My previous roommate had a dead spider in the tail light of his RX300. He didn't care enough to do anything about it.

Is the spider between the lense and the clear plastic exterior of your taillight? If it is, that would be pretty tough to get out with compressed air since there's no way to access that are of the taillight without breaking the seal. The bulb covers can get you inside the lense area, but the exterior area is sealed off pretty good. Maybe there were spider eggs that got in your taillight assembly during manufacturing?