Why are some bites and stings extremely painful or dangerous while others are simply itchy, red, irritating papules?

The host reaction largely depends on whether the bite or the sting is defensive, toxic, or for feeding purposes. The highly visible, buzzing yellow bee or wasp stings primarily in defense of the hive. The venom causes rapid onset of severe pain for immediate negative feedback and defense.

The bite of spiders, on the other hand, contains toxins and proteolytic, digestive enzymes; thus spiders subjugate and then liquify their prey for food. Unfortunately, these venoms are often quite painful and destructive when injected into humans.

The bites of ectoparasites that require blood for nutrition or reproduction, such as the mosquito, are almost always painless and asymptomatic until allergic or irritant reactions take place. The salivary products are often allergenic and cause itching and redness.