Insects are the most successful animal group. They are extremely diverse and abundant. They are present in the sea, in freshwater, on land and in the air. They occupy every ecological niche. Arthropods are not only extremely diverse but are also numerous as individuals. There are approximately 200 million insects for every human being on the Earth. (1) They have a huge impact on humans and our activities. They are pests of both food and animals, and are responsible for disease transmission. They also create important food sources and are beneficial through processes such as pollination and scavenging.
The high degree of insect diversity is due to a combination of high rates of speciation and low rates of extinction. (2) The orders with the highest species richness include Coleoptera (beetles), Hymenoptera (ants, wasps and bees), Lepidoptera (butterflies and moths), Diptera (true flies) and Hemiptera (bugs). (2) Many factors contribute to the success of the insecta such as their small size, their protective cuticle, an efficient nervous system (the blood-brain barrier and sensory neuromotor refinement), the evolution of flight and a high reproductive rate.
The size of an organism determines how the environment will affect it. Most insecta are small; the average is around 3mm in length. The physical environment tends to favour these sized animals. For example; “You can drop a mouse down a thousand-foot mine shaft and, on arriving at the bottom, it gets a slight shock and walks away. A rat is killed, a man is broken and a horse splashes. ” (J. B. S. Haldane) This is due to the ratio of surface area to volume; the bigger they are, the harder they fall.
Also, small animals are relatively more powerful than large ones. The cross-sectional area of insect muscle is large relative to their mass. For example; an ant can carry a load many times heavier than its own body weight. Their size has also allowed insects to evolve to occupy many more ecological niches than larger organisms. There are two conflicting theories as to why insects are so small. The first is that their small size is a limitation imposed by their method of gaseous exchange via tracheae.
(1) This theory suggests that the tracheal system is unable to diffuse gas from active muscles, through the spiracles to the external environment in a large insect, and that further enhancements to the system would compromise water balance as most large insects are narrow and there is limited distance for gaseous exchange. The second theory suggests that the tracheal system does not limit size and that it is an efficient system. (1) If needed, suitable modifications, such as air sacs, would evolve.
The larger size, however, could present problems at moulting as the insects weight would do damage to the soft cuticle, and make them more vulnerable to predators, but this could be overcome by moulting in water or maintaining body shape with hydrostatic pressure. Small size is a result of predation pressure. A closer examination of the past may reveal why insects today are small in size. Early insects were much bigger than they are today. (1) For example, Meganeura (a Carboniferous dragonfly) had a wing span of 75 cm.
Large size may have evolved as a result of predatory insects, which grew bigger until they were no longer functionally viable, or more likely because of the existence of vertebrate predators, e. g. : reptiles. Insects then became smaller to avoid predation. The smallest insects are parasitic wasps which are 0. 2mm long. (2) The limitation of the smaller size range is restricted by a number of physical factors. Small organisms obtain oxygen by diffusion which is not possible across the insect cuticle.
The tracheal system is effective; pairs of spiracles open to branching tracheae and smaller tracheoles. The smallest tracheoles, regardless of the size of the insect, have a diameter of 0. 2 micrometers, to allow for efficient diffusion of oxygen. In animals, even though cell size tends to be much the same regardless of animal size, there must be a minimum number of cells to produce a functioning unit. To relieve this problem, insects have very economical wiring. For example, the nerve axons are branched giving insects polyneural, multiterminal innervation through ‘twiglet’ endings.
(4) Insects are ecothermic, as the large surface to volume area makes it difficult to keep cool and expensive to keep warm. (4) They can regulate their temperature to some degree. For example, moths flutter their wings to warm the muscles. Desert beetles run fast on long legs to minimise contact with the hot sand. Microclimates are important for arthropods as large animals cannot take advantage of them. These include cooler, moister habitats near plants or under stones. The cuticle is another key factor that plays a role in the success of insects.
Arthropoda possess a chitinous cuticle. (2) This rigid exoskeleton provides much protection. It is made up of a complex glycoprotein of bound chitin and protein. Chitin is an unbranched polymer of high molecular weight, containing an amino-sugar polysaccharide composed of ? (1-4) linked units of N-acetyl-D-glucosamine. (3) It provides both mechanical and biological protection. It allows the attachment of muscles by apodemes, which are ingrowths serving as attachment sites.
(3) The cuticle facilitates movement at the joints by the presence of the arthrodial membrane, which consists of thin, flexible and untanned cuticle. (2) The cuticle has an ability to form hard claws and biting mouthparts for defence and feeding. It is also capable of providing information from the external environment through chemoreceptors and mechanoreceptors. Lipids (waxes) of the epicuticle prevent water loss. This is vital to terrestrial arthropods in soil dwelling arthropods for example.
Surface lipids may deter predators and parasites and control temperature, and may also serve signalling purposes (pheromones). Another advantage of the cuticle is the presence of resilin; a uniquely elastic protein. (4) It comprises of polypeptide coils linked by amino acids. Resilin can completely recover its shape after deformation. This is especially important for flying insects, for example, in the claws of a scorpion, where there is no extensor muscle. The insect nervous system and blood-brain barrier also contributes to their success.
(2) The nervous system of any animals integrates information about the external environment and internal physiological conditions. To work properly, this requires a chemically stable environment with little fluctuation in osmotic pressure, ionic concentration, etc. (1) A feature of the CNS of insects and other arthropods is the presence of a clearly-defined blood-brain barrier; the nervous system requires a private pool, not communal bathing. Arthropods such as Chelicerae have a full blood-brain barrier, where as Crustaceans have a partial one and myriapods, very partial.
Neural lamellae are in place which sheathes and supports the brain, ganglia, ventral nerve chords and major peripheral nerves. The perineum acts as the blood-brain barrier, located inside the neural lamellae. It is made up of glial cells. Ions in the haemolymph are prevented from diffusing in and out of the CNS by tight junctions between glial cells. The fluid immediately around the CNS is ionically isolated. (1) The nervous system can function efficiently at all times regardless of fluctuations in the ionic concentration of the haemolymph.
(2) Another factor which contributes to the success of insects is Flight. Flight allows insects to disperse. It enables them to colonize new habitats. It also gives insects the ability to escape from enemies and environmental hazards. Insects evolved wings about 300 million years ago in the Carboniferous era. Only 0. 06% of extant insect species are primitively wingless; Thysanura and Archaeognatha. (1) There is lots of debate about the origin and the evolution of wings, as winged insects appear suddenly in the fossil record with no good intermediates.
Another contribution to the success of insects is their reproductive methods. Many insects have high reproduction rates and short generation times. This means they can evolve faster and adjust to environmental changes more rapidly than slower breeding animals (Lecture notes). For example, bruchid beetles can produce about 80 offspring every 21 days. After 432 days, the beetles would equal 1. 4 x 1029 individuals. This would occupy a volume equal to that of the Earth. Insects may also become isolated in small populations.
This can lead to genetic isolation and potentially the formation of a new species. High reproductive rates also lead to adaptations, for example, insecticide-resistant insects as a result of the use of insecticides. This ability to deal with change is due to high genetic heterogeneity within insect species. From the main points outlined above, it is clear why insects are such a successful group. Due to their size, cuticle, CNS and blood-brain barrier and their methods of flight and reproduction, insects are the most successful animal group.
Cite this Insects – Animal Group
Insects – Animal Group. (2016, Jul 06). Retrieved from https://graduateway.com/insects-animal-group/