They use Pyrethrins.
"The pyrethrins are a pair of natural organic compounds that have potent insecticidal activity."
Most bug sprays use synthetic forms of this.......
"Pyrethroids are axonic poisons and cause paralysis of an organism. The chemical causes paralysis by keeping the sodium channels open in the neuronal membranes of an organism. The sodium channel consists of a membrane protein with a hydrophilic interior; this interior is effectively a tiny hole which is shaped exactly right to strip away the partially charged water molecules from a sodium ion and create a thermodynamically favorable way for sodium ions to pass through the membrane, enter the axon, and propagate an action potential. When the toxin keeps the channels in their open state, the nerves cannot de-excite, so the organism is paralyzed."
The effect of causing all of their muscles to contract uncontrollably is the reason they limp, roll, etc. They are losing control of their muscles. The effect is similar to what happens to Humans that are bit by animals possessing neurotoxins, such as a Black Widow Spider. You would experience muscle cramps and possible asphyxiation due to your diaphragm losing its ability to function.
Bigger animals (spiders) take longer to die than smaller animals (ants) because there is more animal to kill. Also, as an animal grows, your surface area to volume ratio decreases. i.e, big animals have a lot of inside space, but not a lot of "skin". Thus, bigger animals have less surface area to absorb substances compared to their inside volume.
So just spray them extra to lessen their suffering by expediting their death. You hate to kill animals, but pests can track germs, and bite Humans, causing injury. Caring for animals doesn't mean letting them overtake your residence and property. You are also an animal, and as such you must protect yourself just as they would.
What did the spider or ant do to harm you? Did either of them bite you or were they innocently going about their lives and had the misfortune of being seen by you?
If you're such a noble philarthropist who doesn't want them to suffer, don't kill them.
The contents inside the can kill it.
a) Simply buy the can of bug killer you want (usually around ￡1 - ￡3).
b) Locate the insect and press down on the spray releasing it on to the creature.
c ) Watch the insect die a painfull death
Bug spray attacks the nervous system of bugs.
Well it burns, imagine getting sprayed in the face with pepper you would freak out and fall to the ground."then die" if it was a poison strong enough to kill you.
it takes a while because the poison to kill the ants doesn't need to be as strong to kill the spider.
Its brain pretty much melts or explodes or it thins out their blood and they internally bleed :]