INSECT ORGANIZATION: STRUCTURE AND FUNCTION

These animations have been prepared with the objective of serving as teaching tutorials to assist students in conceptualizing the complex dynamics of physiological processes -- especially as they relate to insects. I am sharing these animations with my colleagues that wish to link to them, or refer them to their students for the purpose of illustrating course lecture topics.

As a courtesy, to help me evaluate the usefulness of the animations, if you use them as an educator in your teaching or as a student in your studies, please send me an e-mail at: llkeeley@tamu.edu, and let me know if they are helpful. I would also point out that these are works in progress, and will be updated occasionally with more, or remodeled, scenes to improve their usefulness for instruction

Instructions for playing the animation

The movie starts automatically on opening and the title page takes several seconds to play.

The control panel embedded in the movie allows you to control any scene.

The MENU button on the control panel allows you to jump to any scene without viewing the preceding scenes.

The END button takes you to the end of present scene and the next scene starts, so you can jump from scene to scene.

Scene 1: Exterior Organization

The outside of the insect body is a hard covering called the exoskeleton. The exoskeleton is made of protein and chitin. The exoskeleton acts like both a skin and a skeleton.  It gives the insect form like a skeleton, but like a skin, it protects against water loss, injury and infection by micro-organisms. 

The body is divided into three major sections: the head, thorax and abdomen.  Each of these sections is further divided into segments.

 Insects have six legs.  One pair of legs is attached to each of the three segments that form the thorax.

Only adult insects have wings.  Flies have one pair of wings attached to the thorax.  All other insects have two pairs of wings attached to the thorax.

The antennae are olfactory organs for detecting odors.

The compound eyes are the main organs for vision.

The spiracles are openings in the side of the insect that allow air to enter into the respiratory system

Scene 2: Respiration

The respiratory system carries oxygen to the cells. 

Spiracles are openings in the body wall attached to a network of silvery-white  tubes called trachea. Air enters the spiracles and trachea carry oxygen to the individual cells. Blood does not carry oxygen in insects, only the tracheal tubes.

Scene 3: Fat body

The major tissue for metabolism and storage of nutrients is the fat body. Most of the fat body is in the abdomen surrounding the gut where it can quickly take up absorbed nutrients.  Some fat body is scattered throughout the body, near other tissues that use its products. The metabolic function of the fat body is similar to that of the vertebrate liver.  Like the liver, it stores excess nutrients, and it synthesizes proteins, lipids and carbohydrates that circulate in the insect blood.

Scene 4: CNS

The nervous system conveys and integrates information about the internal and external environments and determines behavior. The insect central nervous system consists of the brain and the ventral nerve cord.

The brain consists of dumb-bell shaped visual lobes with nerves that project outward to the retina of the compound eyes. 

The ventral nerve cord consists of a series of ganglia – usually one per segment.  Ganglia are thickened regions of the nervous system that contain the cell bodies of the nerve cells. Ganglia are connected by paired nerves called interganglionic connectives.

Scene 5: Endocrinology

Like other animals, insects possess an array of hormones that regulate their diverse physiological and biochemical processes.

Hormonal sources include: The neuroendocrine system, the corpora allata, the prothoracic glands, epitracheal glands.  Other endocrine cells are found scattered in various tissues, for example the gut and ovaries.

The neuroendocrine system consists of nerve cells that secrete hormones located in the ganglia that comprise the brain and the ventral nerve cord. Neurohormones are the master regulators and control most physiological and metabolic processes including secretion of the hormones that regulate molting, metamorphosis and reproduction. Corpora cardiaca are major neuroendocrine structures attached to the brain.  The corpora cardiaca store and secrete neurohormones synthesized by brain neurosecretory cells. The corpora cardiaca also contain intrinsic neurosecretory cells that synthesize and secrete neurohormones.

Corpora allata secrete juvenile hormone and are attached to the corpora cardiaca. Juvenile hormone prevents larval insects from undergoing premature metamorphosis into adults at each molt. Juvenile hormone also stimulates egg formation in most adult female insects.

Prothoracic glands are a grapelike cluster of cells surrounding the trachea in the first thoracic segment. Prothoracic glands secrete ecdysone, a hormone that stimulates the molting events necessary for insect growth. Prothoracic glands deteriorate in adult insects because adults no longer molt. In some species, for example mosquitoes, ecdysone is produced by ovaries in adult females and stimulates egg formation.

Like the prothoracic glands, epitracheal glands are groups of secretory cells associated with trachea. They secrete hormones that regulate molting behavior.

Endocrine cells are also found in the gut wall and may affect feeding activity.

Scene 6: Circulation

The circulatory system transports essential metabolites from the fat body to the cells, carries wastes to the excretory system and provides immunity to harmful organisms.

Insects have a simple, open circulatory system.  The circulatory system consists of a dorsal vessel running the length of the body.  The dorsal vessel is divided into a posterior heart that contains intake valves called ostia and an anterior aorta.  The open space of the body is called the hemocoel.  The hemocoel is filled with hemolymph. Hemolymph is pumped forward by the heart into the aorta.  Hemolymph is pumped out of the aorta into the head and flows from the head back through the open hemocoel. Hemolymph re-enters the posterior heart through the ostial valves. The hemocoel is always full of hemolymph and the heart ensures its mixing. 

Auxiliary pulsatile hearts at the base of the antennae, legs and wings pump hemolymph through these appendages.

Scene 7: Digestion – Excretion

Insect digestive systems are adapted to the diversity of the food, but they have common structures. The digestive system is divided into three main sections.

The foregut conducts and stores food prior to digestion.  Food moves from the foregut into the midgut where it is digested and the nutrients are absorbed into the body. 

Malpighian tubules are not part of the digestive system.  Malpighian tubules are the insect excretory organs and absorb all circulating metabolites and waste products from the hemolymph. 

Undigested food from the midgut and Malpighian tubules excretions are passed into the hindgut.  The hindgut absorbs usable metabolites into the hemolymph.  Waste products are excreted.

Scene 8: Reproduction

Most insects reproduce sexually and lay many eggs. 

The female reproductive system consists of paired ovaries made up of separate tubules called ovarioles.  Ovarioles are divided into chambers called follicles.  The follicles contain oocytes that are becoming mature by depositing yolk.  Mature, oocytes are present in the basal follicle  Mature oocytes are chorionated in the follicle, then passed into the lateral oviducts to the common oviduct.  Sperm are released from the spermatheca to fertilize the egg as it passes through the common oviduct for oviposition.   Accessory glands secretions assist egg laying.  These products may be venoms, as in the case of wasps, or cement to fix the egg to the oviposition site.

Testes also consist of follicles where the sperm are matured as they progress from the tip to the base of the follicle.  Mature sperm pass from the follicle to the vas deferens, into the seminal vesicle.  Male accessory glands produce products that mix with the sperm to protect and preserve the sperm.   Some insect species produce a spermatophore that encloses the sperm and is passed to the female during mating.  Other insects transfer sperm without a spermatophore.  Accessory gland secretions may prevent the mated female from mating again by forming a temporary plug or by transferring chemicals that suppress mating behavior.

Scene 9: Flight Muscles

Insect flight occurs by muscles that deformation of the rigid, thoracic box.  Contraction of longitudinal flight muscles causes the thorax to arch and flips the wings out over a fulcrum point into the downstroke.  Contraction of indirect, dorso-ventral flight muscles snaps the thorax back and flips the wings back in over the fulcrum into the upstroke. 

The up-down snapping action of the rigid thorax wall can occur rapidly producing 500-1000 wingbeats per second.  Muscles attached directly to the base of the wing cause wing folding.