Insect Life Cycles

Insects have an exoskeleton, or rigid skin, rather than an internal skeleton like humans. Since the exoskeleton cannot stretch appreciably to accommodate increases in size, larval insects grow in stages referred to as instars. Between each instar they must moult the old exoskeleton away by first absorbing as much material as they can from it then swallowing water or air until the old skin breaks open and the larva crawls out. The old exoskeleton left behind is called an exuvium (plural, exuviae). The larva will continue to swell until the new skin hardens thus ensuring room for a period of growth. Only larvae go through this process, once the adult emerges from the final moult growth ceases.

In general the life cycles of aquatic insects are of two types referred to as hemimetabolous and holometabolous.

Hemimetabolous:

The mayflies, dragonflies, damselflies, stoneflies and bugs all have hemimetabolous lifecycles. In this type of life cycle after hatching the larva develops through a series of instars until maturity. After each successive moult between instars the external wing buds become larger. In the final moult the adult emerges with fully developed wings. Above is a series of larvae and the adult of the Giant Water Bug (Lethocerus americanus).

Note the increase in size and similarity to the adult the larval stages have. In some groups apterous (non-winged) adults occur. The adult is sexually mature although usually some period of time before mating occurs for the teneral adult to feed, disperse, set up territories and/or develop adult color patterns.

Mayflies are unique in having a subadult (subimago) emerge from the last larval moult. This stage, although it can fly, is not sexually mature in most cases and requires a further moult to reach sexual maturity.

Holometabolous:

The holometabolous aquatic insects include the caddisflies, beetles, spongillaflies, alderflies, true flies, wasps and moths. In this type of life cycle the larval growth in similar to hemimetabolous larvae but wing pads and other adult features are not present. The larva remains relatively unchanged in appearance other than an increase in size. There are also usually a definite number of larval instars. The last larval instar moults into a pupa. In this stage major restructuring of the body occurs to reach the adult form. Once this tissue development is complete the fully winged adult emerges. Below are the life stages of a blackfly and mosquito.