Introduction
The evolution of wings in different species of insects is based on their biological ancestry and adaptation to the environment (Brodsky, 2009). Research findings into this evolution have been done across various disciplines such as paleontology, physiology, and geology. According to evolutionary studies carried out on ancient fossils, the origin of modern insects is thought to be closely related to wingless bristle-like creatures known as silverfish. Two theories have been put forward by scientists regarding fossil research on the evolution of insects’ wings.
Main text
Scientists believe that the wings could have developed from the gill-like projections possessed by the insects which lived in water according to episcopal theory (Engel, 2005). The theory states that the gills evolved over a long period after which they extended 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 paranodal theory states that the wings just emerged from protrusions at the back of an insects’ body living on land (Hall, 1998). It is quite remote to imagine that wings evolved from insects living in water since the modern insects live in terrestrial habitats. Insects are thought to be the first creatures to live on land. Winged insects emerged about three hundred million years ago according to the fossil record; however, no clue exists about the fossil background of the original insect ancestors.
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 existing ones. This fossil period that is characterized by the emergence of the dragonfly matches 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.
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. The wings were specialized to translate the force generated from the axillary 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, Reynolds & Woiwood, 2001). In addition, there are also indirect muscles.
Flying demands a lot of metabolic energy which translates to mean that insects have evolved a complex physiological system to respond to their flight energy requirements.
Wings are also 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 records possessed numerous pairs of them all over the 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.
The present-day insects should 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 the 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.
Evolutionists equally believe that wings developed from gills. This theory states that wings are independent structures that developed through a unique process away from any of the existing appendages (Hall, 1998). According to Carpenter, wings developed from gills through the evolution of gene expression with similarities between winged and wingless insects. However, it is difficult to obtain a direct link between the insect ancestors and the modern winged insects concerning their evolution of wings. Wings should therefore have evolved at a specific period in evolutionary history despite their structural complexities.
This 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 a flight are virtually the same in many insect species. The pterygotes’ 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 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 that 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.
The wings were folded to shield the insects especially for those species which did not evolve muscles relevant for flight. The wings are flapped by both the direct and indirect muscles during flying. Direct muscles are attached to the wings at the thoracic segment. For the insect to remain afloat, 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.
Indirect musculature is independent of 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 afloat (Hall, 1998).
For the insect to fly, 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).
These structural and physiological modifications which have occurred during the evolution of wings have resulted in a complex flight mechanism peculiar to 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. The 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.
Most insects fly during the day since they rely on the sun on direction, effective maneuvers, and to create a proper navigation pattern (Thomas, Reynolds & Woiwood, 2001). Other insects such as the moth use the moon to fly at night in addition to sunlight by maintaining the angle at which the radiations meet the eye. This enables them to fly in a typical straight line. This means that the use of 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 concerning the brightest light around.
The wings were also an adaptation to the terrestrial habitats where insects lived with plenty of vegetation. The insects, therefore, needed the wings to negotiate through the trees and grass as well as respond to unfavorable environmental conditions. The insects can mate with suitable mates while on air due to the proximity of their reproduction organs (Brodsky, 2009).
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 is taken by the neuron to regenerate the action potential, therefore, determining 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 the 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 subsequent 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 a minimal number of flight muscles on a lean insect body for both sustained flight schedules and energy conservation. The insects have to possess a streamlined 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, Reynolds & Woiwood, 2001). Additional fuel reserves are obtained from the fat storage tissues.
Normally, immense fuel reserves in the hemolymph and glucose present in the flight muscles are oxidized to propel the insect during short-lived flights. When the flight takes a longer time, fats are utilized from their energy reserves to supplement the extra energy requirements under the control of the endocrine system (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.
As such, the evolution of wings has enabled insects apart from other invertebrates to develop an elaborate flight regime for their 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.
Winged insects, therefore, emerge suddenly according to fossil evidence with already established physiological structures. A close examination of the insect wings reveals that muscles are not directly attached to the wing surface but rather coordinate the flight by exerting force on the thorax. 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.
However, the wings themselves are not complex but simple structures that 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 sleek forces required to lift the wing for flight. The wings, therefore, act as perfect aerofoils 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).
As much as the development of wings appears to be a remote and simple process, their level of structural modifications associated with its physiological systems throughout evolutionary history has resulted in a well-designed flight regime. The evolution of wings from body structures that were previously existing is therefore viable given 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. This then allows the insect to cut across the frictional drag in the air sufficiently to take off the ground while still in support of its weight.
The evolution of wings was therefore a necessary fete for the modern insect 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.
Conclusion
Overall, the insect should have evolved the wings during its existence on the planet 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.
Reference list
Brodsky K., A. (2009).The evolution of insect flight, New York: Oxford University Press.
Engel S., M. (2005). Evolution of the insects,Cambridge: Cambridge University Press.
Hall K., B. (1998). Evolutionary developmental biology, Boston: Birkhäuser.
Hutchins E., R. (2006). Insects, Arizona: Prentice-Hall.
Thomas D., C., Reynolds D., R., and Woiwood I. (2001).Insect movement: mechanisms and consequences: proceedings of the Royal Entomological Society’s 20th Symposium, New York: CABI.