Here we attempt to reconstruct the sequence of events that led to the formation of three regulatory piRNA clusters, namely, 20A, 38C and flamenco in the Drosophila melanogaster genome. Both the 38C and flamenco clusters include inverted sequences, which potentially form double-stranded RNA hairpins. We present evidence in favor of the well-known hypothesis of piRNA clusters as "transposon traps". According to this model, the presence of the only copy of the transposon in the genome indicates that its expression is suppressed by an RNA-interference mechanism immediately after the mobile element enters the piRNA cluster. We also discuss high the structural variability of piRNAs in Drosophila clusters and cases of horizontal transmobile elements between related species.