He was a successful businessman and with time became one of the richest men in Sweden, head of an early industry employing around 3,000 people.

He published his main work on entomology, Mémoires pour servir à l'histoire des insectes, in French in eight volumes between 1752 and 1778.

In his writings, De Geer among other things brought the importance of insects as pollinators to the attention of the scientific community, and criticised the idea of spontaneous generation.

Its sumptuous and rare works, including a collection of music scores, indicate that it was also intended as a way to raise De Geer's social status as an aristocratic collector.

[8] Charles De Geer was born on 30 January 1720 in Finspång in Sweden, but moved to the Dutch Republic when he was three years old.

[10] At the age of sixteen, he organised the garden at the family estate according to the principles of Joseph Pitton de Tournefort, kept pigeons and collected insects and butterflies.

[11][12] Pieter van Musschenbroek's brother Jan also constructed a microscope and other scientific instruments for the young Charles De Geer.

[1] At the age of ten, Charles De Geer inherited the estate of Lövstabruk in northern Uppland, in Sweden.

Instead, his father assumed the guardianship, while delegating much of the every-day business to his eldest son (and Charles' older brother) Louis, who lived in Finspång in Sweden.

[1] Lövstabruk (also spelled Leufstabruk, Löfstabruk) developed as an ironworks, rather than an agricultural estate; it forms part of a group of iron works in northern Uppland managed largely by Walloon immigrants to Sweden.

In an instruction he left behind for his successor, De Geer stressed the importance of not putting too much trust in administrators and inspectors, not to fret over details too much, to maintain good relations with the clergy and the crown, and to treat the workers well.

[25] Charles De Geer was an ardent book collector, and despite his success as an industrialist he is perhaps more remembered for his library, apart from his work as a scientist.

A letter from his father dated 1736, when Charles was 16, depicts "a young man with a childlike fascination for nature and the reference library of an established scientist.

[22] It has been remarked that "this is not the library of a typical adolescent, but a well-considered collection of handbooks, classical literature and scientific works.

[22] He also bought books from booksellers in Sweden, though from 1746 most purchases were made by the Luchtmans bookdealers in Leiden, in the Dutch Republic.

[31] De Geer's extensive connections with the Dutch book market have been described as a "splendid example of how the globalization of the time impacted small villages far from the large cities.

"[32] Around a quarter of the books in the library are on natural history, notably entomology, and contains several of the standard works on biology and zoology of the day.

[48] It thus remains "a kind of time capsule from the 18th century" where ephemera like playing cards and hand-written notes have been found between the leaves of the books.

[39] The interest in natural history and in particular entomology De Geer had displayed from an early age developed into full scientific activity after his return to Sweden.

Earlier scholars, such as Isidore of Seville in the Middle Ages, had held that spittlebugs were produced by the spit of cuckoos, something De Geer showed was manifestly false.

In a speech given to the Academy in 1754, he developed a deeper critique of the idea of spontaneous generation, supported by a "rich flood of observations and arguments", as entomologist Felix Bryk writes in his biography of De Geer.

[52] De Geer also brought the importance of insects as pollinators to the attention of the scientific community through his writings, which Bryk describes as one of his "most notable accomplishments.

[39] His main contribution as a scientist was however in species research, in the footsteps of René Antoine Ferchault de Réaumur.

[55][56] De Geer paid particular attention to insects which were considered pests or otherwise had some concrete impact related to human activity.

[51] He kept in contact with several other naturalists of his day – apart from with Réamur himself, with Bonnet, Pierre Lyonnet, Abraham Trembley and Carl Alexander Clerck.

[5] After his death, his collections of insects and birds were donated by De Geer's widow to the Royal Swedish Academy of Sciences.