The question of insect wings evolution has always been controversial in the field of evolution. It is interesting especially since fossil evidence for winged insects remain full of missing links. The evolution of wings in different species of insects is based on their biological ancestry and adaptation to the environment (Brodsky, 2009). “The origin of modern insects is thought to be closely related to wingless bristle-like creatures known as silverfish” Engel et al (2004). Two major theories try to explain the evolution of insect wing.
The epicoxal theory states that wings could have developed from the gill-like projections possessed by the insects that lived in water (Engel, 2005). Gills evolved over a long period of time extending into the trachea of an adult insect in the form of flaps. These wing-like flaps enabled the primitive insect to jump over short distances but soon evolved further into the modern wings for flying, gliding and diving.
On the other hand, the paranotal theory states that the wings just emerged from protrusions at the back of an insect’s body living on land (Hall, 1998). This seems more reasonable since most modern insects live in terrestrial habitats. Insects are thought to be the first creatures to live on land. They emerged about three hundred million years ago according to the fossil record. (Berlinsk, 1996).
Research into the fossil history of the dragonfly, for instance, reveals a close relationship in terms of physical features between the dragonfly fossils of the Carboniferous period and the extant species. This fossil period that is characterized with the emergence of the dragonfly matches with the time when other insects evolved but without wings. Insects therefore existed before they developed wings but then underwent extensive modifications during their evolution to acquire their modern wings (Hutchins, 2006). Insects evolved wings to fly.
However, some arguments are of recent emerging that wings are thought to evolve as modified limbs. This theory states that wings developed from a section of the legs of wingless insects. “Dorsal wings therefore developed from ventral legs through evolutionary modifications,” (Brodsky, 2009). As such, the first insects to develop wings according to fossil record possessed numerous pairs of them at the sides of their body segments. Modern insects, with just a few pairs of wings, must have undergone structural modifications in the process of evolution, which finally reduced the original number of wings on the primitive insects.
Anatomy and Physiology of Wings and Energetics
The development of wings went hand in hand with other morphological modifications, which included the development of strong thoracic muscles required for propelling the insect in the air. Wings were specialized to translate force generated from the auxiliary apparatus for flying. Wings are attached to the direct muscular system of the insect which either contracts or relaxes to adjust the position of the wings for movement in the air (Thomas, et al, 2001). During flight, insects use a lot of energy, thus, insects have evolved a complex physiological system to respond to its flight energy requirements.
Insects use the sunlight for maintaining the angle at which the radiations meet the eye. This is why most insects fly during the day but remain dormant at night since they rely on the sun for direction, effective maneuvers and to create a proper navigation pattern (Thomas et al 2001). Other few insects, however such as the moth use the moon to fly at night. This enables them to fly in a typical straight line. This means that the use a candle or a lamp for illumination could successfully distract the moth from its typical unidirectional flight pattern. However, the insect will always orient its flight in the straight path with respect to the brightest light around.
The wings were also an adaptation to the vegetative terrestrial habitats. Insects therefore needed the wings to travel through the trees and grass as well as respond to unfavorable environmental conditions. Insects can mate with suitable mates while in air due to the proximity of their reproduction organs (Brodsky, 2009).
The wings were folded when the insects are at rest to shield the insects especially for those species which did not evolve muscles relevant for flight. Wings are flapped by both the direct and indirect muscles during flying. Direct muscles are attached to the wings at the thoracic segment. The various direct muscles attached to the different wings must be coordinated by the brain which processes signals necessary for the contraction or relaxation of the muscles (Engel, 2005). The brain of an insect must have therefore evolved together with the evolution of insect wings to ensure that the frequency of the flight schedules and maneuvers in the air are properly harmonized.
During insect flight, the wings have to be folded to the strategic aerodynamic geometry necessary for flight to occur. The muscles therefore provide the support required to shoulder the wings to ensure that the wings can be adjusted properly by the various muscles before and during flying. As such, the insect can fly at faster speeds through an elaborate pattern that involves beating the wings. The evolutionary process should therefore have taken a procedure that synchronized wing development with other physiological body changes associated with the brain and the muscles (Hutchins, 2006).
According to fossil evidence, winged insects therefore emerge suddenly with already established physiological structures. The wings are typically connected to the trachea, neurons and the circulatory system (Engel, 2005). This is perhaps an evolutionary outcome that enabled the wing tissues to be supplied with metabolites, oxygen and signals from the general body systems always independent of the muscular contractions for flight. Indirect musculature is independent of the wing morphology. The wings are joined dorsally to the thorax at the tergum. In other species, the wings are attached in both the tergum and the thorax which are then propelled by muscles connecting both the ventral and dorsal segments. The wings are therefore coordinated by the muscles during flight with little brain involvement. The brain just initiates the original take off by instructing the muscular contractions that lift the insect above the surface after which the muscles sustain the rhythmic contractions that keep it borne (Hall, 1998).
The nervous system is also associated with the muscular contractions that lift an insect off the ground as well as sustained flight. When nerve impulses are generated by the immediate environment stimuli reach the brain, instructions are ordered from the brain to the wing muscles for an appropriate response. Muscle contractions in insects that flap their wings gradually normally respond to impulses from the nervous system.
The involvement of the nervous system in determining the frequency of the wing beat thereby sets the maximum speed at which flight occurs (Engel, 2005). This is attributed to the time taken by the neurons to generate an action potential for conduction of stimuli as well as the subsequent delocalization of the electrical impulse to the brain. The time taken by the neuron to regenerate the action potential therefore determines the peak velocity at which wing beat can be attained. This is because the nervous impulses coordinate the periodic muscular contractions necessary for propelling the wings of an insect.
The number of muscular contractions is directly proportional to the nervous impulse (Hall, 1998). On the other hand, other insects beat their wings faster than the time it takes a neuron to generate an action potential for conduction of stimuli. The muscles associated with insects that fly at such high speeds contract more rapidly independent of nervous impulses except for the initial signal for flight and the landing signal. These powerful and rapid muscular contractions require considerable metabolic energy for such demanding activity.
The evolution process associated with this type of actively flying insect resulted in minimal number of flight muscles on a lean insect body for both sustained flight schedules and energy conservation. Insects have to possess a contoured mechanism to reduce resistance body shape to remain afloat as well as make all of the important maneuvers associated with their active flying. This physical shape allows for the aerodynamic geometry to be attained in the wing morphology, which permits for the steady flow of air currents through the wing (Hutchins, 2006). As such, the insect can fly up and down as well as sideways.
Hormonal control of flight is equally feasible. For the insects to distribute the metabolic energy from their oxidized fuels to the flight muscles, hormones are involved. Carbohydrates, proteins and fats are utilized after their metabolic oxidation in the hemocoel. Oxidation is provided through the open circulatory system through diffusion to the metabolic pathways located in the flight muscles (Thomas et al, 2001). Additional fuel reserves are obtained from the fat storage tissues.
Normally, immense fuel reserves in the haemolymph and glucose present in the flight muscles are oxidized to drive the insect during short-lived flights. When the flight takes longer time, fats are utilized from their energy reserves to supplement the extra energy requirements under the control of the endocrine system. This is because; glucose is easily oxidized to yield energy in the short period (Brodsky, 2009). Evolutionary changes that brought about the development of insect wings therefore took into account the physiological processes that are required to initiate the flight as well as regulate it.
Structural modifications associated with its physiological systems have resulted in a well-designed flight pattern. The evolution of wings from body structures that were previous existing is therefore viable in view of the energy and regulation requirements that a flying insect must have (Hutchins, 2006). The muscles can propel the lean insect in the direction of motion when wings are folded properly to assume the aerofoil symmetry.
Wing Evolution and Genetics of development
Present day insects could have undergone a reduction of the multiple wing pairs through repression of the homeobox genes associated with regulation of segment characteristics in insects (Engel, 2005). This genetic repression of genes encoding structural proteins unique to unnecessary wings occurred during evolution resulting in polymorphism of the control genes. The control genes therefore were suppressed from expressing transcription factors that could have developed many wings on segment appendages under different environmental stimuli.
The development of wings from gills can be illustrated by the shared characteristics that different insects have in their wings. For instance, a critical study at the veins of insect wings throughout the orders shows similarities in the phylum Pterygota. The thoracic muscles which propel during flight are virtually the same in many insect species. The pterygote’s wings were located on the thorax and abdominal insect regions at the beginning of the evolution process, which later resulted in variation in the wing numbers ranging from three to one pair across its orders (Hutchins, 2006). This is as a result of the suppression of the various homeotic genes on the insect body. As such, the evolution of insect wings followed a process that was initially independent of gene modifications but which later underwent segment migration associated with these homeotic genes. The evolutionary history of wing development therefore provides a clue to the current diversity in insects whose origins cut across paleontological, physiological and biomechanical research findings.
Paleontology explains the evolution of wings from the limb excites which were initially moveable and later modified through the process known as vortex flaking for flight purposes. This theory is supported by the similarities found between the sensory apparatus located on the legs and the insect wings appendages (Thomas, Reynolds & Woiwood, 2001). The wings, which are dorsal appendages, must have developed from legs being ventral appendages through an evolutionary process which was characterized by dorsal relocation around the body of the insect. The particular portion of the leg which evolved into the insect wing is not known.
Wing development could also have occurred by extending dorsally from the thorax. A critical study between homology and paleontology could also bring forward another evolutionary perspective linking the origin of wings to structural modification of ancestral insect legs. The wings are currently possessed by all insect orders in various forms which can be used in their classification (Brodsky, 2009). The wings are so important to the modern insect that their absence could lead to survival challenges as well as difficulty in reproduction. This crucial relevance of wings to the insect was taken into account during its evolutionary process.
As such, the evolution of wings has enabled insects apart from other invertebrates to develop an elaborate flight regime for its survival. The development of wings through the modification of their legs should have taken advantage of the elaborate muscular, nervous and circulatory systems. It is also possible that ancestral insects evolved wings necessarily from primitive body structures independent of the body systems currently known to sustain and control modern insect flight. However, no scientific findings have been recorded to support a viable insect ancestry.
However, the wings themselves are not complex but simple structures which evolved for easy and faster flying. The various maneuvers made by the insect in the air at different speeds are possible through the simple body with not much weight. This also allowed a quick generation of the forces required to lift the wing for flight. The wings therefore act as perfect wings capable of interpreting the dynamic muscular forces and torque resulting from the forces of inertia as the insect flies in different directions in the air (Hall, 1998). These structural and physiological modifications which have occurred during the evolution of wings have resulted in a complex flight mechanism that is designed to specially fit insects. The interaction between the muscles and the wings together with the brain allows the insect to negotiate navigation patterns in the air in virtually all directions with ease. T Wings have therefore enabled the insect to protect itself against predators by simply flying away from danger as can be observed by flies. In addition, the insects can migrate over long distances for food and mates.
Evolution of wings was therefore a necessary element for the modern insect which for the sake of its existence and adaptation to the environment. It is possible that insects were among the first creatures to dwell on the land and therefore were not in any danger, but the structural modifications associated with the insect illustrate a coordinated response to stimuli. The development of wings initiated and directed further evolutionary changes in the insect body necessary to accommodate its crucial role (Thomas, Reynolds & Woiwood, 2001). The body systems appear to be linked to the wing’s dynamic morphology and physiology.
Overall, the insect probably evolved wings in response to its needs and relevance in various habitats. The internal mechanisms therefore adjusted with neighboring environmental changes to determine the pattern and process of wing development.
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