Differential investment in brain regions of polymorphic diurnal and nocturnal Myrmecia ants

Many nocturnal insects have evolved optical adaptations to capture more light at night but these adaptations are not sufficient to explain visually guided behaviour seen at night. Hence, there must be additional neural adaptations to explain this behaviour. At the same time, the size of brain neuropil can be significantly constrained by the size of the animal. Here, I asked if the size of functionally distinct brain regions change in two congeneric polymorphic bull ants, the diurnal M. gulosa and the nocturnal M. midas. I took advantage of the extreme intraspecific size range in both species to also ask how body size affects brain region scaling. I found that the diurnal species invests more into the optic lobes, while the night-active species invests more into the antennal lobes and mushroom bodies, in complete contrast with predictions. The higher order processing neuropils of the central complex did not differ significantly between species, except in the central body lower. I also found neuropil volume changes as body size changes, with larger individuals having larger neuropils and smaller individuals having smaller neuropils. These results show that there are adaptations in the brains of diurnal and nocturnal bull ants that may help with activity in different temporal niches and that the volume of these regions does not exist independently from size.