I haven't had any experience with spider bites and the dogs but do know that fire ant bites can cause raised up places on the skin. The hair generally pulls away and a "bump" appears. Venomous spiders are a different story. The two that we have to worry about in the Southeast are the brown recluse and the black widow.
The brown recluse has a violin shape to the abdomen and is a dull brown in color. These spiders like dark places and are often found under lumber or in darkened corners of sheds or dog houses. Generally, a swelling will occur where the bite is. The spider's venom contains an enzyme that destroys cell membranes in the wound area with affected tissue gradually sloughing away, exposing underlying tissues. Within 24 hours, the bite site can erupt into a "volcano lesion" (a hole in the flesh due to damaged, necrotic tissue). The open wound may range from the size of an adult's thumbnail to the span of a hand. I've seen the results of a brown recluse bite in a human and it is a terrible wound. On a dog, the sunken, ulcerating sore may heal slowly up to 6 to 8 weeks.
Black widow spider venom is categorized as a nerve toxin. One bite can be sufficient to affect severely or even kill a small pet. The venom, a neurotoxin, generally affects small dogs more severely than large dogs. Signs of a black widow spider bite occur immediately after the bite. The primary prevention is to limit your pet's access to areas that might house black widow spiders.
I can't think of anymore lethal animals that the South harbors but if you think of something that needs to be posted that's gloomy and scary, send it along. I'll try to get some cheerful posts going shortly!