Black widow spiders can be identified by the yellow-red hourglass shape on their belly. They carry a venomous neurotoxin that can progress from local to severe, diffuse muscle cramping (even mimicking acute abdomen). The venom accomplishes this by releasing acetylcholine and norepinephrine at the synapse. As you can imagine, with increased neurotransmitter release, you will get sympathomimetic hyperactivity - diaphoresis, restlessness, ptosis, htn, tachycardia.
Analgesia and muscle relaxation is the treatment with opioids and benzos. In severe cases or patients with significant comorbidities, you can give antivenin. Be cautious as 25% of pts will develop anaphylaxis from antivenin and the syndrome is otherwise self-limited.
Other fun facts:
Alaska is the only state that does not have black widow spiders.
Female spiders sometimes eat the male after copulation (hence the name).
Sources
Rosh Review
Medscape: Widow Spider Envenomation