Difference Between Larva and Pupa in Insect Life Cycle

Insects generally undergo metamorphosis to reach adulthood. Metamorphosis is defined as a biological process in which the animal undergoes physical development after its birth. The body structure of the animal changes vastly through cell growth and differentiation.

Larva

The Larva is the young form of many animals before they undergo metamorphosis to grow into adults. This is a kind of indirect development in many animals which include amphibians, insects, etc. All these animals typically have a larval stage in their lifecycle. The appearance of the Larva is very distinctive when compared to that of their adult form. The Larval stage of an insect general includes many structures that are not found in their adult form. The environment adaptability, as well as their food and diet, also is completely different from that of the adult version of the animal.

The larval population of a given amphibian or an insect has more susceptibility to survive from predators when compared to that of their adult population due to the distinctive environment where they live. For example, the larval stage of a frog is a Tadpole that lives exclusively in water and cannot survive outside the aquatic environment unless and until it reaches the adult stage which is called a frog that can live on both land and water.

While some larvae are capable of surviving alone until they mature into adults, there are some larvae that need the help of the adults to be fed. These include the Hymenoptera species in which the female adults are responsible for providing food to the larva.

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Pupa

The pupal stage comes after the Larval stage. The lifecycle of an insect undergoes 4 different stages which are egg, larva, pupa, and imago (adult stage) respectively. The pupal stage of an insect is controlled and regulated by the hormones of the insect, unlike the larval stage. The stage where the Larva becomes the Pupa is called the Pupation. The cells present in the larval stage start growing rapidly. They then become various organs of the insect-like the eyes, wings, legs, etc of the insect. Every variety of insects has a different name for their pupa. For example, the pupae of the butterflies are called the chrysalis. Usually, the pupae are enclosed within a cavity called the shells or cocoons. 

There are Two Types of Pupae: Obtect and Exarate

• In an Exarate pupa, the appendages are free whereas 

• In an Obtect pupa, the appendages are very much attached to the body wall like the cocoon.

Again based on the mandibles the pupa can be classified under two varieties:

• Adecticous pupa, where the pupae are without the articulated mandibles. 

• Decticous pupa, where pupae are with articulated mandibles. 

Generally. The pupae are immobile and hence do not have much defence mechanism to defend themselves from the predators. That is when their external covering comes to their rescue. Either the pupa is covered with the shell or cocoon or they conceal themselves from the environment by going underground. In some varieties, the pupae are protected by other insects whereas a few varieties of pupae secrete toxic substances to protect them from becoming a meal of the predators. 

Depending upon different varieties of insects, the pupal stage may last up to a week or even years before they reach adulthood.

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Difference between Larva and Pupa

The insects and a few amphibians undergo the lifecycle which includes the larval stage and pupal stage but the appearance and duration in each case are different from the other.

Complete Metamorphosis

When insects progress through the pupal, egg, larval, and adult stages, they are said to have complete metamorphosis. Butterflies, flies, moths, wasps, ladybugs, bees, and various beetles are examples of insects that go through a complete metamorphosis. Four steps are required for complete transformation. The more general word metamorphosis refers to two distinct processes, one of which is full metamorphosis, also known as holometabolous development or holometabolous, which is nearly entirely unique to winged insects. There are significant variations between larval and adult forms throughout full metamorphosis. This transition necessitates a substantial amount of energy and is divided into a number of alterations at various phases of the insect's lifespan. It is simply not possible to undergo a complete morphological and anatomical transformation in one sitting since the procedure demands so much energy.

Incomplete Metamorphosis

Not all insects go through so many stages of development. Incomplete metamorphosis refers to insects whose body shapes are the same from hatching to adulthood. They hatch from their eggs as small, underdeveloped replicas of their adult selves. These young are referred to as nymphs. Nymphs molt, losing their exoskeletons, allowing them to develop until they reach adulthood. Insects that undergo partial metamorphosis include grasshoppers, dragonflies, and praying mantises.  Incomplete metamorphosis occurs in termites, praying mantis, and cockroaches. Incomplete metamorphosis allows some of the earlier stages of the insect to reproduce successfully.

The Four Stages of Complete Metamorphosis

Egg Stage of Complete Metamorphosis: 

The egg contains all of the genetic information required for development and function, including the blueprints for imaginary discs. Imaginal discs are seen in insect embryos and develop into anatomical features of adult species. Insect eggs are generated in vast quantities and deposited on protected, concealed surfaces by the female ovipositor. The larval form's nutrition determines where the egg is placed.

Larval Stage of Complete Metamorphosis:

An insect larva's worm or maggot shape is generally a long cry from its adult form. Its principal purpose after hatching from the egg is to absorb energy in preparation for the massive morphological changes of the following stage of full metamorphosis. Larvae may also have imaginal discs or imaginal buds, which develop into adult anatomy. The majority of larvae will go through at least one instar, or larval stage, where the larva must shed its skin to allow it to grow.

Pupal Stage of Complete Metamorphosis:

The imaginal discs of the insect embryo and larva become active during the pupal stage. A meticulously scheduled sequence of cell death and cell growth occurs, in which larval cells die and are broken down to supply energy for the many processes required in the development of an adult insect. Adults must be able to reproduce, and the reproductive organs mature during this period.

Imago Stage of Complete Metamorphosis: 

Eclosion is the process by which an adult insect emerges from its pupa. Hormones secreted at the pupal stage loosen the shell wall, allowing the adult beetle to emerge. The pupal case is left behind as an empty shell, and the adult insect is exposed to the elements and predators for a period of time. Because all of the adult insect's wings are wrinkled and moist, it is unable to fly.