In what appears to have been a hysterical spree of runaway selection, male flies went to ridiculous lengths to outdo their rivals in contests for females. Those females, for some reason, liked broad heads. The outcome was the insect equivalent of a hammerhead shark. The stalk-eye trait arose partly from allometry, but it was also likely abetted by hormonal stimulation. Genetic analyses are ongoing, but we can venture a guess based on evidence amassed in 2012: stalks were probably targeted for growth due to a higher-than-normal density of receptors for insulin-like growth factors. Stalk eyes evolved convergently in snails, crabs, and other crustaceans.
leopard | cheetah compared with butterfly | anglefish | zebra | mouse
cloud leopard | giraffe | ant | beetle | treehopper
stalk-eyed fly | ladybird | snake |

The Interactive Fly resides on the web server of the Society for Developmental Biology.