[2] Described in 2010, the species is a workerless and obligate parasite of the related ant Mycocepurus goeldii.

[1] M. castrator is a relatively minuscule species, with the average wing length being about 1.07–1.23 millimeters.

[3] It has only been found in nests of M. goeldii, which is a widely distributed species, known from Brazil to northern Argentina.

[1] M. goeldii workers will feed M. castrator queens in a process called trophallaxis.

[1] The species mates inside the nest, as opposed to engaging in a nuptial flight like most ants.

Female alates begin to lose their wings three hours after mating, after which they congregate and engage in social grooming.

Less common are cases like M. castrator's, where two species diverge without the benefit of geographic isolation, known as sympatric speciation.

This led them to rule out the possibility of recent interbreeding, and conclude that sympatric speciation had occurred.