Black widows and brown recluse are both venomous spiders. Black widow spider venom is a nerve toxin. One bite can be sufficient to severely affect or even kill a small pet. They are typically shiny black and have a distinctive red hourglass shaped area on their stomachs.
Brown recluse venom destroys the tissues surrounding the bite. With or without treatment, the wound may take weeks to heal. A distinctive fiddle-shaped mark on their back usually identifies brown recluse spiders. Prompt veterinary treatment is recommended if you suspect a venomous spider has bitten your dog.