Bed bug bites show up as an allergic reaction to the tiny insect's saliva, appearing between one and three days afterward in the form of a raised red welt with an annoying itch, explains Dr. Andrea Bledsoe for Everyday Health. In contrast, a spider bite shows up almost instantaneously, similar to a bee sting.
With a bed bug bite, the itchiness and the welt stick around for a few days, according to Bledsoe. Bed bug bites can show up at any point on the body, but they generally show up on areas that are not covered, such as the hands, arms, face and neck. They also appear in rows of at least two, if not more.
While most spider bites are not poisonous, some people have an allergic reaction that takes the form of chest tightness; trouble breathing and swallowing; and swelling in the face, notes Bledsoe. A poisonous bite, such as a bite from a brown recluse or black widow, can lead to a much more dangerous reaction. The bite may not cause pain, but within an hour, swelling and pain set in, followed by muscular rigidity and pain, nausea, vomiting and trouble breathing. It is crucial to seek medical attention if a person thinks he was bitten by a poisonous spider.