Insect life cycle is generally complex involving several stages of the larval and pupal development. Adults are generally quite different from the larval forms. When the larvae undergo considerable change to become adults it is called metamorphosis. Insects show various types of metamorphosis as described below.
1. PALEOMETABOLA, AMETABOLA OR ANAMORHOSIS
This type of metamorphosis occurs in orders Protura, Diplura, Collembola and Thysanura that include insects such as telson-tails, campodeids, spring tails and silverfish. The nymph upon hatching from the egg is similar to the adult in general morphology but there are only 8 abdominal appendages and cerci are lesser in number. Size is small. As the nymph grows and molts its segments in abdomen increase gradually to become 11 in adult. These nymphs live in the same environment as the adult and feed on the same diet as the adult. The insects are wingless both in larval as well as in adult stages. Some biologists do not consider it as metamorphosis or called it Anamorphosis as there is little change during development.
2. HEMIMETABOLA
There are 15 orders of insects which demonstrate this kind of metamorphosis in which juveniles are similar to the adults and there is a gradual change from nymphs to adults. The transformation takes place in the growth of wings as external buds and development of secondary sexual characters. Nymphs generally live in the same environment as the adults. The examples are: grasshoppers, locusts, cockroaches, dragon flies, Mayflies, earwigs, lice, bugs, thrips etc. Hemimetabola includes two categories as given below.
(i) HETEROMETABOLA
These insects do not show any dormant stage during development and nymphs are active throughout their growth stage. Wings develop externally.
(a) Archimetabola
Those insects whose larvae are aquatic while adults are a flying terrestrial insects show differences in the morphology of their nymphal stages owing to their aquatic habitat. Such nymphs are called NAIADS which are generally carnivorous in habit. Examples are: Odonata (dragon and damsel flies), Ephemeroptera (may flies) and Plecoptera (stone flies). The larvae breathe with tracheal gills attached on the tip of abdomen. Wings develop as buds externally on the thorax. The last nymphal instar climbs on a blade of grass and molts outside water to give rise to adult.
(b) Paurometabola
The immature stage is called a nymph and lives in the same habitat as the adult and eats similar food. The examples are: Orthoptera (grass hoppers and locusts), Dictyoptera (cockroaches and mantids), Hemiptera (bugs) and lice. The wings develop externally and become larger after each molt and size gradually increases. There is no dormant stage in the development.
(ii) NEOMETABOLA
This type of metamorphosis is found in Thysanoptera and coccids. Nymphs live in the same environment as the adults and feed on the same kind of food. But they have a dormant or resting stage after the larval development is completed. This dormant stage is different from the pupal stage of the Holometabolous insects as the there is no covering or cocoon or pupal case outside the resting larva. Owing this unique development thrips are assigned a different category of Neometabola.
3. HOLOMETABOLA
This type of metamorphosis is found in endopterygotes, namely, Lepidoptera, Hymenoptera, Coleoptera, Siphonoptera, Diptera, Trichoptera etc. Larvae and adults are completely different in general feature and eat different type of food and live in a different type of habitat. The larva after completing its development transforms into a dormant stage called pupa, which is sometimes enclosed inside a cocoon made of silken threads. The development of the adult takes place inside the pupal case and cannot be seen from outside. The adult is a flying insect that emerges after rupturing the pupal case. This type of development can be seen in silkworm, butterflies, houseflies and beetles.