Growth and Development in Insects | Zoology

The eggs in insects have less food materials than those in birds and reptiles. Therefore, unlike birds, the young ones of insects emerge at an early stage of development and are much smaller than, and quite different in body shape, from the adults. Moreover, the young have a covering of inflexible exoskeleton which they have to shed a number of times in order to grow.

The young larva at the time of its emergence tears open the egg-shell with spines or egg-bursters located on its head or on other parts of its body. These spines are retained till the first skin is cast off by the process known as ecdysis or moulting. Before moulting, the insect stops feeding and becomes quiescent for a short time.

The epidermal cells enlarge and divide mitotically. The old cuticle is detached and a new layer of epidermal cells secretes a new cuticle. The space between the old and the new cuticles is filled with moulting fluid which contains two enzymes, protease and chitinase.

These enzymes dissolve the old cuticle, not touching the new one and the dissolved fluids are absorbed by the epidermis. The old skin splits along the median dorsal line in the thorax, and the insect frees itself by muscular movements, leaving behind the exuviae. The interval between two moultings is known as the stadium and the body form in a particular stadium is called an instar.

During development from the egg to adult, the changes in form are known as metamorphosis. In some groups of insects, the changes are slight and only the size of the body increases, being typical of ametabolism, e.g. silverfish of the sub-class Apterygota. In others the changes are very drastic and the immature stages look quite different from the adult.

The direct development or incomplete metamorphosis is known as hemimetabolism and the indirect development or complete metamorphosis is known as holometabolism. The former is most prevalent in the Exopterygota and the young ones are commonly called nymphs. The latter is characteristic of Endopterygota and the young ones are commonly known as larvae.

Among hemimetabolous insects, most of the external structures and internal organs remain essentially the same and gradually transform into those of the adult; the young ones are called nymphs, and in some cases, naids (Odonata). In the holometabolous insects, there is the pupal stage in between the larva and adult.

In this stage, there is extensive destruction of the larval organs through histolysis and the tissues are broken up and dissolved through the activity of phagocytic blood-cells. The break down products are absorbed into the blood and provide material for the reconstruction of adult organs which are initiated from certain latent cells whose activity remains suppressed throughout the larval period.

Among the holometabolous insects, there are four distinct developmental stages; the egg stage, the active larval stage with variable number of legs, the resting stage or the pupa, and the winged adult. The larvae of this group of insects display an enormous range of structural variations and are adapted for life in a wide variety of environments.

The endopterygote larvae may be divided into a number of forms (Fig. 1.20). The highly specialised form is known as the protopod larva. It is common among the endoparasitic Hymenoptera and Diptera. This form, in a way resembles the embryonic stage because the body segmentation is ill-defined and the appendages are either rudimentary or absent.

The polypod type of larva is structurally of the more advanced form and is provided with thoracic legs and abdominal prolegs (unjointed). The apodous form lacks all the legs and its head may be well developed, reduced or absent altogether. The oligopod type is well- developed with three pairs of legs on thorax.

Larvae of insects in the orders Lepidoptera and Hymenoptera are polypods. Larvae of the sawfly, Athalia lugens proxima (Klug), have 6-8 pairs of prolegs including those on the last abdominal segment. This type of larva is a defoliator and is known as a pseudocaterpillar. Among lepidopteran larvae, the caterpillars bear five pairs of prolegs on third to sixth, and on the tenth abdominal segments.

Many caterpillars are pests of crops; the larva of cabbage butterfly, Pieris brassicae (Linnaeus), is of this type. The semiloopers bear three pairs of prolegs on the fifth, sixth and tenth abdominal segments. These larvae are known as semiloopers because as they crawl they form a semiloop in their body and then they stretch straight.

These larvae are normally defoliators and are pests of crops, for example, larva of the cabbage semilooper, Thysanoplusia orichalcea (Fabricius). Looper bears two pairs of prolegs on the sixth and the tenth abdominal segment, and it moves on a leaf surface by forming a full loop. Larvae of the inchworm (Family Geometridae) are the typical loopers. They feed on the roots of grasses.

Apodous larvae with the reduced head portion are known as hemicephalous. Larvae of the honey bees, Apis spp., are of that type. Larvae of the red wasp, Vespa orientalis Linnaeus, on the other hand, have fully developed head and are known as eucephalous. In some cases, however, the head may be totally absent and such a condition occurs in the larvae of the housefly, Musca nebulo Wiedemann; the larvae are known as acephalous.

The mouth in them is carried far inwards and communication with the exterior is by means of a secondary passage or atrium. Among the oligopods, there are further many types. The campodeiform larvae have well developed legs and sensory organs; their body is flattened and elongated which suits their predatory form of living. Larvae in the family Coccinellidae are of this type, e.g. the lady-bird beetle, Coccinella septempunctata Linnaeus, are predatory on aphids.

Scarabaeiform larvae are cresentic in form; the head and legs are well developed; abdomen is soft and fleshy and is inflated. Larva of the ber beetle, Adoretus pallens Arrow, is of this type. The click-beetle larvae, belonging to the family Elateridae, are known as elateriform. These larvae are elongated, cylindrical, smooth and hardened. They are destructive root feeders of many crops.

Triungulin form is typical of the oil beeties, Meloe spp., in which body is elongated and the legs are very well developed with three claws on each leg. These larvae undergo hypermetamorphosis and are parasitic within the nest of solitary bees or on the egg masses of grasshoppers.

Like larvae, the pupae are also of various types (Fig. 1.21), which are grouped according to the presence or absence of functional mandibles which might be used by the adult to emerge from the cocoon or the pupal cell. In the decticous type of pupa, the functional mandibles are present, whereas in the adecticous type, the mandibles are not functional.

The latter are subdivided into two types, the exarate with free appendages, and the obtect with appendages glued to the rest of the pupal body. An exarate pupa enclosed in a puparium is called coarctate and the silken protective case of obtect pupa is called cocoon.

Hormonal Control of Metamorphosis:

The growth and moulting in insects are controlled by hormones. The neurosecretory cells in the brain, under certain stimuli, secrete a hormone prothoracicotropic hormone (PTTH), which activates the prothoracic glands. As a result of this activation, the growth and differentiation hormone (GDH) or ecdysone is secreted.

This hormone induces the insects to moult. The corpora alata produce the juvenile hormone QH) or neotenin. High concentration of JH results in larval-pupal moult, lower concentration results in larval-pupal moult and absence of JH results in pupal-adult moult.

The two other hormones which also play a role in development are the eclosion and tanning hormones. The eclosion hormone released by the brain controls the process of eclosion in insects while the tanning hormone or burscion regulates the process of tanning and sclerotization of the new cuticle.