Spiders can control how much venom they will inject when the deliver a spider bite and they can also refrain form injecting any venom at all. In some species, such “dry spider bites” seem to be very common when the spider is trying to scare away an attacker.

Producing venom is an energy consuming task and spiders who have learned how to control the amount delivered in each individual spider bite have therefore gained an evolutionary advantage. Generally speaking, spiders will deliver a spider bite either to harm prey or to defend itself. When killing or paralysing prey, the spider will typically determine how much venom that particular animal will require and adjust the amount accordingly. When the spider tries to fend of an attack, regardless of whether it is a real attack or just a perceived one, the objective is instead to scare away the aggressor.

No spider species are known to prey on large mammals, and humans are therefore only bitten by spiders that try to defend themselves from a real or perceived threat. It is theoretically possible for a spider to confuse a human body part, such as a finger, with prey, but such scenarios are not likely to occur.

The fact that spiders only bite humans in defence does not mean that spiders can not behave aggressive when encountering humans. Some spider species will only deliver a spider bite as a last resort, but there are also many species that will not hesitate to deliver a spider bite even in situations that are only mildly threatening. The extremely dangerous Brazilian wandering spiders are for instance known to deliver a spider bite rather than fleeing when they stumble upon a human that they perceive as a threat. Even the aggressive Brazilian wandering spiders will however usually give you a chance to escape before they spend valuable venom on a spider bite. They will make a resolute threat display and assume that you understand what it means. Unfortunately, many humans do not understand animal threat displays. It is also easy to miss the camouflaged spider that is trying to scare away your foot in the South American jungle, and the threat display will therefore go unnoticed. 

If you want to decrease the risk of receiving a spider bite, you can for instance:

• Shake your clothes before you put them on to shake out spiders that may be hiding in them.
• If you have left linen out to dry, it is advisable to shake them as well before you start folding them. The same is true for picnic blankets and similar articles.
• Be careful when you unpack your suit case, especially if you have travelled to another part of the world with a different fauna. If you are bitten by a spider that is not native to your area, your local hospital might not have the proper antivenom in store. If you need medical treatment, it is very important to inform your doctor that you might have been bitten by an imported spider since this will increase the chance of a proper diagnose.
• Be careful when you clean out spaces that are generally left undisturbed, e.g. closets, sheds and garages. Spiders are also known to stay hidden behind furniture.
  

Read more about the bites from different spiders::
Wolf Spider
Poisonous Spider
Brown Recluse
Black Widow
Black Widow bite facts