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> It occurs at the fetal level. A 5-10% ratio is maintained by a pheromone:
> 
> https://phys.org/news/2018-10-ants-growth-seemingly-useless-huge.html
> 
> "Scientists at McGill have found the answer to a question that perplexed Charles Darwin. So much so, that it actually led him to doubt his own theory of evolution. He wondered, if natural selection works at the level of the individual, fighting for survival and reproduction, how can a single colony produce worker ants that are so dramatically different in size—from the "minor" workers with their small heads and bodies, to the large-headed soldiers with their huge mandibles—especially if, as in the genus Pheidole, they are sterile? The answer, according to a paper published today in Nature, is that the colony itself generates soldiers and regulates the balance between soldiers and "minor" workers thanks to a seemingly unimportant rudimentary "organ" which appears only briefly during the final stages of larval development. And only in some of the ants—the ones that will become soldiers. 
> 
> ***
> 
> "Rajendhran Rajakumar the first author adds, "What we discovered was that these rudimentary "organs" are not a secondary effect of hormones and nutrition, but are instead responsible for generating the soldiers. It is their passing presence that regulates the head and body of soldiers to grow at rapid rates, until you get these big-headed soldiers with huge mandibles and big bodies."
> 
> ***
> 
> "Indeed, they found that they were able to scale the size of soldier ants by cutting away differing degrees of the imaginal wing discs, with a corresponding decrease in the size of the heads and bodies of the soldier ants. It was clear confirmation that the rudimentary wing discs play a crucial role in the development of soldier ants.
> 
> "The researchers also discovered that the colony as a whole maintains the balance between soldiers and minor workers by regulating the growth of the rudimentary wing discs in larvae. Earlier research had shown that the ratio of "minor" workers to soldiers remains constant in all colonies of the Pheidole genus, with a proportion of "minor" workers at 90-95 % to 5-10% soldiers. The McGill team has found that the soldier ants maintain this ratio by halting the growth of the rudimentary wing disc with an inhibitory pheromone when there are too many soldiers. However, the colony is able to ramp up the number of soldier ants very quickly if it is under threat or the numbers of soldiers have dropped for some reason, because the rudimentary wing discs that play such a crucial role in regulating the number of soldier ants appear only in the final stages of larval development."
> 
> Comment: This shows an automatic control over soldier numbers by a pheramone release. I can  predict the next finding: the ant colony estimates the ratio of soldiers by analyzing the concentration of a pheramone the soldiers release within the colony and adjust fetal ratios appropriately. A great example of design.