The ability of some insects to imitate the leaves and stems of plants has fascinated collectors and researchers alike. Wings, legs and other body parts can all contribute to a very effective disguise, a phenomenon known as mimesis. There has been speculation, of course, about the adaptive origins of the observed characters, but very little data is available on which to build anything robust. The fossil record is meagre. The earliest example before this year has been the Eocene leaf insect Eophyllium, already fully formed and functional (noted here). It conveyed no evidence to support a gradual transformation model. Since living examples of leaf mimesis relate to angiosperm plants, it has been inferred that leaf mimesis is a trait that post-dates the appearance of angiosperms in the Cretaceous.
"Given the phylogenetic placement of these families and genera among their respective orders, such mimicry of angiosperm models likely appeared subsequent to (rather than along with) the radiation of flowering plants. Accordingly, it has been considered that leaf mimesis is a mid-Cretaceous or younger phenomenon."
However, new research changes this perception. The findings concern lacewings - a group not known for exhibiting leaf mimesis. The fossil specimens come from the late Middle Jurassic, which is significant because this was a period of Earth history before angiosperms appeared and before they became dominant. The plants then were gymnosperms and many of them had pinnate leaves.
"Two extraordinary fossil lacewings, Bellinympha filicifolia [. . .] and Bellinympha dancei [. . .] from the Jiulongshan Formation in northeastern China, preserve wings that are dramatically modified to resemble pinnate leaves. These are the earliest evidence of leaf mimesis and predate Eophyllium by nearly 120 million years. Bellinympha demonstrates that lacewings of the late Middle Jurassic (165 million years ago) already had evolved highly specialized mimicry of pinnate leaves as a strategy for avoiding predators such as contemporaneous mammals, pterosaurs, dinosaurs, birds, spiders, and other predacious insects, suggesting a much richer biotic world in the Mesozoic Era."
The fossil lacewings have wings that resemble the feather-like leaves of mesozoic gymnosperm plants (Credit: Natl Acad. Sci. Source here)
These fossils are not the earliest lacewings, which go back to Permian times. The authors suggest similarities with Holozamites and Nilssonia, which are Cycadales, and with leaves of some cycadophytes. The finds are the earliest known witnesses to leaf mimesis.
"The numerous pinna-like markings on the wing membranes are remarkably similar to the pinnate leaves of Mesozoic Cycadales and Bennettitales: the dark posterior median region of the wing resembles the rachis of the leaf, whereas the oblique stripes resemble the pinnae. Among the two species, B. filicifolia has a more obvious pattern of pinnate leaves due to a darker and better-defined coloration, whereas B. dancei has a more pronounced central rachis-like region, with a similar zigzag shape near the apical area, but the oblique pinna-like stripes are less developed. Pinna-like markings on the forewings imitate contemporaneous pinnate leaves of Cycadales and Bennettitales that are frequently found in the Daohugou strata."
The research has the effect of emphasising the early appearance of specialisation in insects, including the association between insects and their environments. There are already indications that ecosystems were far more complex than previously thought (for example, see here). Lacewings have been lacewings since the Permian, and this example of mimesis so early in their history is suggestive both of a plasticity of form and also of a possible loss of this plasticity over geological time.
"These species reveal a unique pattern that seems to have disappeared in modern insects and adds to the growing body of evidence documenting that the evolution of insects was more complex before the radiation of angiosperms. These enigmatic scenarios of interactions between insects and gymnosperms were lost during the course of evolutionary history and show a more rich association of insects and their surrounding environment in past geological epochs than previously has been surmised."
Initial complexity followed by relative stasis characterises this group of insects. The Nature briefing on this research refers to the authors suggesting "that the biotic world of the Mesozoic period was more complex than previously thought". Darwinian gradualism gets no support from this research. Abrupt appearance, early specialisation and complex ecosystems are all confirmed.
Ancient pinnate leaf mimesis among lacewings
Yongjie Wang, Zhiqi Liu, Xin Wang, Chungkun Shih, Yunyun Zhao, Michael S. Engel, and Dong Ren.
Proceedings of the National Academy of Sciences, Published online before print August 30, 2010 | doi: 10.1073/pnas.1006460107
Abstract: Insects have evolved diverse methods of predator avoidance, many of which implicate complex adaptations of their wings (e.g., Phylliidae, Nymphalidae, Notodontidae). Among these, angiosperm leaf mimicry is one of the most dramatic, although the historical origins of such modifications are unclear owing to a dearth of paleontological records. Here, we report evidence of pinnate leaf mimesis in two lacewings (Neuroptera): Bellinympha filicifolia Y. Wang, Ren, Liu & Engel gen. et sp. nov. and Bellinympha dancei Y. Wang, Ren, Shih & Engel, sp. nov., from the Middle Jurassic, representing a 165-million year-old specialization between insects and contemporaneous gymnosperms of the Cycadales or Bennettitales. Furthermore, such lacewings demonstrate a preangiosperm origin for leaf mimesis, revealing a lost evolutionary scenario of interactions between insects and gymnosperms. The current fossil record suggests that this enigmatic lineage became extinct during the Early Cretaceous, apparently closely correlated with the decline of Cycadales and Bennettitales at that time, and perhaps owing to the changing floral environment resulted from the rise of flowering plants.
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