In this passage from his commentary on Aristotle’s Generation of Animals, Michael of Ephesus tries to explain a reference to insects generated in wool. Michael thinks Aristotle cannot mean wool, as in the material made from sheep’s hair; rather, he believes Aristotle was talking about silk, specifically the raw silk of a silkworm's cocoon.
Now, Michael is almost certainly wrong about this. The method of producing silk was unknown in the west in Aristotle’s time and would remain so until the reign of Justinian (6th century CE). But in trying to explain what Aristotle meant, Michael ends up giving an amazing description of the process of silk-making during the Byzantine middle-ages.
“[Generation] happens in the same way as [it does among caterpillars] in the case of the other [insects] that are generated in wool and not from copulation.” (Aristotle, De generatione animalium III 9, 758b21)
“By ‘wool’ he means what is now in fact called ‘silk’* by many people. For a certain kind of worm produces this silk. There is really nothing to stop [someone from] observing their generation. Certain winged animals copulate with one another (the males obviously with the females), and from their copulation something worm-like is produced, something which nevertheless does not have the ability to sense [i.e., is not yet really alive].** The women whose job is to produce the silk collect [these worm-like things] and place [them] in the folds of their robes, warming them, until the worm acquires sensation and becomes an animal. Once they become animals, the women place them into a sieve and give them leaves of mulberry to eat.*** By feeding on these leaves, the worms grow and so produce a cocoon around each of them, and it is the cocoon which the women unwind into silk. Then the worm dies. And after a time, out of the cocoons that have broken open, a certain winged creature emerges, resembling those that generated the worms. And it goes on in this way forever. For from this winged creature in turn a worm is produced; and from this worm, a woolen cocoon and a winged [creature]; and again from this winged creature a worm, and so on forever.”
758b21 «Τὸν αὐτὸν δὲ τρόπον συμβαίνει καὶ ἐπὶ τῶν ἄλλων τῶν μὴ ἐξ ὀχείας γινομένων ἐν ἐρίοις.»
Ἔρια λέγει νῦν καὶ τὴν καλουμένην ὑπὸ τῶν πολλῶν μέταξαν· σκώληκες  γάρ τινες ταύτην τὴν μέταξαν γεννῶσιν. ἴσως δὲ οὐδὲν κωλύει τὴν τούτων γένεσιν ἱστορῆσαι. ζῷά τινα πτηνὰ ὀχεύουσιν ἄλληλα, τὰ ἄρρενα δηλαδὴ τὰ θήλεα, ἐκ δὲ τῆς τούτων ὀχείας γεννᾶται σκωληκώδη τινά, ἀναίσθητα μέντοι, ἃ δὴ συλλέξασαι αἱ περὶ τὴν μέταξαν πονοῦσαι γυναῖκες καὶ ὑπὸ τὸν κόλπον ἐμβιβάσασαι θερμαίνουσιν, ἕως ἂν αἴσθησιν λάβῃ καὶ ζῷα γένηται. ζῴων δὲ γεγονότων, τίθενται αὐτὰ εἰς κόσκινα καὶ διδόασιν ἐσθίειν φύλλα συκαμίνων, ἐξ ὧν φύλλων τρεφόμενα αὔξονται καὶ οὕτως ἐργάζονται τὸ κέλυφος κύκλῳ ἕκαστον αὐτῶν, καὶ ἔστι τὸ κέλυφος ὃ ἀναλύουσιν εἰς μέταξαν· εἶτα ἀποθνήσκει. καὶ μετὰ χρόνον τινὰ τοῦ κελύφους ῥαγέντος ἐξέρχεται ζῷον πτηνὸν ὅμοιον τῷ γεννήσαντι τὸν σκώληκα, καὶ τοῦτο ἀεὶ οὕτω γίνεται. πάλιν γὰρ ἐκ τοῦ πτηνοῦ τούτου γεννᾶται σκώληξ, ἐκ δὲ τούτου ἔριον κέλυφος καὶ πτηνόν, καὶ πάλιν ἐκ τοῦ πτηνοῦ τούτου σκώληξ, καὶ οὕτως ἀεί.
Michael of Ephesus, In de generatione animalium commentaria, 153,29-154,13 Hayduck
*Silk had been produced in the Eastern Roman Empire since the time of Justinian (483-565 CE). In his History of the Wars, Procopius reports that Justinian wanted to solve the “silk question”: how to acquire silk without having to buy it from their Persian enemies (VIII.vi.1-8). Some monks who had recently returned from India came to Justinian with an answer. They had visited a land north of India called Serinda (Σηρίνδα, China), and discoverd the secret of silk production and how it might be produced by the Romans. Silk, they said, was produced by grubs. And while it was impossible to bring the grubs back from China alive, they could (and eventually did) bring back their eggs, hatched them in Byzantium, and began an industry that would last almost a thousand years.
**Michael is no doubt talking about the eggs laid by the silk moth (bombyx mori). Michael refuses to call them eggs, considering them instead imperfect worms. This is why the women who produce silk need to warm them: to finish the process of bringing the worm to life. He refuses to call them eggs, because, at least according to Aristotle, the immediate offspring of metamorphosizing insects are worms, a stage of life that precedes the egg. The egg itself for Aristotle (and Michael when he is interpreting him) is what we call a pupae or cocoon. Procopius in the passage cited above, does not hesitate to call the things laid by silk moth ‘eggs’, and it is remarkable to me that Michael would think they are anything else. But he seems to endorse Aristotle’s view that what the silk moth produces are tiny non-animals that need to be warmed into life, only to die when they become the egg of a different kind of animal.
***In researching this, I had no idea how silk was actually produced, so I went to youtube. I found this video from the “High Fashion Silk Company” in China which claims that “in ancient times, farmers tucked the [silkworm] eggs into their clothes so the larvae would grow up healthy”. And this video of a traditional silk-farm in Cambodia shows the silk-growers feeding the silkworms mulberry leaves in something that resembles a sieve, like Michael describes. I think the resemblance of these techniques and those described by Michael is just brilliant.