Insecta

Insects (Class Insecta) are arthropods that have existed for hundreds of millions of years. Insects inhabited Earth since before the time of the dinosaurs. Many modern insects had already evolved to very similar forms even before the dawning of the dinosaur and lived alongside them and beyond up to the present day. Like today, prehistoric insects were an important part of the food chain in their time.

The differences between modern and prehistoric varieties can be essential, and, like many other creatures of prehistory, the latter tended to be much larger than their contemporary equivalents. This size difference is thought to be due to higher atmospheric oxygen levels (allowing diffusion through spiracles over greater distances) and higher temperatures (enhancing metabolism).

Since insects have chitin exoskeletons rather than mineralized bones, their burial processes differ compared to the fossils of much larger vertebrates such as dinosaurs. Many insect remains are found preserved in the hardened sap of ancient trees (amber).

Examples of prehistoric insects include:


 * Eobelinae
 * Palaeodictyopteroidea
 * Rhyniognatha
 * Meganeura

Evolution
The relationships of insects to other animal groups remain unclear. Although more traditionally grouped with millipedes and centipedes, evidence has emerged favoring closer evolutionary ties with the crustaceans. In the Pancrustacea theory, insects, together with Remipedia and Malacostraca, make up a natural clade.

Other terrestrial arthropods, such as centipedes, millipedes, scorpions and spiders, are sometimes confused with insects since their body plans can appear similar, sharing (as do all arthropods) a jointed exoskeleton. However upon closer examination their features differ significantly; most noticeably they do not have the six legs characteristic of adult insects.

A phylogenetic tree of the arthropods and related groups.

The higher level phylogeny of the arthropods continues to be a matter of debate and research. In 2008, a Tufts University student and a faculty lecturer uncovered what they believe is the world's oldest known full-body impression of a primitive flying insect, a 300 million-year-old specimen from the Carboniferous Period. The scientists presented the fossil at the Second International Congress on Ichnology, in Krakow, Poland in September 2008. The oldest definitive insect fossil is the Devonian Rhyniognatha hirsti, from the 396 million year old Rhynie chert. This species already possessed dicondylic mandibles, a feature associated with winged insects, suggesting that wings may already have evolved at this time. Thus, the first insects probably appeared earlier, in the Silurian period.

The origins of insect flight remain obscure, since the earliest winged insects currently known appear to have been capable fliers. Some extinct insects had an additional pair of winglets attaching to the first segment of the thorax, for a total of three pairs. So far, there is no evidence that suggests that the insects were a particularly successful group of animals before they got their wings.

Late Carboniferous and Early Permian insect orders include both several current very long-lived groups and a number of Paleozoic forms. During this era, some giant dragonfly-like forms reached wingspans of 55 to 70 cm, (22-28 in) making them far larger than any living insect. This gigantism may have been due to higher atmospheric oxygen levels that allowed increased respiratory efficiency relative to today. The lack of flying vertebrates could have been another factor. Most extinct orders of insects developed during the Permian era that began around 270 million years ago. Many of the early groups became extinct during the Permian-Triassic extinction event, the largest mass extinction in the history of the Earth, around 252 million years ago.

The remarkably successful Hymenopterans appeared in the Cretaceous but achieved their diversity more recently, in the Cenozoic. A number of highly-successful insect groups evolved in conjunction with flowering plants, a powerful illustration of co-evolution.

Many modern insect genera developed during the Cenozoic; insects from this period on are often found preserved in amber, often in perfect condition. Such specimens are easily compared with modern species. The study of fossilized insects is called paleoentomology.