Big spiders. I don't like big spiders!
Despite my background in science research, I have no trouble suspending belief for a well written story, . . . within reason of course. However, nothing makes me want to put a book down more than over sized spiders. Yet numerous bestselling authors and countless video games utilize these creatures. Not only do they use spiders, but also insects and other arthropods.
Let's explore the logic behind making jumbo arachnids. If small spiders scary people than larger spiders will scare them even more. Really? Is a joke funnier if you shout it? Well, maybe. If we are being honest, wouldn't we be laughing at the person yelling regardless of the joke? In the same vein, when we cringe from huge arachnids aren't we really cringing from a large creature threatening to overpower our favorite characters, regardless of what the creature is?
There is a reason I have trouble suspending belief with extra large spiders, as well as other arthropods and insects. It is physically impossible for them to be huge and bulky. The largest known spiders are the Giant Huntsman spider of Lao (leg span of 12inches) and the Goliath, aka Bird-eater, of South America (leg span of 11 inches.)
I could take a stab at explaining the science behind the size limitations, but this interactive tutorial from Berkeley does a better job than I ever could.
The main points from the Berkeley site are that arthropods, including spiders, can't be large because of problems with molting, exoskeleton strength, and respiration. Molting for large animals would cause them to be weak without their exoskeleton to support them. This puts them at high risk of exposure to the elements and predators. The exoskeleton wouldn't be able to support the weight of the muscles needed for movement. The respiration system for spiders, book lungs, would be possible but not very efficient. For insects and other arthropods, their respiration system wouldn't work at all.
Let's say a writer has just finished her novel about alien spiders coming down to earth and terrorizing civilization. She is putting the finishing touches on her book when she comes across this post. She could ignore it, the story is flawless with well developed characters and an amazing story line. Do readers really care if it is realistic or not? They are for the thrills not the science.
Maybe.
Or she could make a few minor anatomical and physiological changes to make large spider-like creatures. Start by giving them an endoskeleton, along with muscles for movement. That takes care of molting problems and exoskeleton strength concerns. Next, replace the exoskeleton with plated-like structures, similar to reptiles or armadillos. With this, you can still have molting, complete with shedding skin. As an option, you could add hair like projections on the legs and body that allow them to sense vibrations of moving prey, the way the Goliath spider does. Finally, give them mammalian lungs. Book lungs are nice, depending on the size, but a real set of pipes may be just what the doctor ordered. There are other benefits to this, being able to growl and have vocal communication.
For physics sake, don't have them scaling vertical walls or hanging from webs. Some spiders, like the Sydney funnel-web spider, use their webs as trip-line for prey, springing on their prey and inject highly toxic venom. With the eight muscle bound legs surely they can climb, jump and even put their proverbial shoulders into any obstacles in their way. Other fun things you could add are muscular venom sacs. These could easily flex to shoot venom, keeping in mind they would only have a short supply of venom in one go. With a few changes, large spider-like creatures can be used for those spine chilling moments without insulting the reader's intelligence.
The only exception to these rules apply to the arthropods of the sea. Aquatic environments have different physics and allow for larger arthropods. But these same creatures can't leave the waters without facing the same challenges.