IDAHO FALLS, Idaho (AP) One of the world's foremost authorities on spiders has been missing for six weeks and police say they fear the worst.

Police have had two investigators working on the case since 48-year-old Darwin K. Vest vanished June 3.

''We believe it's unlikely he's committed suicide or gone somewhere on his own,'' Lt. Steve Roos said Monday.

Only days before his disappearance, Vest had notified his sister, Rebecca, that an Oregon business was interested in selling the hobo spider trap kits he had invented.

The hobo spider had special meaning for Vest and his sister: They named the spider, whose bite was thought for years to be that of a brown recluse.

Vest was an authority on spider, snake and plant poisons. The self-taught scientist and owner of Eagle Rock Research in Idaho Falls has testified about poisonous bites in court cases across the country, lectured and written on the subject and been featured on the Discovery Channel.

Vest collected spiders and milked them for their venom. The process involved Scotch-taping an anesthetized arachnid to a board, delivering a shock and using a pipette to pick up a bead of venom.

He apparently vanished as he was walking back to his house near the Snake River. He had played his weekly trivia game with friends at a bar and then stopped off at another place, apparently for a nightcap.

''Nobody's heard anything more about him,'' his cousin Farrel Vest said.

The dark-haired, bearded man was known for taking walks at night, and was mugged after a similar evening out several years ago, he said.