 Myrmecia ants are easily noticeable , due to their large mandibles , and large compound eyes that provide excellent vision , and a powerful sting that they use to kill prey . Each of their eyes contains 3 @,@ 000 facets , making them the second largest in the ant world . Size varies widely , ranging from 8 to 40 millimetres ( 0 @.@ 31 to 1 @.@ 57 in ) in length . The largest Myrmecia species is M. brevinoda , with workers measuring 37 millimetres ( 1 @.@ 5 in ) ; M. brevinoda workers are also the largest in the world . Almost all species are monomorphic , but M. brevinoda is the only known species where polymorphism exists . It is well known that two worker sub @-@ castes exist , but this does not distinguish them as two different polymorphic forms . This may be due to the lack of food during winter and the fact they could be incipient colonies . The division of labour is based on the size of ant , rather than its age , with the larger workers foraging for food or keeping guard outside the nest while the smaller workers tend to the brood . 
