Fig. 3.—Empty Case of a Newly-moulted Cockroach.
The change from the worm-like caterpillar to the butterfly is a great one, and, if we did not know it so well, would be startling. This change is known as a 'complete metamorphosis.' The dragon-fly is another insect with a complete metamorphosis. How the dragon-fly moults you will see in the illustration (fig. [1]): even an acrobat might envy him!
Carefully examine the series of figures from a to d. The empty case at a shows the last stage of the larval life. Out of this case the young dragon-fly is just emerging. In c he has gained his freedom, and is stopping to take breath and allow his wings to expand. By the time this has taken place, they will be nearly as long as the body (as in d).
The locust furnishes us with an instance of what is known as 'incomplete metamorphosis.' In other words, the young, when they emerge from the egg, are very little different from the parent form. The youngest locust in the illustration (fig. [2]) is obviously a locust, though he lacks wings; but there is no promise of the butterfly in the worm-like caterpillar.
Fig. 2.—Stages of Locust's Growth.
The cockroach, like the grasshopper and the locust, only undergoes an incomplete metamorphosis. The empty case of a newly-moulted cockroach is shown in fig. [3]. The slit(s) along the back marks the spot where the insect crept out.