The worker ant, under feasible conditions, will live for about half a year.
(http://www.sonic.net/~petdoc/lifespan.htm).
There were a few emperors who reigned for a brief period of time, due to old age and assassination, among other things. These men were not able to do much for the Empire, but they got the pleasure of having their name inscribed in history due to this [possibly foolish] act.
Marcus Salvius Otho, a Man with Great Ambitions
Before becoming an emperor Otho was one of the men around Nero, but due to an icident with a woman he was forced to stay in Lusitania. When Galba rose in revolt against Nero, he joined him, though he later caused his assassination. After the death of Galba the people cried out to Otho to become emperor, which he accepted. At the same time, however, another man named Aulus Vitellius Germanicus was declared emperor by the legions of the Rhine. After the Battle of Bedriacum Otho assassinated himself rather than fight on and have more men die meaninglessly.
Pertinax – 86 Days of Distrust (192-93 A.D.)
His life before and during his reign as Emperor
Pertinax was born the son of a slave, and he eventually joined the Roman army. He was raised to the equestrian rank under Marcus Aurelius’ reign, and later on became a Senator. Moesia, Dacia, and Syria were also once governed by him. Pertinax went from rags to riches, an incredible feat for his time. When he was elected Emperor after the assassination of Commodus, he convinced the Senate to forgive Laetus, the Praetorian Prefect, who had arranged for the assassination of Commodus and proclaimed Pertinax emperor. The Praetorian Guard itself, however, were uneasy with him. “A payment of 12,000 sesterces per man assisted the Praetorians to make up their minds, but for an anxious moment it seemed that it would not work” (Southern, 26). At the festival to swear loyalty to Pertinax some members of the Praetorian Guard attempted to nominate another candidate for emperor, but he ran away due to fear. Pertinax also removed any people he found unnecessary in the Imperial house, and he reinstated those who had been unfairly demoted in the reign of Commodus. He tried to improve the economy by means of fixing the value of the coin and encouraging people to farm, but by the orders of Laetus he was assassinated, the second emperor killed under his orders. Pertinax had not chosen a successor, so there was no true heir to the throne. Laetus feared gaining a position of power, so this free spot was open to the next candidate, Didius Julianus.
Didius Julianus, the Man who proved that Money cannot buy Happiness (March-June 193)
After the death of Pertinax, the title of Imperator was available to anybody, worthy or not. Flavius Sulpicianus, who was the father-in-law of Pertinax and prefect of Rome, tried to buy the title, but lost to a man namaed Didius Julianus. What was the price of emperor? 25,000 sesterces per Praetorian Guardsman. This was a win-lose situation for Julianus, however, and when Septimius Severus marched on Rome there was little he could do.
Other Men…
Petronius Maximus (455), Anicius Olybrius (472), and Glycerius (473-4).
Sources
The Penguin Historical Atlas of Rome by Schris Scarre
The Roman Empire from Severus to Constantine by Pat Southern
Wiki for backup/pictures
Notes
-I consider only the men before the fall of the Western Empire (486 at Soissons, 480 after Nepos' death, or 476 after Augustulus' abdication) to be “Roman Emperors,” though it can be said otherwise
-Feel free to point out any other noteworthy men
- :Crandar: (I had to )