Insects are built upon a segmental plan and their characteristic feature is a hard, jointed exoskeleton. Moreover, the exoskeleton is a significant structure that provides many functions for the insects such as the support and shape of the body soft tissue, protection from the predator and reduces the water loss in the desert and freshwater environment. Furthermore, this exoskeleton feature provides mechanical advantages for muscle strength and movement agility. Significantly, this armor-like feature mainly provides great protection against any physical and chemical attack which theoretically contribute to the survival of the species throughout the extinction events (Meyer, 2017).Chapman (1998) explains that the cuticle-formed exoskeleton covers continuously over the insect’s body which consists of sclerites, a series of hard plates connected by flexible membranes which are articulated to each other to give precise movement. The insect’s segments are divided into three units which are head, thorax, and abdomen in which each basic segment parts might be significantly modified or lost. Each unit is retained and modified to perform a specific function- the thoracic segments are retained for walking, the head is equipped with appendages for sensory and feeding purposes and as for the abdomen for mating and oviposition.
As for the composition of the exoskeleton, a polysaccharide chitin and sclerotin are mainly the primary fabrics that bind with many other protein molecules that form the body way which gives elastic and flexible behavior with dual properties of rubber-like elasticity and metal rigidity (Meyer, 2007; Betancourt, 2010). Interestingly, the ratio of these components which varies on each body part allows for a firm, secure protection especially in the head capsule which contains vital organ for the organism. The strong and rigid nature of the exoskeleton provides some insects with very complex and unique body design. Some insects have spikes for warning and protection while others possessed fierce predatory claws. Insects also have colourful iridescence and camouflage, which are made possible by the possession of the exoskeleton. Across the geological time scale, recent insects exhibit similar body structure like their past ancestors. Dragonflies for example bears similarity of having similar segmented body anatomy with their common ancestor which is the largest insects that ever lived, Meganeuropsis permiana with their fossil record ends with the Permian-Triassic extinction event.
Insects are the only invertebrate that successfully learned how to fly using their wing to colonize globally through time. Their wings benefit them in so many ways such as protective shells, social purposes musical instrument(grasshopper), camouflage, signals for recognition, mating and warning mechanism and most notably for flying (Venton, 2011). Wings can take on almost any color, texture, or appearance such as metallic or even transparent. According to Chapman (1998), primarily the structure of the wings is determined by the need of optimizing favourable aerodynamic forces during the flight. The shape of the wings is influenced by the need of aerodynamic consideration and ecological factors.
The writer further explains that the succession of the insects is mainly influenced by the attributes of the wings, found in numerically dominant pterygotes. Based on fossil evidence, one longest-standing hypothesis had been made imputes to the origin of wings to postulated lobes, derived from paranota- thoracic terga. Each winglet/proto-wings are formed by the fusion of epicoxa lobe of basal leg segments of the respective ancestral leg which are exite (outer appendage) and endite (inner appendage) with both lobes undergo tracheation and articulation during the fusion. Furthermore, this hypothesis of an exite-endite model for wing origin is substantiated by a molecular study (Gullen & Cranston, 1994).
Besides the exite-endite model, another recurring view can be reconciled with the hypothesis which that wings derived from tracheal gills of ‘protopterygote’- an ancestral aquatic arthropod. One comparison that can be made according to this hypothesis is that the abdominal gills of aquatic mayfly nymph may be analogous to the abdominal winglets of protopterygote and thoracic wings. Winglets are vital for gas exchange, ventilation and swimming assistant for aquatic juveniles while the terrestrial adult benefits the structure as an aerodynamic function. All early wings hypothesis proposed that protowings are not mainly used for flying but rather for other function such as swimming and leg protection. Eventually, new theory explains that aerodynamic function such as gliding and running-jumping to flying contributed to the progression of flight-functional uses of the wings. This unique adaptation of the insect’s wings helps to sustain the survival of the species throughout geological time which answered the reasoning behind the diversity of insects in fossil record.
Plant-Insect Diversity Interrelation
The diversity of insects is interrelated with plants, or angiosperm specifically. Briefly, several numbers of insects can occur in one distinct species of plant. They may not be competitors although they share the same resource. In addition to that, they are often separated temporarily as a result in phenology differences between them. (Schoonhoven, Jermy & Loon, 1998). For example, about eight species of insects incorporated with one species of plant, stinging nettle (Urtica dioica). The difference in the population build-up of the insects caused by the different life cycle patterns of the plant. The diversity of angiosperm and insects are shown by the act of mutualism by both. The flowering plant produces nectar that is one of the source food for the insects.
According to Schoonhoven, Jermy, and Loon (1998), about 67 percent of all flowering plants experience pollination by insects. In return, the plants provide food to the insects in term of nectar and pollen for as a gift of pollen transfer made by the insects. Positively, both angiosperm and insects benefit from each other. In fact, the absence of one of them in this process will affect the other completely. So, the relative abundance of angiosperm will enhance the diversity of insects. Somehow, some rare insect species also connected with this mutualism. Furthermore, it is proven that the rarity of the insects can be deduced by the diversity of the angiosperm in that specific locality. The non-optimal plant quality for the insects causes the decline in the remaining species densities (Schoonhoven, Jermy & Loon, 1998). It is also known that the density of the insects, especially herbivores will be much lower compared to the food resources available, or in other word, plants. Apart of it, the other mechanism in which plants can increase the insect’s species richness is co-evolution (Mayhew, 2007).
The co-evolution can be explained in term of the relative speciation in both plants and insects. Co-evolution can be subdivided into two. According to Gullan and Cranston (1994), specific or pair-wise coevolution is described as the evolution of one species(insect) because of the trait of the other species(plant), in which it was originally evolved because of the first species trait. In contrast, diffuse or guild coevolution is described as group evolutionary changes reciprocally rather that species, or pairs. Speciation of the plants in that locality will encourage the speciation in insects . As this happens, the opportunity for insect’s diversity in the future will be based on host switching. Contradict to this idea, Schoonhoven, Jermy and Loon (1998) states that the adaptation of the insects to new host plant occurs as a result of the plant abundance. This idea reveals that if new species emerges from the insect population and change to a new plant instead of the parental host plant, its abundance will be higher following the abundance of new host plant. Based on these two ideas, it is preferable that the insect diversity richness follows the host plant abundance. Higher plant abundance in certain locality will cause the increasing insect diversity.
Insect Diversity with Tropical Distribution
Larger tropical distribution can contribute to the species richness of the insects. A lot of studies being conducted in this diverse tropical system in proving the correlation between the diversity of the plants and insect diversity. One of the recognized biodiversity hotspot, Cape Floristics Region in South Africa contains about 10000 plant species in 90000 km2 (Kemp & Ellis, 2017). In this region, it is proven that there is a positive relationship between the plant diversity and insect diversity. However, this correlation only valid in smaller sampling scales as the outcome will be different in larger scale. The author also states that when comparing the plant diversity structurally, tall plants are utilized by less insect species rather than shorter or medium-sized unbranched plants. It it most probably related to the zoning of the system. Insects mostly occupy in relatively low zones while birds in the higher zones. Relatively intermediate latitudes show the peak of the species richness, where the temperature as well as the plant abundance are conducive and suitable for various species of insects to live in. Specifically, the insect fossils can be found higher in ancient environments where its plant diversity is higher. Tropical paleoenvironment will be one of the environment that can contribute to the relatively higher insect fossils.