I just found an instance in Dhammapada where a monk was reborn as an insect. Check this out:
Dhammapada Verse 240
Tissatthera Vatthu
Ayasava malan samutthitam
tatutthaya tameva khadati
evam atidhonacarinam(1)
sani kammani nayanti duggatim.
Verse 240: Just as rust is formed from iron, and corrodes the iron from which it is formed, so also, his own deeds lead the transgressor to a lower plane of existence (duggati).
1. atidhonacarinam - transgressor: i.e., one who transgresses or indulges too much in the use of 'dhona', the four requisites of a bhikkhu.
The Story of Thera Tissa
While residing at the Jetavana monastery, the Buddha uttered Verse (240) of this book, with reference to Thera Tissa.
Once there was a thera named Tissa in Savatthi. One day, he received a set of fine robes and was very pleased. He intended to wear those robes the next day. But that very night he died and because he was attached to the fine set of robes, he was reborn as a louse and lived within the folds of the robes. As there was no one to inherit his belongings, it was decided that this particular set of robes should be shared by other bhikkhus. When the bhikkhus were preparing to share out among themselves, the louse was very much agitated and cried out, “They are destroying my robes!” This cry was heard by the Buddha by means of his divine power of hearing. So he sent someone to stop the bhikkhus and instructed them to dispose of the robes only at the end of seven days. On the eighth day, the set of the robes which belonged to Thera Tissa was shared out by the bhikkhus.
Later, the Buddha was asked by the bhikkhus why he had told them to wait for seven days before sharing out the robes of Thera Tissa. To them the Buddha replied, “My sons, Tissa had his mind attached to this particular set of robes at the time of his death, and so he was reborn as a louse and stayed in the folds of the robes. When you all were preparing to share out the robes, Tissa the louse was very much in agony and was running about to and fro in the folds of the robes. If you had taken the robes at that time Tissa the louse would have felt very bitter against you and he would have to go to niraya. But now, Tissa has been reborn in the Tusita deva world, and that is why I have allowed you to take the robes. Indeed, bhikkhus, attachment is very dangerous; as rust corrodes iron from which it is formed, so also, attachment destroys one and sends one to niraya. A bhikkhu should not indulge too much in the use of the four requisites or be very much attached to them.”
Then the Buddha spoke in verse as follows:
Verse 240: Just as rust is formed from iron, and corrodes the iron from which it is formed, so also, his own deeds lead the transgressor to a lower plane of existence (duggati).
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So it seems at the time of death, if any defilement pops up its head, it becomes the cause of next birth immediately.
