In his seminal Book of Sharks, Richard Ellis writes of the world’s largest fish, the whale shark Rhincodon typus, "there is a general (albeit erroneous) understanding that if a fish can be ‘40 feet long, why not 50, and if 50 feet, then why not 60’. "
The same logic seems to apply to Titanus. The largest reliably measured, and published, specimen of Titanus giganteus is a male from French Guiana that was 16.7cm long. But the popular literature abounds with references to 18, 20 and even 22cm specimens. There is a widely circulated rumour that the Natural History Museum has a 20cm example of this species kept under lock and key. This is false, as, no doubt, are most of these outsize ‘records’.
At present males of this species are believed to be larger than females, because all the largest known specimens are males. But females will probably prove to be larger than males - the sex ratio of known specimens means we are comparing the extremes from thousands of male specimens with the extremes from a few dozen female specimens.
In similar species, such as the British and north European Prionus coriarius, where both sexes are quite well known, the females are on average a few percent larger than the males. There is no reason to believe the situation in Titanus is any different.