A strange order of insects, close to the coleopterans, has been given the name of phasmids (“phantoms”), indicating their ability to remain invisible to neighbours by imitating the forms and colors of the trees on which they alight. Stick insects and leaf insects belong to this order. These astute animals are taken as examples of mimetic adaptation, but they proved embarrassing when paleontologists started following their fossil traces and found them where they were not supposed to be. How these ghost insects – these incredible mimickers of leaves and sticks – ever came into the world remains a mystery, an unsolved scientific detective story. The reigning utilitarian interpretation would have it that these insects, before they were like leaf surfaces or dry sticks, got mixed up with leaves and twigs and, through mutation after mutation, came to resemble their background, until they arrived at the point of becoming the perfect models we see now (to the point that we don't see them!) on plants. Unfortunately for them and for the theory, these artful imitators derive no benefit from their mimetic capacity and are prey for their enemies, which have no difficulty in detecting them. But the most unforeseeable surprises have come from paleontology. The oldest phasmid fossils (they go back in Baltic amber to the Tertiary – i.e. about 50 million years ago) look identical to present-day species, showing that no gradations have occurred. It is thought that those phasmids originated from Chresmodids of the Upper Jurassic in Germany, fossils of which are encountered in deposits dating back some 150 million years. But the oldest fossils of stick or leaf insects (protophasmids) go back to even remoter periods, in the Permian (250 million years ago, in the Paleozoic). One might argue that these insects completed the process of imitating leaves at an extremely gradual rate beginning at a still earlier time. Yet things do not work out this way. Plants with flowers and leaves (phanerogams and latifoliae) appeared on earlier than the Cretaceous – in other words about 100 million years ago, long after the first protophasmids. This chronological anomaly places the imitators earlier in time than the objects of the imitation, leaving entomologists and paleontologists disconcerted.That's a long paragraph! The two key points he is making are:
1. Having a leaf-like/stick-like appearance doesn't convey a survival advantage.
2. In any case, insects appeared that looked like leaves and sticks before leaves and sticks evolved.
I have little doubt about the initial guess at an evolutionary story that would be told concerning these insects. But given that it doesn't work, is there an alternative?
