While studying medicine Swammerdam had started to dissect insects and after qualifying as a doctor, Swammerdam focused on insects.
His father pressured him to earn a living, but Swammerdam persevered and in late 1669 published Historia insectorum generalis ofte Algemeene verhandeling van de bloedeloose dierkens (The General History of Insects, or General Treatise on little Bloodless Animals).
The treatise summarised his study of insects he had collected in France and around Amsterdam.
He countered the prevailing Aristotelian notion that insects were imperfect animals that lacked internal anatomy.
Following the publication his father withdrew all financial support.
As a result, Swammerdam was forced, at least occasionally, to practice medicine in order to finance his own research.
He obtained leave at Amsterdam to dissect the bodies of those who died in the hospital.

At university Swammerdam engaged deeply in the religious and philosophical ideas of his time.
He categorically opposed the ideas behind spontaneous generation, which held that God had created some creatures, but not insects.
Swammerdam argued that this would blasphemously imply that parts of the universe were excluded from God's will.
In his scientific study Swammerdam tried to prove that God's creation happened time after time, and that it was uniform and stable.
Swammerdam was much influenced by René Descartes, whose natural philosophy had been widely adopted by Dutch intellectuals.
In Discours de la methode Descartes had argued that nature was orderly and obeyed fixed laws, thus nature could be explained rationally.

Swammerdam was convinced that the creation, or generation, of all creatures obeyed the same laws.
Having studied the reproductive organs of men and women at university he set out to study the generation of insects.
He had devoted himself to studying insects after discovering that the king bee was indeed a queen bee.
Swammerdam knew this because he had found eggs inside the creature.
But he did not publish this finding.
In 1669 Swammerdam was visited by Cosimo II de' Medici and showed him another revolutionary discovery.
Inside a caterpillar the limbs and wings of the butterfly could be seen (now called the imaginal discs).
When Swammerdam published The General History of Insects, or General Treatise on little Bloodless Animals later that year he not only did away with the idea that insects lacked internal anatomy, but also attacked the Christian notion that insects originated from spontaneous generation and that their life cycle was a metamorphosis.
Swammerdam maintained that all insects originated from eggs and their limbs grew and developed slowly.
Thus there was no distinction between insects and so called higher animals.
Swammerdam declared war on "vulgar errors" and the symbolic interpretation of insects was, in his mind, incompatible with the power of God, the almighty architect.

Swammerdam therefore dispelled the seventeenth-century notion of metamorphosis —the idea that different life stages of an insect (e.g. caterpillar and butterfly) represent different individuals or a sudden change from one type of animal to another.
While studying medicine Swammerdam had started to dissect insects and after qualifying as a doctor, Swammerdam focused on insects. His father pressured him to earn a living, but Swammerdam persevered and in late 1669 published Historia insectorum generalis ofte Algemeene verhandeling van de bloedeloose dierkens (The General History of Insects, or General Treatise on little Bloodless Animals). The treatise summarised his study of insects he had collected in France and around Amsterdam. He countered the prevailing Aristotelian notion that insects were imperfect animals that lacked internal anatomy. Following the publication his father withdrew all financial support. As a result, Swammerdam was forced, at least occasionally, to practice medicine in order to finance his own research. He obtained leave at Amsterdam to dissect the bodies of those who died in the hospital.
At university Swammerdam engaged deeply in the religious and philosophical ideas of his time. He categorically opposed the ideas behind spontaneous generation, which held that God had created some creatures, but not insects. Swammerdam argued that this would blasphemously imply that parts of the universe were excluded from God's will. In his scientific study Swammerdam tried to prove that God's creation happened time after time, and that it was uniform and stable. Swammerdam was much influenced by René Descartes, whose natural philosophy had been widely adopted by Dutch intellectuals. In Discours de la methode Descartes had argued that nature was orderly and obeyed fixed laws, thus nature could be explained rationally.
Swammerdam was convinced that the creation, or generation, of all creatures obeyed the same laws. Having studied the reproductive organs of men and women at university he set out to study the generation of insects. He had devoted himself to studying insects after discovering that the king bee was indeed a queen bee. Swammerdam knew this because he had found eggs inside the creature. But he did not publish this finding. In 1669 Swammerdam was visited by Cosimo II de' Medici and showed him another revolutionary discovery. Inside a caterpillar the limbs and wings of the butterfly could be seen (now called the imaginal discs). When Swammerdam published The General History of Insects, or General Treatise on little Bloodless Animals later that year he not only did away with the idea that insects lacked internal anatomy, but also attacked the Christian notion that insects originated from spontaneous generation and that their life cycle was a metamorphosis. Swammerdam maintained that all insects originated from eggs and their limbs grew and developed slowly. Thus there was no distinction between insects and so called higher animals. Swammerdam declared war on "vulgar errors" and the symbolic interpretation of insects was, in his mind, incompatible with the power of God, the almighty architect.
Swammerdam therefore dispelled the seventeenth-century notion of metamorphosis —the idea that different life stages of an insect (e.g. caterpillar and butterfly) represent different individuals or a sudden change from one type of animal to another.
Jan Swammerdam