For Darwin, slave-making ants were a powerful example of the force of instinct. He used the case of the ant Formica sanguinea in the On the Origin of Species to show how instinct operates—how it directs behavior and prevails over action—and to speculate about how it might have developed evolutionarily. Darwin corresponded about slave-making ants with various people, both before and after the publication of On the Origin of Species. As he was preparing to write On the Origin of Species, he exchanged letters with entomological experts who classified the ant species Darwin collected and advised him on how best to observe the slave-making activities of F. sanguinea.
A circle of friends and experts
After Origin of Species was published in 1859, friends, acquaintances, and strangers wrote to Darwin about his treatment of the remarkable phenomenon of slave-making ants. Those who wrote to Darwin asked for additional clarifications, offered observations of slave-making ants, and expressed their opinions of Darwin’s theory of evolution. Two of the most notable participants were Joseph Hooker, Darwin's long time friend and Director of the Botanic Gardens at Kew, and Henry Walter Bates, a young naturalist who traveled to the Amazon with Alfred Russel Wallace. The case of F. sanguinea intrigued Darwin's network of scientific friends and acquaintances. Reading the correspondence alongside Origin provides insight into Darwin’s research and writing process, his life and character, and the broader public reaction to his work.
You can download the entire packet of reading materials here: slave-making ants source pack
Darwin, Charles. On the Origin of Species. 1859. London: John Murray. (See: Chapter 7 “Instinct”)
Excerpts from Charles Darwin's Notebook C, p. 166
Excerpts from Charles Darwin's Notebook M, p. 46, 52-59
Letter 2226—Frederick Smith to Darwin, 26 Feb 1858
In this letter, Smith, a prominent entomologist at the British Museum, identifies the species of an ant described by Darwin in a previous letter and advises him on how best to observe the slave-making activities of F. sanguinea.
Letter 2235—Darwin to Frederick Smith, [before 9 Mar 1858]
This letter contains a list of questions, mostly about the behavior of slave-making ants, which Darwin sent Smith. Darwin routinely sent research queries to his network of correspondents; in this way he used his letter-writing as a tool for gathering information and confirming his own scientific observations. In this case, because of its specialised content, the questionnaire is directed only to Smith.
Letter 2456—Frederick Smith to Darwin, 30 Apr 1859
Here Smith answers a number of Darwin’s queries on ants (this is not the same list as in Letter 2235a). He gives Darwin more advice on how to observe F. sanguinea, and shares some of his own observations, which Darwin included in this discussion of slave-making ants in Origin.
Letter 2413—Charles Darwin to Emma Darwin, [25 Apr 1858]
Written from Moor Park, a hydropathic medical establishment where Darwin was being treated, this tender letter offers a glimpse of the affection Charles and Emma felt for each other. Darwin had been amusing himself observing the ants at Moor Park and mentions that he has sent a specimen to the British Museum to confirm whether it might be F. sanguinea. If so, it would be the first observation of the species outside its known habitat in Britain.
Letter 2265—Charles Darwin to William Erasmus Darwin, [26 Apr 1858]
Writing to his eldest son, William Darwin, Charles Darwin describes the ability of the Moor Park ants to distinguish between ants from their own hills and strangers from other communities.
Letter 2306—Charles Darwin to Joseph Hooker, 13 [July 1858]
In this famous letter to Joseph Hooker, Darwin's close friend and the assistant director of the Royal Botanical Gardens at Kew, Darwin speaks frankly about the impact of Alfred Russell Wallace’s letter on his work. Darwin reveals that Wallace has independently arrived at many of the same conclusions regarding the evolution of life. In an oft overlooked passage, Darwin describes his enjoyment in observing the behavior of F. sanguinea.
Letter 3266—Charles Darwin to H. W. Bates, 25 September 
In a letter full of advice on publishing scientific works, Darwin encourages the young naturalist H. W. Bates, who travelled the Amazon with Alfred Russell Wallace, to include entomological details in his account of his explorations. Darwin notes that his own remarks in Origin on the instincts of slave-making ants had attracted significant attention.
1. How would you characterize Darwin's central question regarding instinct and its relation to the development of the mind? In what ways do Darwin's notebook excerpts provide insight into Darwin's interest in this topic?
2. By what mechanism does Darwin believe instinct acts? How does this mechanism relate to how he believes instinct played a role in the evolution of the mind?
3. How did Darwin gather his observations on the behavior of ants? Why do you think he was particularly interested in slave-making ants? In what ways might his abolitionist background be relevant to this subject?
In order to more fully explore Darwin's observations of Formica sanguinea, we suggest an activity involving direct observation of ants either in your local area or in an ant farm which can be ordered from a supplier such as this one. Some suppliers will even provide the species Darwin studied so closely! Have your students make detailed observations of the ants' activities and their social behavior. Can they duplicate any of Darwin's observations?
Here's an example of a classroom activity performed at Harvard:
Charles Darwin wrote about the parasitic behavior of slave-making ants in Chapter VII of On the Origin as an example of how animals evolve instincts through natural selection which aid their chances of survival. As he wrote:
to my imagination it is far more satisfactory to look at such instincts as…ants making slaves…not as specially endowed or created instincts, but as small consequences of one general law, leading to the advancement of all organic beings, namely, multiply, vary, let the strongest live and the weakest die.
After reading Chapter VII of On the Origin and Charles Darwin’s letters about slave-making ants, the class visited the Harvard Museum of Comparative Zoology’s ant collection. The collection is the largest in the world: it boasts a collection of approximately 1 million ant specimens, including over 6,000 distinct species of ants. Through this field trip, the class was able to learn first-hand about slave-making ants.
If you do not have access to an ant collection, there are many other ways to bring Darwin’s observations of animal instincts to life. For example, you could visit a beekeeper to learn about how bees cooperate to make hives. Alternatively, you can observe ants in an ant farm – you can order one from a supplier such as this one. Some suppliers can even provide the species Darwin studied! While observing the ants, have your students make detailed observations of the ants’ activities and their social behavior. Can they duplicate any of Darwin’s own observations?