Tuesday, 16 August 2011

New Fossils Shed Light On The Origin Of Insect Wings

Insect wings are truly incredible pieces of biological machinery. They consist of a thin membrane just a few hundreds of micrometres thick supported by a series of even thinner veins embedded just within the flesh. These also rely on thin membranes and the pressure of the blood inside for support. These are attached to the insect by a joint that is a marvel of microscopic engineering. Yet still they can beat over one hundred times a second and support a body that weighs more than themselves.

However the biggest mystery surrounding them is simply how they evolved in the first place. The earliest creatures to live on the land came around 400 million years ago in the form of 8 and 12 legged crustaceans that we think were rather like centipedes. How such creatures evolved delicate wings and bodies that could sustain flight is an evolutionary problem that baffled scientists for years. Apart from the fact that insect wings rarely preserve due to their delicate nature, they evolved so far back in time that, at that point, the land was barren and fossils could not have formed on it.
One of the holotype fossils of Coxoplectoptera

Any fossils that might tell us how insect wings evolved are almost non existent. However this situation changed recently. A series of spectacular insect fossils, named Coxoplectoptera, were discovered by a team of palaeontologists from the Stuttgart Natural History Museum from lower Cretaceous formation in South America. While the fossils are lower Cretaceous, almost 320 million years since the first ever terrestrial insects, they have a surprising morphology that provides us with clues as to the evolution of winged insects.

The fossils were very odd creatures. They had the same system of wing veins as mayflies, which made the team think that they are the first ever mayflies. They have also the thorax and wing design of a dragon fly and the legs of a preying mantis - a rather patchwork creature indeed. The importance of Coxoplectoptera lies with the arrangement of its wings. It seems that the wings evolved from its back thoracic plates. This simple fact is very useful as it can give us a rough date for the origin of insect wings.

The fossil of Rhyniognatha hirsti
with the mandibles labled 'M'
All insects have a head, thorax and abdomen. The first large land based group of arthropods were the trigonomotarbids. We think that they gave rise to the arachnids, with just a head and abdomen and the insects with their three part body system. The oldest known insects come from a prehistoric peat bog called the Rhynie Chert in the form of Rhyniognatha hirsti. This creature is around 400 million years old and is so advanced for its time that scientists think that the first insects could be even older than 400 million years.

Rhyniognatha also shares many characteristics with winged insects, suggesting that they could have already evolved. While this establishes a rough date for the origin of winged insects, it does not explain why they evolved. Flight is very energy consuming and insects absorb a lot of oxygen to be able to fly. Just 15 % of the atmosphere 400 million years ago was composed of oxygen. Flight would have been difficult and impractical in such conditions.

If we reduce the problem to the most simple construct of evolution - reproduction - a logical solution based upon the lifestyle of Coxoplectoptera can be formed. Scientists believe that this creature lived, partially, in water as an ambush predator. Rivers, ponds and lakes would have been its main habitat. So why did it need wings? Let us assume for the moment that winged insects evolved in a place such as the Rhynie Chert.

The pools would have been inhabited by predatory trigonomotarbids - not a problem if you were also a large predator. The problem comes with reproduction. The actual copulation is very quick, there is still no threat from an ambush attack. However if the fertilised eggs are left alone in a pool of dangerous predators, they are unlikely to survive.

However if you can fly away from the danger and deposit the eggs on a leaf or rock, then they are far more likely to survive - the prime objective in evolution. Scientists think that Coxoplectoptera would have had a lifestyle similar to the Rhynie Chert model. This combined with its morphological characteristics, has given palaeontologists a great new insight into the origin and evolution of the beautiful and diverse arthropod group that are the insects.