One standard definition of sentience is does the organism in its functional state respond to painful/pleasurable stimuli ..
By that criterion ants are certainly sentient. They are drawn to honey. They avoid heat etc.
But...here comes the nematode issue. As others have pointed out on this and other forums Nematodes are the cosmos's way of stopping Buddhists becoming too triumphalist about diet purity..
Nematodes are sentient by any definition. They respond to stimuli. They have separate digestive tracts. Many species of nematodes reproduce sexually.
And they live in all plant tissue...all plant tissue. Organic fruit and veg have more nematodes per gm than inorganic f and v. If you cook fruit and veg the nematodes die. If you eat raw fruit and veg you eat live nematodes.
These are not bacteria.They are small multi celled animals. Some are visible to the naked eye.
So, does this mean that we can hunt deer or kill chickens because we are going to eat sentient creatures anyway ?
Of course not. When we eat a carrot we have no intention of killing nematodes..as we would have if we kill a chicken. When we eat carrots the death of nematodes is an unintended consequence. Therefore no vipaka ensues.
The point being that in conditioned existence the death of sentient creatures is an inevitable result of needing food. Its one big chain. Which means that no Buddhist should point the finger at another Buddhist over the food issue. Its all a matter of degree. Purity as another member pointed out recently is impossible in this area.
The going for refuge is the door of entrance to the teachings of the Buddha.