Exceptions from rules
Rules (laws) are generally not one hundred percent valid in science and there are frequently a number of exceptions. This is caused primarily by the fact that, as they are formulated, they are excessively simplified and thus imprecise. Take, for example, the rule that females generally select males is valid only assuming that females invest more valuable resources into reproduction than males. However, this is not true for a number of species. For example, it does not hold for giant water bugs of the Belostomatidae subfamily, where the female lays her eggs on the back of the male and he then carries them, defends them and ensures that they get enough oxygen for three weeks. The total weight of the eggs is twice that of the male and care for them is a great burden on him. Females can copulate with a number of males, but a male decides whether he will accept eggs from a female or not. As the area of the backs of males is a factor limiting reproduction, fierce competition occurs amongst females for males willing to accept batches of eggs. The adage “the exception proves the rule” should properly be “the exception tests (or allows testing of) the rule”. In this form, it is a profound truth – when we study the individual exceptions from rules, we should always discover their cause (in the above case, the cause of the deviation from the rule is the fact that the males, and not females, invest the more valuable resources into offspring). If we can find the reason for the individual exceptions, we confirm that we have understood the nature of the rule properly.