Dominance and factors leading to the predominance of insect

DOMINANCE OF INSECTS.

Today, the insects constitute the largest class not only of the animal kingdom but also of the whole living world. The numbers of known species of insect is estimated to vary from 700,000 to 1,500,000 which constitute 70-90% of the known species of the animal kingdom. Over a period of several hundred million years, insects as a class, have acquires many distinctive structural, developmental, physiological and behavioral perfections which have enabled them to occupy this dominant position.

FACTOR LEADING TO THE PREDOMINANCE OF INSECT:-

a.    Exoskeleton

b.    Small size

c.     Quick speciation

d.    Hexapod locomotion

e.    Differentiation of body

f.      Functional wings

g.    Tracheal respiration

h.    Development characteristics

i.       Chemical communication

j.       Physiological adaptation

Characteristics of above mentions categories:-

a.    Exoskeleton:- The hard, chitinous, exoskeleton provides area for muscles attachment, protects muscles and internal organ from mechanical injury and prevent desiccation of the body. The exoskeleton has also turned the appendages into good tools for digging, preying, oviposition, etc. lastly, the exoskeleton maintains the shape of the body much more efficiently than does the endoskeleton.

b.    Small size:- During the evolutionary course the insects have developed a large number of smaller individual. This step immensely increases the chances of survival of the species. Smaller individuals are able to survive on small quantity of food and can take refuge in small niches against the exigencies of climates as well as biotic stress factor. Larger numbers of individuals increase the chances of variability and mutation. The muscular strength of insect is phenomenal due to their smaller size. A fiea with a body length of about 12 mm can jump a horizontal length of more than 30 cm and a height up to 20 cm.

c.     Quick speciation:- The possession of chitinous exoskeleton and the aerial mode of the  life probably been the largest contributors to so much speciation in insects. Due to mechanical rigidity of the exoskeleton, even minutes variation in the external genitalia lead to reproductive isolation and consequent fixation of the species. The acquisition of wings has helped insects to lead life under much diverse ecological habitats which has provided the trigger for variations. The vast increase in the number of species has immensely increased the economic potentialities of the insect as a class.

d.    Hexapod locomotion:- insects share the jointed nature of the legs with other arthropods but the numbers of such legs has reached s ideal stage of evolution in insects. During locomotion, the insect body rests on a tripod, while the other three legs move forward. The tripod mode provides stability as well as easy maneuverability to insect locomotion.

e.    Differentiation of body:- Differentiation of the body into head, rigid thorax and legless abdomen is a particularly favorable arrangement for terrestrial arthropods. The mobility of the head (in contrasts to arachnida) may have facilitated the development of various types of mouthparts (and hence varied feeding habits) and also better orientation during flight and leading. The consolidation of thorax offers rigid attachment for the legs and yet flexible base for the insertion of wings. Functions of digestion, excretion and reproduction have been regulated to usually distensible and flexible abdomen.

f.      Functional wings:- The power of flight greatly increased the chances of survival and dispersal of insects. Functional wings increased the feeding and breeding range, and provided a new means of eluding natural enemies. sturdy wings of locusts and some butterflies enable them undertake intercontinental migration. The structural perfection of these wings can be gauged from the fact that a hive bee can fly at 9 km/hr. a hoverfly at 12 km, a hawk moth 17 km and a butterfly up to 90 km/hr.

g.    Tracheal respiration:- Tracheal system supplies oxygen directly to each and every cell of the body. It has been made possible by invagination of a relatively enormous surface area permeable to both oxygen and water. In this system, water loss, so critical to small terrestrial animals, can be greatly reduced because contact with the external environment is only via the spiracles.

h.    Developmental characteristics:- the full life history of the homometabolous insects consists of four well-defined stages: the egg, the larva, the pupae and the adult. Two of these stages, the mainly reproductive adult stage, are so different from each other not only in structural details but also in requirements of the food and habitat that competition between the parent and the offspring for food or shelter is eliminated. Further by the interpolation between the larval and adult stages of a quiescent stage of pupa with great resistance and practically no food requirement, the two active stages are the best fitted into different season optimum for them. Complete metamorphosis has enabled insect to combine the advantages of two different ways of life and at the same time avoid many of the disadvantages of both. The voracious feeding and digesting ability of larvae have enabled insects to be prolific reproducers.

i.       Chemical communication:- Small size of the insects limits their ability to locate food , sexual partner and avoid danger through visual and tactile stimuli. But a dazzling variety of insects behavior is elicited in response to minute amounts of chemicals emanating from their conspecifics, other animal, plant and even dead organic matter. Pheromones are utilized by insects for sexual attraction, alarm, aggregation, caste determination and a variety of other functions. On the other hand , alleochemicals provide information regarding host plants ,prey ,natural enemies suitability of a habitat for food and oviposition etc. the sensitivity of the insects to communication chemicals can be judged from  the fact that their antennal receptors have been shown to respond to even a single molecule of the sex pheromone.

j.       Physiological adaptation:- Terrestrial insects face an acute shortage of water , and excretion in the form of ammonia and urea require enormous quantity of water for elimination from the body. These insect Have developed the ability to excrete from the body in the form of uric acid to conserve water. Diapause is another enormously valuable adaptation to fluctuating environmental conditions. Physiological changes occurring prior to onset of dipause are useful for tiding over unfavorable conditions. In spite of the presence o a large number of repellents, aniferrdants, toxicants, development disruptors and, metabolic inhibitors in plants, insect are able to survive and multiply on these plants. This ids dur to different escape and detoxification mechanisms developed by insects. Surprisingly, even plants (chrysanthemum, derris, nicotiana, azadirachat, melia, etc) containing highly toxic chemicals are utilize as host plants by various insect species. Insects even utilize some of the toxic secondary as cures for identification of their host plants. The versatile behavioral and metabolic pathway of insects have helped them in survival, multiplication and speciation.