Metamorphosis in insects
Although all insects molt and change body shape through their life, some of them undergo only minor changes while most undergo extreme changes in structure and function. Juvenile insects molt frequently during their journey to adulthood. A few insects exhibit no change in body part proportions or modification of the body after molting, but most insects accompany these molts with simple or dramatic alterations of body form called metamorphosis. These changes may be accompanied by addition and/or subtraction of body parts, or they may simply be alterations of body proportions.No Metamorphosis
A few insects are ametamorphic and do not exhibit any metamorphic changes. The aphids (Homoptera) are prime examples of this. An aphid is produced when an unfertilized egg hatches inside the mother, grows to a young female that looks exactly like her mother except in size, and the offspring emerges onto the host plant the mother is feeding on. After eating for about a day, the young aphid has molted one or more times, and begins to produce her own offspring – even before she has reached full size or has finished molting. This mode of reproduction is so efficient that under favorable conditions, an aphid colony can multiply to ten times their numbers (1000%) in three days.Gradual Metamorphosis
The simplest form of metamorphosis is a gradual change in body form that occurs between molts. The juvenile stages of these insects closely resemble the adult stages and only trained entomologists (insect scientists) can distinguish the several stages of their life history. A newly hatched praying mantis (Mantis religiosa) or grasshopper looks very much like an adult without wings. It has six legs, of which the front pair are modified to capture food just as in the adult mantis. Antennae, mouth parts, and abdomen are the same as in the adult but their relative proportions differ.The proportion of the head gradually becomes smaller and that of the abdomen longer with each molt. The reproductive organs and wings of the adult remain inactive as imaginal discs for several molts. When wings begin to develop during the last two molts before the adult molt, they appear first as small external buds, become about one third the adult size with the next molt, and finally reach adult size and function after the last molt at which time, the insect is also reproductively mature.
Incomplete Metamorphosis
A more extensive form of metamorphosis is termed incomplete or abrupt metamorphosis. Insects that use this mode of development pass through several nymphal stages before the nymph molts to the winged adult. These insects are usually aquatic with external gills. At the beginning of life, insects that undergo incomplete metamorphosis, such as aquatic dragonfly and damselfly (Orthopterans) nymphs, called naiads, go through a molting process similar to gradual metamorphosis. These insects have six legs, a head with compound eyes and antennae, and a small abdomen with gills at the posterior end. The head becomes proportionately smaller and the abdomen larger with successive molts. As they approach adulthood, wings appear on the back, much as they do if the animal were to undergo gradual metamorphosis.As these insects metamorphose, the insect abruptly changes its body form. The gills are lost and wings develop. But the changes in leg form, head shape, abdominal morphology, development of spiracles for air breathing, and reduction in the size of mouth parts, drastically alter the appearance of the insect. Looking at the penultimate naiad and the adult, most naive observers would not guess they were related – never mind being the same individual.
Complete Metamorphosis
The complete form of metamorphosis involves three very different life stages once the insect hatches from the egg: larva, pupa or chrysalis, and adult or imago. If these insects are aquatic, they may or may not have gills. Those without gills must rise to the surface of the water to breathe. The larval forms of most insects exhibiting complete metamorphosis is more worm- like than insect-like with tiny or no legs, and often use a worm-like wriggling movement to move. Except for the caterpillars of butterflies and moths and the grubs of some beetles, these larvae usually have a reduced or even absent head definition, often have no eyes or antennae, and sometimes appear to be mostly abdomen with a mouth at one end and anus at the other.At some point in their development, these insects molt to form a “resting” stage called a pupa or chrysalis. This is a stage incapable of coordinate locomotion. Feet, antennae, and wings (when present) are closely attached to the animal's outer surface and, although appearing for the first time, are nonfunctional.
The metamorphosis from the pupa to the imago is sometimes as dramatic as the metamorphosis of an insect with incomplete metamorphosis. The relatively inactive pupal shell splits and the adult draws itself out of the pupal shell. The adult then, as in those with incomplete metamorphosis, pumps body fluids and air into its extremities, causing them to swell and transform to the adult form. After a period of hardening, when the antennae are capable of supporting themselves and the wings and legs can support the insect in flight and walking, the insect walks or flies off to feed, find a mate or mates, and reproduce.
Metamorphosis of Various Insects
1. Sculptured egg: Chorion with reticulate markings and ridges. Eg: castor butterfly
2. Elongate egg: Eggs are cigar shaped. Eg: Sorghum shoot fly.
3.Rounded egg: Eggs are either spherical or globular. Eg: Citrus butterfly.
4. Nit: Egg of head louse is called nit. It is cemented to the base of the hair. There is an egg stigma at the posterior end, which assists in attachement. At the anterior end, there is an oval lid which is lifted at time of hatching.
3. Ootheca: Eggs are deposited by cockroach in a brown bean like chitinous capsule. Each ootheca consists of a double layered wrapper protecting two parallel rows of eggs. Each ootheca has 16 eggs arranged in two rows. Oothecae are carried for several days protruding from the abdomen of female prior to oviposition in a secluded spot. Along the top, there is a crest which has small spores which permit gaseous exchange without undue water loss. Chitinous egg case is produced out of the secretions of collaterial glands.
4. Egg pod: Grass hoppers secrete a frothy material that encases an egg mass which is deposited in the ground. The egg mass lacks a definite covering. On the top of the egg, the frothy substance hardens to form a plug which prevents the drying of eggs.
5.Egg case: Mantids deposit their eggs on twigs in a foamy secretion called spumaline which eventually hardens to produce an egg case or ootheca. Inside the egg case, eggs are aligned in rows inside the egg chambers 
6. Egg mass: Moths lay eggs in groups in a mass of its body hairs. Anal tuft of hairs found at the end of the abdomen is mainly used for this purpose. Eg: Rice stem borer. Female silk worm moth under captivity lays eggs on egg card. Each egg mass is called a dfl (disease free laying). 
7. Egg raft: In culex mosquitoes, the eggs are laid in a compact mass consisting of 200 – 300 eggs are called egg raft in water.
Looper: They are also called measuring worm or earth measurer or inch worm. In this type only two pairs of prologs are present in sixth and tenth abdominal segments. Eg: Daincha looper.
i. Chrysalis: It is the naked obtect pupa of butterfly. It is angular and attractively coloured. The pupa is attached to the substratum by hooks present at the terminal end of the abdomen called cremaster. The middle part of the chrysalis is attached to the substratum by two strong silken threads called gridle.
ii. Tumbler: Pupa of mosquito is called tumbler. It is an object type of pupa. It is comma shaped with rudimentary appendages. Breathing trumpets are present in the cephalic end and anal paddles are present at the end of the abdomen. Abdomen is capable of jerky movements which are produced by the anal paddles. The pupa is very active.
Types of larvae and pupae
1. EGGS
The first stage of development in all insects is egg. Majority of insects are oviparous. Egg stage is inconspicuous, inexpensive and inactive. Yolk contained in the egg supports the embryonic development. Eggs are laid under conditions where the food is available for feeding of the future youngones. Eggs are laid either individually or in groups. The outer protective shell of the egg is called chorion. Near the anterior end of the shell of the egg, there is a small opening called micropyle which allows the sperm entry for fertilization. Chorion may have a variety of textures. Size and shape of the insect eggs vary widely.
Types of eggs
a) Singly laid1. Sculptured egg: Chorion with reticulate markings and ridges. Eg: castor butterfly
2. Elongate egg: Eggs are cigar shaped. Eg: Sorghum shoot fly.
3.Rounded egg: Eggs are either spherical or globular. Eg: Citrus butterfly.
4. Nit: Egg of head louse is called nit. It is cemented to the base of the hair. There is an egg stigma at the posterior end, which assists in attachement. At the anterior end, there is an oval lid which is lifted at time of hatching.
5. Egg with float: Egg is boat shaped with a conspicuous float on either side. The lateral sides are expanded. The expansions serve as floats. Eg: Anopheles mosquito. Turbo
b) LAID IN GROUPS
1. Pedicellate eggs: Eggs are laid in silken stalks of about 1.25 mm length in on groups plants.
Eg: Green lace wing fly.
1. Pedicellate eggs: Eggs are laid in silken stalks of about 1.25 mm length in on groups plants.
Eg: Green lace wing fly.
2.Barrel shaped eggs: Eggs are barrel shaped. They look like miniature batteries. They are deposited in compactly arranged masses. Eg: Stink bug.
3. Ootheca: Eggs are deposited by cockroach in a brown bean like chitinous capsule. Each ootheca consists of a double layered wrapper protecting two parallel rows of eggs. Each ootheca has 16 eggs arranged in two rows. Oothecae are carried for several days protruding from the abdomen of female prior to oviposition in a secluded spot. Along the top, there is a crest which has small spores which permit gaseous exchange without undue water loss. Chitinous egg case is produced out of the secretions of collaterial glands.
4. Egg pod: Grass hoppers secrete a frothy material that encases an egg mass which is deposited in the ground. The egg mass lacks a definite covering. On the top of the egg, the frothy substance hardens to form a plug which prevents the drying of eggs.
5.Egg case: Mantids deposit their eggs on twigs in a foamy secretion called spumaline which eventually hardens to produce an egg case or ootheca. Inside the egg case, eggs are aligned in rows inside the egg chambers
6. Egg mass: Moths lay eggs in groups in a mass of its body hairs. Anal tuft of hairs found at the end of the abdomen is mainly used for this purpose. Eg: Rice stem borer. Female silk worm moth under captivity lays eggs on egg card. Each egg mass is called a dfl (disease free laying).
7. Egg raft: In culex mosquitoes, the eggs are laid in a compact mass consisting of 200 – 300 eggs are called egg raft in water.
2. LARVAE
There are three main types of insects larvae namely oligopod, polypod and apodous.I. OLIGOPOD:
Thoracic legs are well developed. Abdominal legs are absent. There are subtypes:
i. Campodeiform: They are so called from their resemblance to the dipluran genus campodea.
Body is elongate, depressed dorso ventrally and well sclerotised. Head is prognathous. Thoracic legs are long. A pair of abdominal cerci or caudal processes is usually present. Larvae are generally predators and are very active. Eg: grub of ant lion or grub of lady bird beetle.
i. Campodeiform: They are so called from their resemblance to the dipluran genus campodea.
Body is elongate, depressed dorso ventrally and well sclerotised. Head is prognathous. Thoracic legs are long. A pair of abdominal cerci or caudal processes is usually present. Larvae are generally predators and are very active. Eg: grub of ant lion or grub of lady bird beetle.
ii. Scarabaeiform: Body is ‘C’ shaped, stout and subcylindrical. Head is well developed.
Thoracic legsa re short. Caudal processes are absent. Larva is sluggish, burrowing into wood or soil. Eg: - grub of rhinocerous beetle.
Thoracic legsa re short. Caudal processes are absent. Larva is sluggish, burrowing into wood or soil. Eg: - grub of rhinocerous beetle.
II. POLYPOD OR ERUCIFORM:
The body consists of an elongate trunk with large sclerotised head capsule. Head bears a pair of powerful mandibles which tear up vegetation. Two groups of single lensed eyes found on either side of the head constitute the visual organs. The antenna is short. Three pairs of thoracic legs and upto five pairs of unjointed abdominal legs or prologs are present. Thoracic legs are segmented and they end in claws which are used for holding typically bears rows or circlet of short hooked spines or crochets which are useful in clinging to the exposed surface of vegetation and walking. Abdominal segments three to six and ten typically bear prologs. Eg: caterpillar (larvae of moths and butterflies).
i. Hairy caterpillar: The body hairs may be dense, sparse or arranged in tufts. Hairs may cause irritation, when touched. Eg: Red hairy caterpillar.
ii. Slug caterpillar: larva is thick, short, stout and fleshy. Larval head is small and retractile.
Thoracic legs are minute. Abdominal legs are absent. Abdominal segmentation is indistinct. Larva has poisonous spines called scoli distributed all over the body. Such larva is also called platyform larva.
iii. Semi looper: either three or four pairs of prologs are present. Prologs are either wanting or rudimentaly in either third or third and fourth abdominal segments. Eg: Castor semi looper.
ii. Slug caterpillar: larva is thick, short, stout and fleshy. Larval head is small and retractile.
Thoracic legs are minute. Abdominal legs are absent. Abdominal segmentation is indistinct. Larva has poisonous spines called scoli distributed all over the body. Such larva is also called platyform larva.
iii. Semi looper: either three or four pairs of prologs are present. Prologs are either wanting or rudimentaly in either third or third and fourth abdominal segments. Eg: Castor semi looper.
Looper: They are also called measuring worm or earth measurer or inch worm. In this type only two pairs of prologs are present in sixth and tenth abdominal segments. Eg: Daincha looper.
They are larvae without appendages for locomotion. Based on the degree of development and sclerotization of head capsule, there are three subtypes.
i. Eucephalous: larva with well developed head capsule with functional mandibles, maxillae stemmata and antennae. Mandibles act transversely. Eg: Wriggler (larva of mosquito) and grub of red palm weevil.
ii. Hemicephalous: Head capsule is reduced and can be with drawn into thorax. Mandibles act vertically. Eg: Larva of horse fly and robber fly.
iii. Acephalous: Head capsule is absent. Mouth parts consists of a pair of protrusible curved mouth hooks and associated internal sclerites. They are also called vermiform larvae. Eg: maggot (larva of housefly)
i. Eucephalous: larva with well developed head capsule with functional mandibles, maxillae stemmata and antennae. Mandibles act transversely. Eg: Wriggler (larva of mosquito) and grub of red palm weevil.
ii. Hemicephalous: Head capsule is reduced and can be with drawn into thorax. Mandibles act vertically. Eg: Larva of horse fly and robber fly.
iii. Acephalous: Head capsule is absent. Mouth parts consists of a pair of protrusible curved mouth hooks and associated internal sclerites. They are also called vermiform larvae. Eg: maggot (larva of housefly)
3.PUPA
It is the resting and inactive stage in all holometabolous insects. During this stage, the insect is incapable of feeding and is quiescent. During this transitional stage, the larval characters are destroyed and new adult characters are created. There are three main types of pupae.I. OBTECT:
Various appendages of the pupa viz., antennae, legs and wings pads are glued to the body by a secretion produced during the last larval moult. Exposed surface of the appendages are more heavily sclerotised than those adjacent to body. Eg: moth pupa.
i. Chrysalis: It is the naked obtect pupa of butterfly. It is angular and attractively coloured. The pupa is attached to the substratum by hooks present at the terminal end of the abdomen called cremaster. The middle part of the chrysalis is attached to the substratum by two strong silken threads called gridle.
ii. Tumbler: Pupa of mosquito is called tumbler. It is an object type of pupa. It is comma shaped with rudimentary appendages. Breathing trumpets are present in the cephalic end and anal paddles are present at the end of the abdomen. Abdomen is capable of jerky movements which are produced by the anal paddles. The pupa is very active.
II. EXARATE:
Various appendages viz., antennae, legs and wing pads are not glued to the body. They are free. All oligopod larvae will turn into exarate pupae. The pupa is soft and pale. Eg: exarate- pupa of rhinocerous beetle.
III. COARCTATE:
The pupal case is barrel shaped, smooth with no apparent appendages. The last larval skin is changed into a case containing the exarate pupa. The hardened dark brown pupal case is called puparium. Eg: Fly pupa.
PUPAL PROTECTION
In general pupal stage lacks mobility. Hence, it is the most vulnearable stage. To get protection against adverse conditions and natural enemies, the pupa is enclosed in a protective cover called cocoon. Based on the nature and materials used for preparation of cocoons, there are several types:
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Entomology