There are a bewildering number of insect types in the world but they all basically go through one of two types of life cycles. The more primitive types are called the exopterygotes, a word that simply means their wings (pterygota) develop on the outside of the their bodies. In the more evolved groups, the endopterygotes, the wings develop on the inside.
The exopterygotes are sometimes referred to as the insects that have an incomplete life cycle: hemimetabolous is the scientific word. From egg to adult, these insects look pretty much the same, do not go through a true larval stage and do not seal themselves in a pupa to make the final changes to adult form.
Exopterygote orders include the Orthoptera (grasshoppers and locusts), the Hemiptera (true bugs) and many other smaller groups such as the earwigs (Dermaptera), praying mantids ( ), stick insects ( ), lice ( ) and others. The basic life cycle of these groups is simple and similar, even though the body forms and life styles are quite different. Take a praying mantid for instance. The young mantid hatches from an egg. Its body is tiny but exactly the same as the adult form except there are no wings. Each time the mantid moults, it gets a little larger. As the moults progress, little wings start to form. With each moult, the wings get larger until when it reaches its adult size, the wings are functional and can be used for flight.
Sucking bugs, stick insects, grasshoppers and earwigs all follow the same pattern. The adults mate and the females lay eggs. After a time, the eggs hatch into small but wingless replicas of their parents. As with all Arthropods, their skeletons are on the outside so they have to moult to grow. However during these growth phases, no major body changes take plsce. There is no caterpillar stage in exopterygotes and as the name exo implies, the growth of the wings can be seen as it take place on the outside of the body. In some really simple forms where the life style is the same throughout the insect’s life, the young look just like the adults, just smaller and with no wings turning into short wings.
In some groups though, the young insects, called nymphs, do live different lives from the adults so some changes do occur. These young are called nymphs. Dragonflies, stoneflies and mayflies belong to three different orders but in each, the nymph stages live in water and when they are big enough and the wings have developed over a number of moults, the young insect emerges into the air, where their wings spread and harden and allow them to take up an adult existence, where food takes secondary importance and reproduction and dispersal become of primary importance.
There are fewer endopterygote orders of insects but they are the most numerous and diverse and evolved groups. They include what entomologists call the Big Four: Flies (Diptera), Butterflies and Moths (Lepidoptera), Bees, Ants and Wasps (Hymenoptera) and the Beetles (Coleoptera). There are some smaller groups too such as the Siphonaptera (fleas) and Neuroptera (lacewings). These insects all go through a ‘complete’ or holometabolous life cycle and the larval forms look completely different from the adult forms, with a pupal stage in between.
Take the lovely Lacewings for an example, Order Neuroptera. These insects look superficially like small dragonflies but they can fold their lacey wings back against their bodies and they have a whole life cycle. Neuropteran eggs do not hatch into nymphs, they hatch into Ant Lions, ferocious little beasts with large jaws, that bury their larval bodies in sand pits with only their jaws protruding. When ants slip down the walls of the pit, they meet their end in the jaws of the Ant Lion. When it has eaten enough to grow strong and healthy and has enough stored energy to go through the change, an ant lion makes a pupa and inside that case, turns its self inside out and upside down, to make a totally different beast: the adult Lacewing.
This same process happens in ants, bees and wasps, butterflies and moths, beetles and flies. It even happens in fleas. In flies the stage is legless, fat and we call them maggots. Their chief role of a maggot is to eat and get fat enough to change into the adult form so it can reproduce. In social ants, bees and wasps, the larvae are nurtured and fed lovingly by adults. In beetles, the grubs have legs (usually six) and crawl around gobbling up leaves or tree bark or other forms of vegetation, and in butterflies and moths, we have many-legged caterpillars doing the same thing.
Why have the endopterygotes gone to all this trouble? It would seem that splitting the life cycle into two distinct phases, a feeding phase with one body form and a dispersing, reproducing phase with a totally different form, has given these insects a distinct advantage in the struggle for survival. It is enough of an advantage to overcome the dangers associated with the pupation phase, such as becoming trapped in the pupal case, not having enough energy to make the change or being eaten when one is helpless. The success of the Big Four groups indicates that the advantages far outweigh the disadvantages.
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