I saw this photo in the UC IPM archives.  Since it is a really unusual configuration and the fact that lately bug-vacs have become a bigger part of the lygus management discussion again, I thought this might be something to share with my readers.

To find out more, I contacted Thom Flewell, who wrote this in response:

"I used to have 3 bug vacs with that general configuration. The fans were from  plastics manufacturing and were used to move the pellets that formed the feed stock for producing plastics. Mike McCluney made many Bug Vacs using these fans, usually with one fan per bed. They were very heavy, and demanded about 17 horse power per fan. The tractors had barely enough power to operate the fans and move the machinery down the rows. There were two disadvantages with these machines. The size of the fans required mounting them off the back of the tractor. The aspirator head had to be placed forward of the tractor to keep from scaring the lygus and causing them to drop below the effective suction zone of the machine. The problem was with the distance that separated the head and the fan. Vacuum was reduced significantly along the travel from the head to the fan. The second problem was the weight of the machines. It was not uncommon to have a driver let his attention wander and the tractor then wandered too, going from the hard-packed furrow to the soft and soggy bed. When the wheels left the furrow and sunk in the beds, destruction followed and pulling the tractor back up onto the furrow was like trying to pull it from a swamp onto a railroad track. 

The greatest advantage to these fans was that nothing that enter the volute exited alive. The fan employed flat impeller blades accounting for their inefficiency. But that same inefficiency meant that air and anything entrained in the air flow probably collided either with the blades or the side of the fan before exiting. Only parts remained. "

Thanks Thom!