Why are some spiders not affected by insecticide?

What are the active ingredients in the insecticide?

Well, technically, spiders are not insects. That might not be the reason—but it might.

Insecticide usually don’t contain the compounds to destroy the exo skeleton of Spiders (aranea order, arthropod class, arachnid).

Pyrethoids are often used as insecticides because insects (and spiders) have exoskeletons through which the poison can travel in order to paralyze the organism. If it’s true that arachnids have stronger/thicker exoskeletons than insects, it would make sense that many pyrethoid-based insecticides wouldn’t work as effectively on spiders.

http://en.wikipedia.org/wiki/Pyrethroid

@coelacanth nice answer, but I have also noticed that some insecticides do not work on flies as well. I have seen that when you spray some flies with the spray they fall to the ground but after awhile they would recover and in some cases become immune. The spiders however, when they are sprayed will just go to one corner of their web, stay there a little as if weakened but would soon be back in the center of their web as if nothing happened.

I think it quite possibly has something to do with insects/arachnids becoming resistant to insecticides.

This is taken from a Michigan State University page and explains how insects become resistant to pesticides/insecticides

“Effects of pesticide selection

Repeated use of the same class of pesticides to control a pest can cause undesirable changes in the gene pool of a pest leading to another form of artificial selection, pesticide resistance. When a pesticide is first used, a small proportion of the pest population may survive exposure to the material due to their distinct genetic makeup. These individuals pass along the genes for resistance to the next generation. Subsequent uses of the pesticide increase the proportion of less-susceptible individuals in the population. Through this process of selection, the population gradually develops resistance to the pesticide. Worldwide, more than 500 species of insects, mites, and spiders have developed some level of pesticide resistance. The twospotted spider mite is a pest of most fruit crops and is notorious for rapidly developing resistance to miticides.” (Michigan State University 2011).

The CSIRO (and I am sure other government science agencies) are doing research into resistance to pesticides because it has major implications for agriculture.

I try to save the environment and avoid poisons, so I just swat them with a shoe. Works great on most all small crawly things. I only do it though when they invade my space, in the wild, I just stay out of their way.

I agree @rooeytoo. I don’t use insecticides/pesticides unless I absolutely have to. Spiders can stay unless they are dangerous or we put them outside. The only thing I do use insecticidal products for are cockroaches (which as you know fly here and can be huge). I do put baits down and use surface sprays to control those horrors. Other than that though, window and door screens help keep the bugs down.

The spiders in my house are common house spiders and they usually live around the light bulbs in the ceiling. I do not bother them though because they eat a variety of insects and they really do not pose a threat to me.