Just to clarify, these "reversions" in evolutionary process are actually just introduction of similar genes. In the instance of genetically limiting a population such as in the example of bugs being sprayed with DDT, when you discontinue the limitations provided by the poison, evolutions of genetic structure that do not possess the DDT resistant gene can then survive and reproduce, as opposed to the DDT resistant generations simply becoming non-resistant. The reason the species does not continue to be specifically resistant is due to the introduction of non-resistant individuals surviving long enough to pass on their genes