A sentient being is an organism that experiences, that's what sentience means:)
Dogs are sentient, people are sentient, algae isn't and I am not sure about spiders:)
This is difficult to know, without first hand experience. Spiders have a nervous system, as do insects in general. Humans feel pain and pleasure through the stimuli that is transmitted through the nerves, hence it is easy to imagine that the spiders would experience in like manner.
The so called "lower life forms" react to external stimuli, you can't be sure if they also experience something? You can't rule it out, I would think.
When humans react to external stimuli, we conclude that they experience feelings, do we not ?
It is worth noting that spiders, grasshoppers etc... have tiny brains, something that is like a spinal column, a heart, tiny muscles that move their legs, and so on... You will find descriptions of it in the web, in categories like "Spider anatomy, Grasshopper anatomy".