α-Latrotoxin

• neurotoxic (bad fer teh brain) protein present in the venom of widow spiders (members of the genus Latrodectus) including the infamous black widow
  
• one of seven latrotoxins, is the only one that is vertebrae-specific (only affects animals with a spine, so my chemistry prof should be safe! OH SNAP!), the others being primarily active against insects (the natural prey of the spiders) or crustaceans
• the current theory is that four units of the protein can join together to form pores in cells, with the resulting changes in ion flux signalling the release of neurotransmitters from nerve cells, producing (among other things) muscle contraction, manifested as painful abdominal cramps and breathing difficulties

http://en.wikipedia.org/wiki/Latrotoxin
http://en.wikipedia.org/wiki/Latrodectism