A vestigial organ is one that has been reduced so that it can no longer function in its original capacity. The ancestors of the organism will have had good use of the organ but over successive generations the organ has not been needed and been reduced to the point where it can no longer function.
A good example of a vestigial organ is the wings of some species of stick insects. For example, the adult female Jungle Nymph (Heteropteryx dilatata) has greatly reduced wings and these are not capable of flight.
A photograph of a female Jungle Nymph stick insect showing the vestigial wings.
Photograph by Totodu74
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