I just looked this up:

Harmful Matter—General
Except as provided in this document, any article, composition, or material is nonmailable if it can kill or injure another or injure the mail or other property. Harmful matter includes, but is not limited to:

...

b. All poisonous animals except scorpions mailed for medical research purposes or for the manufacture of antivenom; all poisonous insects; all poisonous reptiles; and all types of snakes turtles, and spiders.