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Insects Evolution
In the process of insects evolution, like that of all other animals, some periods was far more active than that of the others. There have been geological periods of explosive evolution, during which many new forms have appeared. There could be due to the changes of climatic or evolutionary advances of other animals and plants. New methods of feeding and living led to diversity of insect mouthparts and limbs, the origin of wings, metamorphosis, and other changes.
Importance note: The above chart is constructed base on insects that
we find here. The relations between each group of insects are base on their
major features only. Many smaller groups of insects are omitted. There are the
very high degree of speculation on constructing the chart. Be cautions when
using those information. 
- A bug nymph just after molting
There is an important factor that govern the evolution of insects - their molting, that lead to their capability of transformation of body, i.e., metamorphosis, and adapt to change. By molting, insects in different stage can have different shape to make them suitable for their different living environment. Where there is the new habitat, insects are usually the first group of animals to occupy the habitat. Insects are the first animal to fly.
The story of Insects Evolution is the story of how insects explore new habitat.
Move to the land, move to the sky, parasite into the hosts, live with the dinosaurs,
live with the birds, and live with the humans.
First Insect in Devonian
- Three Body Parts
- The first important step in the evolution of insects was the development of definite body plan, i.e., the three body parts - head, thorax and abdomen. This happened approximately 300-350 million years ago the period called Devonian. At that time most arthropods, which the insects evolved from, had only two major body divisions, the abdomen and the cephalothorax. This evolutionary step led to the first insect that looked something like silverfish nowadays.
Compound Eyes
Most insects fly.
One of the pre-required conditions for flying is that there must be a good navigation system, such as a pair of eyes with good eye sight. One of the very earliest identifying features of insects is the compound eye, found only among the insects and some other arthropods.
- Living in water
It is reasonable to believe that insect ancestor was living in fresh water in the early time. Their life style was similar to the dragonfly larvae except their adult stage was not winged and they find their partners and mate in the water. - Molting
- Insects are exoskeleton, the hard skin give their body support but also hinder the
grow. Between each stage of growing, insects moult, leave their old smaller
size skin, expanse and re-grow a bigger size skin. The feature of molting
and re-grow evolved the important feature of insects - metamorphosis.
First Landing
One of the branches of insects landed before they gained their wings. They become the wingless species of insects nowadays, such as the silverfish.
Winged
- The early primitive winged insects not too much different from dragonflies today
Approximately 200-350 million years ago during lower Carboniferous, insects developed wings and the ability to fly. These wings were primitive and could not be folded away. Insects were then dominant in Carboniferous and Permian. From the basic structure of insects wings, it is believed that wings have been evolved only once among the insects.
Under some conditions, insects evolved their wings. They were the first group of animal had this flying power. It is sure that this become the major factor that the insects success on earth. How they gain the wings function is hard to guess. They winged before or after their adult landed is also hard to guess. From the lift cycle of dragonfly, It suggests that the insects winged when their larvae is still living in the water.
There is no fossil evidence to show how their wings evolved. One hypothesis is that wings arose as fixed planes extending sideways from the thorax and that these planes were used for gliding. Later muscles developed, first to control inclination and then to move the wings in flapping flight. Another hypothesis is that wings may have originated from large thoracic tracheal gills.
By guessing, the whole insect wings story could be like this. The insects at that time were living in water. When become an adult, they climb up by the plants, move up about water surface to mate. There are two reasons for them to do this. They use plants about water surface as meeting place, and over there they can avoid the predator in the water during mating. They may climb, with a bit of jumping, between plants to find their mates. It is reasonable to believe that the adult life is very short, die after reproduction, just like the mayfly we have today.
For some insects, beside jumping and climbing, they may have a device that helps a bit for gliding when they jump. The device could be modified from the gill that they used to breath in the water. Under natural selection, the device become larger and more like wings. After a long time they learn to control the device and it become their wings for flying. The adults can fly to new area and explore new habitat. The adults than live in the total different environment as their larvae and they become look different, and with different body shape to cope with the different living condition. Then this group of insects' life cycle become similar to the dragonfly nowadays. They become the most success animal on earth.
Folding wings
Approximately 315 million years ago, soon after the development of wings and flight ability, insects developed wing-flexing mechanisms. This is the ability to fold up the wings over the thorax and abdomen and get them out of the way when not in use. This allowed the insects to run better and have better access to restricted habitats.
Most insects today fold their wings on their back likes this Bush Cockroach
Neoptera ("new wing") - The classification of insects in this way may not reflect 100% the true, but no doubt, this affect the way of thinking in insects evolution a lot. Some scientists put the evolution of folding wings is another important stage in the insects evolution history. I think folding wings is just one of the many evolution steps in insects history, just like the evolution of sucking mouth and jumping legs etc. The dragonflies cannot fold their wings just because it is not necessary for them. The damselflies have the wings folding capability and do not seems have much advantages over dragonflies. Of course if the cockroach cannot fold their wings, they are not going to survival, but the folding function seems not too difficult to have.
Larvae landed
The latest important step in the evolution of insects was the development of complete metamorphosis. The change in body form, from larva to pupa to adult, allowed individuals to take advantage of more than one habitat. Individual insects were now free to exploit new resources and reduce competition between adults and
young. This occurred about 290 million years ago in Upper Carboniferous
The characters of larva and adult became genetically independent; in response to natural selection, therefore, each was able to evolve independently of the other. Mouthparts, limbs, and other morphological features were modified in different directions and in higher groups. Where these differences were extreme, an intermediate pupal stage evolved to bridge the morphological gap between larva and adult. It seems quite probable that the development of metamorphosis occurred more than once during the evolution of insects.
Due to some reasons, such as the water dry out, avoid of predators etc., some larvae of the winged insects move to the land as well. After this, evolution of insects go at least three different ways.
- Larvae and Adult look similar
- After the larvae landed, the living environment of the larvae and the adults became the same. So the larvae and the adults are evolved to the similar shape. This reverse the evolution process mentioned above. The living environment for the adults are the same so both the adults and larvae evolved to similar shape. This is an example of the habitat shape the live from. For the insects with graduate or in-complete metamorphosis, their shape of different stage is similar. This is not because they are the same species and have to have graduate change. This is because they all live in the same environment, so they evolved the similar shape. This group of insects become the Orthopteroids insects with chewing mouths we have nowadays. They include grasshoppers, stick insects, mantids, cockroaches and termites etc. The other group of insects with sucking mouth, the true bugs, are also in the same case.
- Larvae and Adult look different
- After the larvae landed, if the larvae living condition is different from the adult, they evolved the different shape. The cicadas are a good example. Their larvae landed, but they live deep under ground. As the adult they have to come out from the soil to found their mate. They retain their living style as their ancestors, the come up from the soil, not water, and climb to the plants for the final moulting and become an adult.
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- Pupa
- For the insects larvae and adult live in different environment, and the shape of different is too large for graduate changes, some insects evolved the pupa stage to overcome the problem. Between the last larvae stage and the adult stage, they have a stage that the larvae become motionless for a relatively long time. In this stage, there is no motion, no feeding, totally for body transformation. This group of insects, have two completely different body shape, make them gain a lot of advantage including adults and larvae not completing for the same food and living resource, to overcome season changes, and to avoid predator in different stage. This group of insects become another blooming of the insects Class.
Insects evolved the sucking mouth and this leaded to some new orders of insects. Clock here for more discussion on Sucking Mouths.
- Social Insects
Wasps, Bees and Ants are considered the highest form in insects evolution. They evolved the highly structured society. They have their powerful strings. They do not have many predators. They seem dominate in the insect class. They are quite different from others insects. It is believe that the ancestor of hymenopterans, i.e., wasp, ants etc., look like the Sawflies today. We can find the special features in wasps and ants, the early images are still retained in Sawflies. We may deduct the evolution paths of wasps and ants from sawflies. Click here for more information on Evolution of Social Insects.
Other Elements that affected Insects Evolution
- Flowers
- At the end of the Mesozoic Era the first flowering plants appeared. At first, flowers crossed pollen by the help of the wind. Than insects involved in the evolution of flower and insect evolution has progressed together with the flowering plants. As insects began to feed on flowers, flowering plants came to rely more and more on insects for transferring their pollen. Flowers evolved scents and colours to attract insects that could effectively crossing pollen. Insects evolved appropriate mouthpart modifications for extracting nectar from flowers, as well as body hair to carry the pollen.
- Pterosaurs, Bats and Birds
- During the Mesozoic mammals and birds first appeared. At the beginning of Tertiary Period, they had become predominant among the earth's large animals. The warm fermenting excrement and the decaying dead bodies of mammals furnished excellent nutrient media for many insect larvae, notably among the flies and beetles. The adults in both groups found their nourishment in flowers. Some true bugs pierce the skin of birds and mammals and feed on their blood. The sucking lice and the fleas have become so specialized for this type of parasitic existence.
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Spiders
Spiders are the major predators of insects. Let's discuss on how spiders affect the evolution of insects.
