Dionaea, of all the other insectivorous plants he worked on, Darwin spent the most time studying Utricularia (bladderworts). At first he could not work out how they caught their prey, finding it difficult to manipulate the tiny bladders under the microscope, and complaining ‘The negative work takes five times more time than the positive.’ The breakthrough came when aristocratic horticulturist Dorothy Nevill leant him a plant of Utricularia montana. Darwin realised he had mistaken the empty stem tubers for bladders; the real bladders were tiny and transparent and on the roots, and when he discovered that they did indeed capture prey, he exclaimed to Nevill: ‘I have hardly ever enjoyed a day more in my life than this day’s work’.

Thomas Lauder Brunton and Joseph Fayrer shared their research on the effect of cobra poison on animal and plant tissue after Darwin had already started writing and he was able to add that the poison acted far more injuriously on the protoplasm of animals than on that of Drosera, bolstering his argument that there were large differences in the responses of plants and animals to various toxic substances.

Darwin spent the first three months of 1875 correcting his manuscript, and complained, ‘I am half killing myself in trying to get a book ready for the press’. He initially wanted to publish Insectivorous Plants and a second edition of Climbing plants together in the same book but his publisher John Murray found the manuscript so large that it was decided to publish them separately. This was despite setting the technical details in smaller type, a technique to reduce the page count used in Cirripedia and Variation. Julius Victor Carus immediately began a German translation from the proof sheets, which Darwin’s son Francis corrected signing himself ‘The proofmaniac’. Darwin agreed with Murray on an initial print run of 1250, forgoing his profits until 750 or 800 had been sold, because he thought that would be all that would ever sell of such technical book that was unlikely to go into a second edition. 

Insectivorous plants was published on 2 July 1875 and it was immediately clear that Darwin had been too desponding about the potential sales of the work, as 1700 copies sold straight away.[10040]. A second and third thousand copies were printed off quickly with a forth thousand following in 1876.

Darwin’s neighbour Ellen Lubbock celebrated the publication with a poem written from the insects’ point of view:

We saw that you were watching us,

We felt you were our friend,

And as we, in a general way,

Come to a fearful end,

It suddenly occurred to us

That we would have a look

At what you said about us,

So we crawled upon your book.

We now have buzzed all over it,

And find that, as we feared,

Voracious Plants could tell us

How our friends have disappeared.