MOULTING AND METAMORPHOSIS IN INSECTS

MOULTING AND METAMORPHOSIS IN INSECTS In insects, growth is limited by the cuticle Casting the cuticle is commonly known as MOULTING. Moulting involves two distinct processes. The first process is the separation of the old cuticle from the underlying epidermal cells and this is called apolysis. The second process is the shedding of the remnants of the old cuticle, and this is known as ecdysis.

Structure of the insect’s integument

Generally, once an insect has moulted to the adult stage and has fully inflated and hardended its cuticle, it does not normally grow any more, however long it may live. One exception to the rule being the mayflies which have wings as a sub-imago, or ‘dun’ and the moult again to the true adult or ‘spinner’.

METAMORPHOSIS IN INSECTS Insects exhibit several types of metamorphosis that range from no apparent metamorphosis, with only a change in size and sexual maturity, to complete metamorphosis with quite drastic changes in both appearance and feeding habits.

Insects with simple metamorphosis can be further broken down into – TYPES OF METAMORPHOSIS SIMPLE METAMORPHOSIS Insects with simple metamorphosis have three life stages namely – egg, nymph and adult. These insects have wings which if present, develop externally. The young ones of this group of insects are called nymphs and there is no resting stage. Insects with simple metamorphosis can be further broken down into –

AMETABOLOUS INSECTS (No metamorphosis) In the case of ametabolous insects, adults differ from nymphs only in size and in the possession of fully developed reproductive organs. Ametabolous insects include Springtail (Order Collembola) and Silverfish (Order Thysanura).

Silverfish an example of an AMETABOLOUS INSECT

HEMIMETABOLOUS INSECTS (Incomplete metamorphosis) Insects with incomplete metamorphosis include Mayflies, dragon flies and damsel flies, and Stoneflies In these orders, the immature are called naiads. Naiads are aquatic and possess gills for breathing. Nymphs and adults occupy different habitats and feed on different foods.

Incomplete metamorphosis in dragon fly

Adults and nymphs of these insects usually feed on the same foods. PAUROMETABOLOUS INSECTS (Gradual metamorphosis) Insects with gradual metamorphosis include grasshoppers, crickets, mantids, cockroaches, earwigs, termites, aphids. Adults and nymphs of these insects usually feed on the same foods.

Gradual metamorphosis in grasshoppers

HOLOMETABOLOUS INSECTS (Complete metamorphosis) In insects with complete metamorphosis, the wings, if present develop internally. The active immature stages are known generally as larvae. In some orders, larvae are referred to by other names, such as maggots (flies) caterpillars (butter flies and moths), or grubs (beetles). Larvae and adults of these insects live in very different habitats and often feed on different types of food. These insects also have a resting stage known as pupa. Examples of insects with complete metamorphosis include lacewigs, beetle, butterflies and moths, flies, fleas, and saw flies, ants, wasps and bees.

Complete metamorphosis in butterfly

THE INSECTS DIGESTIVE AND EXCRETORY SYSTEMS

THE INSECT’S DIGESTIVE SYSTEM Digestion involves the breakdown of food (large molecules) into smaller and smaller molecules (components) which can be absorbed and assimilated into the body. These smaller molecules can be used by cells of the body for energy, growth, or reproduction. The digestive system or alimentary canal of insects is a long, muscular and tubular structure that starts from the mouth and ends at the anus. It is divided into three distinct regions namely - Foregut, Mid gut and Hindgut.

The foregut is also called Stomodeum. The foregut starts at mouth and ends just after proventriculus but before the cardiac sphincter. The foregut is also called Stomodeum. It is ectodermal in origin and is therefore lined internally with cuticle. Preoral cavity: The space before the mouth Cibarium: The part of the preoral cavity between the epipharynx and hypopharynx Salivarium: The part of the preoral cavity between the hypopharynx and salivary duct Mouth: The opening into the fore gut Pharynx/ Cibarial pump Crop: a food reservoir Proventriculus or Gizzard : the musculated posterior part of the foregut. Cardiac sphincter

The MIDGUT The midgut starts at the cardiac sphincter and ends at the pyloric sphincter. Midgut is also called mesentron. It is endodermal in origin and so has no cuticular lining. The midgut is covered internally by a thin peritrophic membrane. Midgut is where the food is mainly digested. It has digestive cells which secrete enzymes into the lumen of the midgut that aid in digestion of proteins, carbohydrates and fats. In many insects absorption of nutrients occurs through microvilli of midgut epithelial cells by diffusion.

THE INSECT’S EXCRETORY SYSTEM The excretory system is responsible for the maintenance of chemical homeostasis. The excretory system is made up of the malphigian tubules and the hindgut.

THE HIND GUT The hindgut is also called proctodeum. Like the foregut, it is ectodermal in origin and thus has cuticular lining. The hindgut is differentiated into three regions viz., ileum, colon and rectum. The ileum of some insects contains a population of symbionts which use waste products to produce substances the insect needs. The rectum contains rectal pads which help in the dehydration of faeces. The rectum opens out through anus. The main functions of hindgut are the absorption of water, salt and other useful substances from the faeces and urine.

The number of malphigian tubules may vary from two to more than 250. Malpighian tubules The malphigian tubules are found attached between the midgut and hindgut. They float freely within the haemocoel are bathed in hemolymph. The number of malphigian tubules may vary from two to more than 250. The malphigian tubules help regulate and maintain ionic concentrations in the hemolymph They also help to remove toxins and nitrogeneous wastes like ammonia and uric acid. Most aquatic insects excrete ammonia while terrestrial forms or those species that are water-limited excrete uric acid. In sap feeders (liquid feeders) the faeces is liquid like.

The insect’s excretory system