Different types of metamorphosis and Ecdysis

Metamorphosis  
Metamorphosis is the change in growth and development an insect 
undergoes during its life cycle from birth to maturity. There are four basic 
types of metamorphosis in insects.

1. Ametabola: (No metamorphosis) e.g. Silver fish. These insects have only 
three stages in their life namely egg, young ones and adult. It is most 
primitive type of metamorphosis. The hatching insect resembles the adult in 
all respects except for the size and called as juveniles. Moulting continues 
throughout the life.

2. Hemimetabola: (Incomplete metamorphosis) e.g. Dragonfly, damselfly 
and may fly. These insects also have three stages in their life namely egg, young one and 
adult. The young ones are aquatic and are called as naiads. They are 
different from adults in habit and habitat. They breathe by means of tracheal 
gills. In dragonfly naiad the lower lip (labium) is called mask which is 
hinged and provided with hooks for capturing prey. After final moult, the 
insects have fully developed wings suited for aerial life.
3. Paurometabola: (Gradual metamorphosis) e.g. Cockroach, grasshopper, 
bugs. The young ones are called nymphs. They are terrestrial and resemble the 
adults in general body form except the wings and external genitalia. Their 
compound eyes and mouth parts are similar to that of adults. Both nymphs 
and adults share the same habitat. Wing buds externally appear in later instars.
The genitalia development is gradual. Later instars nymphs closely resemble the adult
with successive moults.
4. Holometabola: (Complete metamorphosis) e.g. Butterfly, moth, fly and 
bees. These insects have four life stages namely egg, larva, pupa and adult. 
Majority of insects undergo complete metamorphosis. Larvae of butterflies 
are called caterpillar. Larva differs greatly in form from adult. Compound 
eyes are absent in larva. Lateral ocelli or stemmata are the visual organs.  Their mouth parts and food habit differ from adults. Wing development is internal. When the larval growth is completed, it  transforms into pupa. During the non-feeding pupal stage, the larval tissues  disintegrate and adult organs are built up.
Ecdysis
* Ecdysis or Molting is a very common phenomenon in Insects. Due to the 
presence of hard exoskeleton the body growth is restricted.
* So, in odor to permit further body growth of the animal the old skeleton is 
castoff & a new exoskeleton develops in that place.
* So, ecdysis is an essential process in all the insect.It is known that ecdysis 
takes place only along predator minded exovial lines on the body .
* Ecdysis is a regular phenomenon in all the insects & it continue untill the 
animal become sexually mature with the exception of collembolla and 
Diplurea, where ecdysis is takes place through out the life. The time required 
between two moulting or ecdysis is called inster or intermediate time.
According to the Imms (1957) the number and duration of each inster varies 
from species to species & with relation to the type of food, climatic factors 
&host etc. Ecdysis can be distinguse by three important processes----
1. 

Splitting and sheding of old cuticle
2. Expansion of the body &new cuticle.
3. Darkening and hardening of new cutical.
Stages involved in Ecdysis or moulting:
There are three stages of ecdysis. These are….
. Proecdysis.
. Ecdysis proper.
. Post ecdysis.
Proecdysis
* This is the initial preparative stage in which the body completely stops the 
normal activities and the animal takes a suitable shelter, even the slightest 
moment of the body can be observed.
*Histologycally the beginning of the proecdysis is worked by a significant 
increase in cellsize(epidermis),this is due to the formation of new epidermal 
glands.This glands secret a fluid known as moulting fluid which is used for 
the digestion of old cutical.
Ecdysis proper
* There are two important phenomenon in ecdysis proper.
1. Passive phase
2. Active phase
Passive phase:
It is known that splitting of old cuticle take place along some predetermined 
lines mainly on the dorsal side of the body (Snodgrass, 1949).this line 
known as ecdysal lines.
In some insects the splitting of old cuticle takes place with the help of a 
special organs known as ptilinum, situated at the head region (Cyclorrhorous 
sp.)
Active phase:
During this phase the animals actively crawls out of the old exoskeleton. 
Time require various form a second to half an hour in different groups of 
insects and it require huge amount of energy. Blirs (1953)has proved that 
O2 requirement of the body during the active phase is significantly higher
than the normal stages.
3. Post ecdysal changes
*A lot of changes take place in the new cuticle after the moulting or ecdysis
is completed. This include expansion hardening &darkening of the cuticle.
*The cuticle of a newly molted insect is very soft and colorless except in 
some specialize regions of the body. The hardening &darkening takes place 
gradually and exception in an intermediate process.
*A most important tanning agent is tyrosine which is produced &secreted
from in large amount hymolymph.
Control of ecdysis
*The entire process of ecdysis is controlled by a complicated 
neuroendocrine system. 
These is a moulting hormone known as Ecdyson secreted from prothorasic 
gland (insect)& Y gland (in case of crustacean)situated at the base of eye.