A look At metamorphosis

All insects go pass through some sort of change in the course of their life cycle. In a few, a very few, cases the change is very slight, the newly born insect is practically a miniature replica of its parent, and by increase in size alone it attains the adult stage.

In many cases, and they are more familiar, the insect only completes development by a series of stages, totally different from one another. There are also many intermediate forms between the two, and a few exceptional phenomena which do not rightly fit into any class. These changes whatever, the type, are known as metamorphosis, and a few typical examples will , I hope, make the subject a little easier to understand . 

Take the case of the cabbage white butterfly.

Take the case of the cabbage white butterfly for here we have a subject that most people can observe without difficulty. here we have an example of a complete life cycle from perfect insect to perfect insect. 

After pairing, the female will deposit her eggs upon some plant { particularly Nasturtiums or as its name suggests on cabbages and other members of that family}, which will form the food for its larvae, the next stage in its life cycle. The egg is the first stage of its metamorphosis.

Eggs of the cabbage white butterfly on the unserside of foliage



In due course the eggs will hatch and the caterpillars will emerge. caterpillars are technically known as larvae but here we shall refer to them as caterpillars. They form the second stage of the life cycle. By degrees they grow to such an extent that their skins are in danger of becoming to small. They cease feeding for a short period, their outer skins are shed, and below they are clad with a new and larger skin.

Once more feeding commences and continues till the new skin is to small and then the operation is repeated. This skin casting is termed as moulting and the number of moults are constant in each species, but variable in different species. Some larvae moult but twice while in others as many as twenty times.

To digress for a moment, It is apt to dispel a myth at this point that adult insects, that is the adult in the last stage of their life cycle continue to grow-they do not. We often hear the remark " there is a young house fly!" as a small fly is indicated. As a matter of fact this small house fly is altogether a different species from the common house fly and is usually the lesser housefly which has been the subject of the statement.

Someone may remark that in a collection of insects it is possible to find a number of individuals, which expert entomologists describe as being all of the same species and sex, of varying size. This is true but easily accounted for. The amount of food and the quality of the food in the larval stage has a great bearing upon the ulitmate size of the perfect insect, but this will be the subject of another page

After this digression we return to the caterpillar, which we will presume is fully fed, that is to say it has completed its growth  and passed through its last moult. The larvae become restless, seeks a place where it may undergo the next transformation and makes itself secure by means of silken threads, and passes into the resting pupal stage. The pupa is usually inert. It possesses only feeble powers of movement, and in our example never moves from its original position. At any rate this-the third stage, is outwardly a dormant stage.

However, within the pupal case wonderful changes are occurring, which are complex and far to intricate to go into within the confines of this page. When the changes are complete the pupal skin splits and the perfect insect, known as the imago, emerges, thus having completed its life cycle and ready to breed so that the life cycle can begin again.

So the life cycle described above has four stages, 1-the egg,2-the active larva,3-the dormant pupal stage and 4- the active non-growing adult. Such a metamorphosis is said to be complete and is the norm among Hymenoptera, Lepidoptera, Diptera, Coleoptera and many others. 

Caterpillars of the cabbage white. Note the shed skin. the larvae have almost eaten their way through this Nasturtium leaf.


All insects have to pass through a stage of metamorphosis

Other life cycles are not termed as complete

The life cycle of the Cockraoch, Blatta orientalis is quite different. The females deposits their eggs.The eggs give rise to live active larvae, which grow and moult as do those of the cabbage white butterfly, but whereas the larvae of the Cabbage white bear no likeness to their parents, the Cockroach larvae are very similar to the adults, except that they are smaller. It is by a series of moults that they attain nearly the adult stage. At the final moult they are so similar to the adults that it requires an expert to say whether they are adults or nymphs, as they are called at this stage. The final moult produces the mature adult. The metamorphosis is incomplete as it does not include a resting pupal stage.

There are many other forms of incomplete metamorphosis as another example the Dragonfly can be cited. The eggs give rise to active larvae, quite unlike the parent insect. The larvae transforms into an  active nympth, which bears a considerable resemblance to the imago into which it will develop into. Again there is no resting pupal stage.

The time taken for an insect to pass through its life cycle depends, of course, on the species, and, in the same species it depends on external conditions such as climate and food supply.  Many species of Aphid complete their life cycle within a week, while others take much longer, for example there is a species of locust that takes seventeen years to complete its life cycle.

Certain species in Britain have two broods per season, ie, pass through a complete life cycle twice in 12 months and perform the same operation three times in warmer countries. There are many species were the transformation is habitually shortened. The Sheep Bot fly deposits living larvae missing the egg stage as does the flesh fly.

All insects producing live young are known by the term-viviparous. The common aphid afford a striking and easily observed examples of viviparous insects. They are also Parthenogenesis, ie, reproducing without having been paired. This phenomenon  is rare among the animal kingdom in general, however, in insect land it is fairly common.

On the next page of this series I will review more insects that produce young in a variety of ways including the phenomenon  of alternation in generations.