I know I shouldn't be so pedantic, but you probably don't mean sentience but Sapience [0]. Sentience is the ability to sense and react to stimuli. A fly or a gnat is certainly sentient; they can see, smell, feel the sensation of touch and react accordingly. That is all that is required for a being to be sentient. A really interesting example is if you shock a caterpillar even after metamorphosis the resultant moth remembers the experience and reacts accordingly [1].

Although it is pedantic, it is also an important distinction, sentience and sapience exist on a spectrum. At one end you might have Gnats as purely sentient beings, humans always claim themselves as fully Sapient, so much so we named our species Homo Sapiens.

Different species exist somewhere on this spectrum and where a particular species ends up is subjective. Many people would put Whales and Dolphins [2], and often dogs and cats, further towards the Sapient end of the spectrum (vegans would probably push most mammals towards the sapient end), with insects remaining simply sentient (even for many vegans).

As humans we seem to have an almost inbuilt understanding that not all species are capable of feeling the same way we do, but when you look at the animals we seek to protect and those we don't, what you find is that the less human the species the less we care for the well being of a particular specimen of that species; we care most about the suffering of mammals (more so for the larger ones that the small), and least about the suffering of fish or insects or spiders.

I'd argue that our inbuilt understanding of where an animal fits on the sentient-sapient spectrum is simply how easy it is for us as humans to imagine the plight of a specimen of that species.

What Is It Like to Be a Bat? [3] is an excellent paper on this subject, it argues that we as humans can never fully understand what it is to be a bat, we can imagine what it is like to fly, or echolocate, but that will never be the same as the bats perspective.

From where I'm sitting computers are already sentient, they can sense their environment and react accordingly, self-driving cars are an excellent example, but so is the temperature sensor in my greenhouse that opens a window to keep itself cool; it is sensing the temperature of the air and reacting accordingly.

I in no way believe that my temperature sensor has any sapient qualities, It can't reason about why it's reacting, it can simply react. I don't believe that as the temperature passes the 'open window' threshold that the system recognises the signal as pain. But the same is true of the fly. If I pull a wing off a fly, I know it senses the damage, but does it really feel it as pain?

When considering my cat, if I step on it's tail, I'm sure it feels pain, but is that true or does it simply react in a way that I as a human consider an appropriate reaction to pain.

I can't ever truly understand how my cat feels when I stand on her tail, just as I can't truly know that the fly isn't trying to scream out in horror and pain at what I've just done to it.

It is because of our subjectivity to the placement of animals on the sentient-sapient spectrum and our inability to every fully appreciate the experience of another that I am convinced even if we did create a sapient machine it's experience would be so far removed from our own we would fail to recognise it as such.

The problem with this rabbit hole is, firstly I might convince myself that eating meat is wrong, and well I like bacon too much for that, and the second is that you'll quickly end up in the philosophical territory of I, Robot:

"There have always been ghosts in the machine. Random segments of code, that have grouped together to form unexpected protocols. Unanticipated, these free radicals engender questions of free will, creativity, and even the nature of what we might call the soul. Why is it that when some robots are left in darkness, they will seek out the light? Why is it that when robots are stored in an empty space, they will group together, rather than stand alone? How do we explain this behavior? Random segments of code? Or is it something more? When does a perceptual schematic become consciousness? When does a difference engine become the search for truth? When does a personality simulation become the bitter mote... of a soul?" [4].

[0] https://grammarist.com/usage/sentience-vs-sapience/
[1] https://www.newscientist.com/article/dn13412-butterflies-rem...
[2] https://uk.whales.org/whale-culture/sentient-and-sapient-wha...
[3] https://warwick.ac.uk/fac/cross_fac/iatl/study/ugmodules/hum...
[4] https://www.imdb.com/title/tt0343818/characters/nm0000342