‘And Moses said to them, “Let no man leave of it until the morning.” In spite of this they did not listen to Moses but some of them left of it until the morning, and it bred worms and stank. And Moses was angry with them.'
Each days supply was to be for that day alone, and Moses ordered them not to leave any over until the morning. But some, having learned in the wilderness to preserve food supplies, were disobedient and kept some for the next day. Then to their horror they found it teeming with worms (or ants - the Hebrew word is a general one and can be used of ants or any number of wriggling creatures) and smelling. This counts against seeing it as the excretion of plant lice as, while that is gathered by ants, it does not smell horribly.