The fossil record of insects contrary to what we think, is abundant and very diverse. If outcrops with fossil insects are rare compared to those with other kinds of invertebrates, especially marine ones, then they compensate by yielding large number of specimens and taxa. The fossil insects are often well preserved and articulated, allowing morphological comparisons with Recent forms, adoption of the same systematic system, and inclusion in phylogenetic studies. Fossil insects also occur as disarticulated remains, especially wings, and various trace fossils recording ancient activity. In the fossil record we have feeding traces on leaves, colonial structures such as termite nests and combs, galls, burrows etc. In the same outcrops, insects can be found from different habitats, both aquatic and terrestrial. There is also evidence of palaeobiological associations such as symbiosis, parasitism, commensalism, phoretic associations, and examples of co-evolution. The earliest reference to fossil insects is by Gaius Plinius Secundus - Pliny the Elder (24-79 B.C.). In his work Naturalis Historia, he described amber and the insect inclusions in it. In this period another writer, Marcus Valerius Martalis (40-104 BC) poetically described the occurrence of fossil insect inclusions.

How insects fossilise: Factors which favour the preservation of fossil insects.

Insects, because of their delicate exoskeleton, have usually been considered by palaeontologist as soft bodied organisms! This is true for example of some holometabolous larvae but it is not a good description of the exoskeleton of common adult beetles (Coleoptera). Nevertheless, if we compare insect preservation with invertebrates possessing hard, mineralised exoskeletons, then insects need some special conditions for fossilisation.
As always in the fossil record, the chances of preservation are directly related to the degree of mineralization of the skeleton: in insect, the sclerotisation or hardness of the exoskeleton is significant. For this reason, we often find isolated parts such as tegmina of cockroaches and elytra of beetles at outcrop. Chitin, one of the principal compounds of the insect’s cuticle, is one of the most abundant biopolymers on Earth, it is more resistant to degradation than protein, for example, but it is rarely preserved in the fossil record. Usually, during diagenesis, chitin is transformed to other organic compounds. Another factor that favours the preservation of insect remains is those individuals that lived in habitats close to or forming part of the sedimentary palaeoenvironment such as lakes or lagoons; in the case of amber, those insects living around resin-producing trees. It is worth noting that, in terrestrial strata, the preservation of chitin is more likely than in marine deposits. Insects are often found in rocks formed in lakes because they either live in them, e.g. mayfly and dragonfly nymphs, adults of aquatic heteropterans and coleopterans, or around the lake. Sometimes, it is possible to find insects in lagoons or marine sediments, for example in the Upper Jurassic of Solhnofen, now the Germany, where marine animals such as the horseshoe crab and jellyfish occur with terrestrial insects. In such cases, insects have been transported into the depositional environment.
In terrestrial environments, there are two additional problems for insects: they have insignificant weight and large wingspans. Although they may live around aquatic habitats, when they fall into the water it is necessary for them to break the surface tension and sink prior to arriving on the bottom where they can be buried in sediment and eventually fossilise. The processes which affect the insects from death on the water surface until they arrive ton the bottom are diverse including transport, surface tension, wind, rain, predation, decay, decomposition, disarticulation etc. It is very important whether transport takes place during or just before death. If transport took place some days after death, during decomposition, then insects will be disarticulated. Then we usually find isolated parts of insects at outcrop.
When insects finally arrive on the bottom, it is necessary for them to be buried rapidly. The case the carcasses will then be untouched by scavengers and oxygen levels will be low, favouring delay of insect decomposition. At this time, chemical factors such as pH are important and, in some cases, may be significant for the preservation of the cuticular organic compounds.
The types of sedimentary rocks are very important for insect preservation. Sediments forming under aquatic conditions are, in general, the best for fossil insects. Those rocks originating under tranquil hydrodynamic and anoxigenic conditions, related to the activity of cyanobacteria, favour insect preservation.
Insects can be find exceptionally preserved in metamorphic or volcanic rocks. Moulds of big insects occur in rocks affected by low grade metamorphism in the slates of the Upper Carboniferous of Mendoza, now the Argentina. Cinerites are deposits of volcanic ashes. Sometimes, in lacustrine deposits large numbers of insects are associated with ashes, as in the Oligocene outcrops of Florissant in Colorado.
Fossil insects are found in fine-grained clastic rocks like clays and silts or in limestones and, less commonly, in sandstones e.g. Liassic marine deposits in Luxembourg. In limestones, there may be two calcium carbonate crystalline phases: calcite, as in the lithographic limestones of El Montsec, or aragonite, as in the Miocene oil shales of Rubielos de Mora, both in Spain. Some insects are found in dolomites as at Ribesalbes, now the Spain. Other insects occur in travertine, where casts of caddisfly cases and dragonfly wings can be found. The travertine is formed by the deposition of calcium carbonate in lentic waters under warm climates.
Insects are also preserved in lacustrine diatomites, rocks formed by the accumulation of siliceous diatom algal skeletons as in the Miocene of Shanwang in China and Bellver de Cerdanya in Spain. Accumulations of the calcareous marine algal skeletons or coccolithophorids can also form rocks e.g. in the Upper Jurassic of Solnhofen where there are well-preserved insects and diverse other animals. Fossil insects are also found in rocks rich in silica or oramorphous silica produced by geyser activity. Insects are also found in rocks with evaporites, for example in gypsum crystals like in Messinian of Elba, Italy or in potash. Phosphatic rocks sometimes contain horizons rich in fossil insects as in the Eocene-Oligocene phosphorites of Quercy in France, or in the Oligocene /Miocene of Riversleigh in Australia. In the coal basins of the Carboniferous period are found the majority of the earliest known insects. In these deposits, not many orders of insects are found but the species diversity is high.. The majority of these insects are found as compressions. Asphalt is a product of the oxidation of petroleum when it migrates to the surface of earth; it is sticky and a large number of insects can be found in it, as in amber. The most famous asphalt fossil site is Rancho la Brea, Los Angeles, California. Amber and copal are natural polymers of tree resin, usually of araucariaceans and another Coniferales, as well as Leguminosae. However these are not strictly sedimentary rocks, although amber and copal may be found in them. Mummified insects are found in association with mammals in the Pleistocene of Belgium, in the stomachs of Siberian mammoths, and in Egyptian mummies. Insect remains are found also in the stomachs of bats from the Eocene of Messel. Traces of insects are observed on or into the organic rests. In leafs exists feeding traces and mines, gals, bark boring by beetles in trees, borings in seeds and spores. Into the palaeosoils termite nets, bee combs and another fossil insect traces are also been found. If the kind of sediment is very important for the future conservation and fossilisation of insects, is fundamental the existence of an early diagenetic mineralization, during the diagenesis. The most common minerals that can precipitate in sedimentary environments are calcite, silica, pyrite and calcium phosphate. These minerals can precipitate on the carcasses or in the interior of the organic tissues. Pyritization is a precipitation or substitution of pirite on/in the place of the organic walls of the organisms caused by the activity or the sulphate-reduction bacteria. This process made a copy of the organisms and promotes that remains can be preserved just today. This process can be observed in Cretaceous fossil insects from the Crato Fm in Brazil. In this case the pyritization was preserved the volume of insects. The pyritization have been developed a few time later after burial, favoured by a high rate of sedimentation, presence of sulphates and low rate of organic material in a anoxigenic environments. Nevertheless the pirite was oxidised in contact with the oxigenic environments, and now we can study the insects preserved in an oxide of iron. Nodules and cast formations are another common processes affecting insect’s remains during the diagenesis. Some insects and arthropods are preserved in carbonate nodules as in the Carboniferous of Montceau-les-Mines in France or in the Lower Miocene of Izarra. In some limestone’s we can find only the mould of insects. During the diagenesis, and after the rock lithification, the organic compounds of the insect can be removed. The mould of it external structures can be preserved as impression in the rock. If the grains of the rock are microscopic, mudstones for example, the impression can show extremely well the external characters, mainly the wings; is the case of some Lower Cretaceous carbonate layers in Las Hoyas. As we are reed above during diagenesis the original chemical composition of chitin may be completely altered and transformed to another type of organic compound. Nevertheless in archaeological sites and some ancient outcrops the diagenesis are preserved the original composition. The differences in the proportion of chitin preserved reflect the environment of deposition more than their age.