Spiders that are harmful are the Black Widow, Brown Recluse, Hobo spider, Yellow Sac spider, Sun spider, and the Orb weaver spider. 

Black Widow spiders are abundant in the south states. Adult black widow females are jet black in color and an hour glass design on the underneath side. Their body is about 1/2 inch in length.

Desert Spiders Adults are tan to dark brown with a dark fiddle shaped marking on the top of their body. The length of their body is about 1/4 inch to 1/2 inch in length.

Hobo or aggressive house spider is approximately 3/8 inch to 5/8 inch across and has a chevron marking on the top of the abdomen. 

Yellow Sac This spider is approximately 3/8 inch across the body and these spiders are pale yellow in color, and are known for building sac-like webs in the corners of walls and ceilings. 

The yellow sac spider delivers a painful bite. The venom from a yellow sac spider bite cause redness, and a burning sensation at the site of the bite. Individuals experience severe symptoms such as nausea, vomiting, and headaches.

If you suspect that you have been bitten by a yellow sac spider, it is important to seek medical attention as soon as possible.