When you walk in a park and find some ants walking in a row, bend down and look at the ants and observe what they are doing. Then wave your hands at them and say, "ants, how are you today?". They don't respond back. If you place your finger somewhere near their path, they would ignore it and continue onwards.
You would find that the ants are completely oblivious of your presence.
Similarly, the devas or brahmas who are said to be superior to humans, may be watching us and attempting to interact with us, but we may be completely oblivious of them.
On this page, hungry ghosts or hungry shades (petas) are described as:
peta yoni — here the beings have deformed bodies and are usually
consumed by hunger and thirst
In the Janussonin Sutta, the Buddha states that food offerings to the dead cannot reach them if they are reborn in hell, as animal, as human or as devas (heavanly beings). However, food offerings to the dead would reach them or other ancestors who are born as hungry ghosts or petas. Those who offer this, would not go unrewarded for their generosity, according to the sutta. This appears to be the practice described in the Tirokudda Kanda.
Ven. Nanda Mahathera writes here:
Peta-yoni (pa + ita) lit., departed beings, or those absolutely devoid
of happiness. They are not disembodied spirits of ghosts. They possess
deformed physical forms of varying magnitude, generally invisible to
the naked eye. They have no planes of their own, but live in forests,
dirty surroundings, etc.
Ven. Ajaan Lee Dhammadharo describes them here as:
Hungry shades come in all different shapes and sizes — really
entertaining, the hungry shades. Some of them have heads as big as
large water jars, but their mouths are just like the eye of a needle:
that's all, no bigger than the eye of a needle! Some of them have legs
six yards long, but hands only half a foot. They're amazing to watch,
just like a cartoon. Some of them have lower lips with no upper lips,
some of them are missing their lips altogether, with their teeth
exposed all the time. There are all kinds of hungry shades. Some of
them have big, bulging eyes, the size of coconuts, others have
fingernails as long as palm leaves. You really ought to see them. Some
of them are so fat they can't move, others so thin that they're
nothing but bones. And sometimes the different groups get into
battles, biting each other, hitting each other.
Based on all the descriptions above, it sounds to me like hungry ghosts or hungry shades are microorganisms. It also makes sense that when food offerings are made to the dead, they would be consumed by microorganisms.