All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The Hadeeth that reads: “All flies are in the Fire, except for the bee.”, was reported by al-Bazzaar and at-Tabaraani, and al-Albaani classified it as Saheeh (authentic).
As for the Hadeeth that reads: “The life span of flies is forty nights”; it was reported by Abu Ya’la from the Hadeeth of Anas, may Allah be pleased with him.
Ibn Hajar said: “Its chain of narrators is good.”
But it was only narrated by Sekkeen Ibn Abdul-‘Azeez about whom an-Nasaa’i said: “He is not strong.”
Then Ibn al-Jawzi mentioned it in his book al-Mawdhoo’aat [i.e. the fabricated Ahadeeth].
It appears –Allah knows best –that this Hadeeth is not confirmed.
If we presume that it is confirmed, then it is said that forty days is its maximum life-span. Otherwise, it may die before that, as stated by az-Zarqaani in Sharh al-Mawaahib al-Laduniyyah.
On the other hand, what is meant by the fly in the Hadeeth is more general than the domestic fly whose life-span is mentioned. Rather, it is more general than that.
Lisaan al-Arab reads:
“Black flies that are in the houses, fall into pots and food. Flies also refer to bees.”
Allah knows best.