Black Widow :
In the adult female black widow (the harmful one), you will see an abdomen that is pure black and shiny.
Also on the abdomen, typically you will find an hourglass shape composed of two red triangles. The hourglass can also be yellow or orange.
The adult female is 1/2 inch long, or 1.5 inches long when the legs are spread.
The adult male, which is not poisonous, is only half the size of the female. The body is smaller and the legs are longer.
The male does not have the hourglass shape on its abdomen. Instead, you will see red spots and white lines.
Brown Recluse :
Count the legs. Brown recluse spiders have eight brown legs that are long (about the width of a quarter), thin and covered with fine hairs but not spikes.
Size up the spider. Brown recluse spiders range from 3/8-inch to 5/8-inch in length.
Check the spider's color. Brown recluse spiders are light brown with dark brown markings and legs.
Look for special markings. Brown recluse spiders have a dark inverted violin shaped marking from their head into their bodies.
Check for hairs. Brown recluse spiders have bellies that are covered with many fine hairs that look like velvet.
House Spider :
House spiders are very common and are typically brown or tan with various markings.
Their size varies and can reach 3/8" in length. House spiders are those web-building spiders common in the corners and garages of most homes and buildings, and are responsible for most of the cobwebs found inside.
Cobwebs are actually old webs that have collected dirt such that they become easily visible. These spiders are common in garages, crawl spaces and basements as these areas are less disturbed and tend to harbor more insects. Their bite is not dangerous, and therefore are not considered a threat to man.