10 What is the most important reason for the success of insects on land, and why is this success not mirrored in the ocean? Emily Duke

What is the most important reason for the success of insects on land, and why is this success not mirrored in the ocean?

Emily Duke Upper Sixth

Diptera fly

Insects are a successful class of animals, representing more than half of the wold’s biodiversity (University of Bergen, 2015). They can be found in large abundance in a diverse range of ecological habitats and are found in almost every terrestrial and freshwater environment on Earth. For example, the ice bug can survive at -20oC, while the locust can survive at 80oC (AgriYouthNepal, 2017). Although many insect species spend part of their life on the surface and margins of the ocean, and more so within fresh-water lakes, there is not a single insect species which spends its entire lifetime in the open ocean (Maddrell, 1998). The following essay details the characteristics that provide explanations to the success of insects, and some hypotheses for the failure of insect colonisation within the open ocean. The first, perhaps most obvious reason for success is their fast rate of reproduction. An example of such high fecundity (producing a large number of eggs) is the queen honeybee that can produce 4000 eggs each day (AgriYouthNepal, 2017). Antoni van Leeuwenkoek researched the reproduction potential of insects by rearing Calliphora erythrocephala. The data calculated that a pair of flies could produce 3,869,835,264 individuals by the sixth generation. However, this type of calculation highlights the colossal potential of insect colonisation; it ignores the limiting factors that constrain the size of a natural population (Meyer, 2007).

A method which allows female aphids to reproduce without a male mate is called parthenogenesis. This allows for several generations of aphids to be efficiently produced within one summer alone, without the female being required to find a mate. Some insects, such as bees, can reproduce both sexually and asexually. For example, worker bees are produced from fertilised eggs, whereas drone bees are produced from unfertilised [T]he ice bug can survive at eggs (Tipton, 1976). Most insects are oviparous, which means they lay -20oC, while eggs; however, others are the locust can survive at 80oC. viviparous and give birth to active young. An example of parthenogenic metagonadic viviparity (when the offspring develops in the hemocoel, where nutriment is derived from maternal tissues, causing the larva to consume the mother’s internal anatomy) is in Diptera, a type of fly (Hagan, 1948). The benefit of having multiple methods of reproduction is that insects are not at a disadvantage should they experience a reproductive set-back, such as finding themselves without a mate or having their young predated on. The combination of high fecundity and fertility means that female insects can produce large numbers of offspring within a typically short lifetime (2-4 weeks) which provides the genetic resources to adapt quickly to the changing environment. Possibly the most interesting example of insect adaptation in recent history would be resistance to the insecticide in the house fly, Musca domestica, in the United States after World War II. Public 9 health officials made an effort to eradicate the housefly using an insecticide called DDT. However, the few resistant flies survived, containing an enzyme capable of detoxifying DDT and passed the resistance onto their offspring – a modern example of natural selection. However, the house fly is not alone in its resistance to pesticides; significant levels of pesticide resistance has been reported in over 500 species (Meyer, 2007). Other adaptations include physiological adaptations in defence, such as releasing poisonous and unpleasant odours (AgriYouthNepal, 2017), or having spines, such as on the katydid grasshopper (Tipton, 1976), or behavioural defence mechanisms, such as the Colorado potato beetle which ‘drops dead’ at any sign of threat (AgriYouthNepal, 2017).

Another important explanation for insect success is not only that insects fly, but that they are the only invertebrates that can do so. According to fossil records, insects acquired this ability 100 million years before the first flying reptiles. This means that they would have had a successful method of escape from their prehistoric predators (Meyer, 2007). Flight allows insects not only to escape predators but also allows them to expand to new habitats to exploit new resources (AgriYouthNepal, 2017). For example, when the insect reaches a new habitat, they are able to quickly adapt to suit the new ecological niche through generations of natural selection (Jacobs, 2013).

Flight is an efficient use of energy, so insects are capable of flying for long periods. The metabolic cost of flight (the number of calories required per unit of lift) is similar to that of birds and bats, although an insect’s flight musculature produces more than two times the power per unit of muscle mass. This incredible efficiency is due to the high elasticity of the thorax and allows insects to travel by flight for long periods of time, over a large distance. For example, the locust Schistocerca gregaria can remain airborne for up to 9 hours without rest (Meyer, 2007).

There is a vast variety of theories as depth, the total pressure is approximately 10 to why insects haven’t colonised the atmospheres which would reduce the volume oceans, ranging from competition of gaseous air, resulting in system failure. from crustaceans filling niches Another strategy used by crustaceans is to to there being no become transparent. This flowering plants or trees in the open ocean. However, the [T]he queen honeybee... is unobtainable for insects because their “glistening” air-filled tracheal system primary explanation as to why insects have failed to colonise the can produce 4000 eggs would ruin their transparency (Maddrell, 1998). open ocean appears each day. However, in using this to be that insects fail hypothesis, it overlooks why to escape from their insects can survive in saltwater ocean predators: fish. Other animals, environments in-land. An analysis was such as crustaceans, have developed carried out, collecting insects and comparing strategies to avoid predation, such as seawater samples to waters from the Salt descending to the ocean bed. Insects, Range in Punjab, Pakistan, where numerous however, are naturally buoyant insect samples were found despite the salinity due to their air-filled respiratory being 2-5 times higher than seawater. The key system so they cannot do this. The difference was the comparatively low calcium respiratory system relies on the content of the seawater. The calcium content presence of gaseous air and at 100m of the waters from the Salt Range varied from 1.77% to 31.67%, except San Sakesar Lake, where the calcium content was 0.11% and no insects were found. It is thought that the high calcium content in the highly saline waters of the Salt Range that allow insects to keep their body fluids at equilibrium. This suggests that low calcium levels have played a role in limiting insect colonisation of the open ocean (Pruthi, 1932).

To conclude, insects are highly specialised animals that are capable of adapting rapidly to environmental changes. The most important point leading to success is that their short life cycle, along with high fecundity and fertility, allows for a species to adapt through rapid generations of natural selection, developing into highly specialised and highly adaptable species. However, their specialisation is also their downfall in the open ocean, in that their adult form is so specialised to suit terrestrial life that adaptations, such as their air-filled respiratory system, are unsuitable for marine life. To colonise the open ocean, the first step would be for insects to develop suitable adaptations to sustain life in low calcium concentration waters. The next hurdle would be avoiding marine predators, such as fish and crustaceans. However, this is a lesser concern, given that insect species have been able to colonise both fresh and saltwater bodies of water in-land. In all, despite their incredible success on land, insects are simply too specialised to be suited to an entire lifetime in the open ocean.

De Haviland Seaplane