TOPIC 6: INSECTS

Insects are so abundant that they constitute about half of all known species.

Insects belong to a large phylum of vertebrates called Arthropoda.

Arthropods have a body divided into segments, have jointed legs and a skin cuticle made of chitin.

Other classes of arthropods include crusteceans like the crab, myriapods like the centipedes and arachnids like the spiders.

Insects have three body parts, three pairs of legs, one pair of antennae and a pair of compound eyes.

Examples of insects include cockroaches, houseflies, locusts, grasshoppers, butterflies, moths, mosquitoes, bees and termites.

A diagram of a human and an insect comparing the three main body parts head thorax and abdomen.

The Cuticle:

The cuticle protects the body, helps in muscle attachment and reduces water loss by evaporation.

The cuticle however reduces the rate of growth of insects. The insect can only grow after the cuticle has been shed or removed. This type of growth which is continually interrupted by formation of a new cuticle is called intermittent growth.

The process by which insects shed off their cuticle is called ecdysis or moulting.

The process of ecdysis:

A moulting hormone called ecdysone is released.

The inner layers of the cuticle are digested by enzymes secreted from the epidermis.

The produced fluid is absorbed back into the body.

Blood is forced to the thorax so that it swells and splits the old cuticle.

Another hormone called the juvenile hormone that inhibits moulting is released.

The insect then increases in size (grows).

A new cuticle is then gradually formed.

Breathing in insects:

Insects have a branching system of tubes called tracheae. The tracheae open to the outside by pores called spiracles. The tracheae divide repeatedly ending in trachioles.

The tracheae and tracheoles from the tracheal system of insects.

The entrance to the spiracles has muscles which control their opening and closing.

To take in air (inspiration), the muscles at the entrance to the spiracle relax.

The spiracle opens.

Air is conducted from the atmosphere to all living tissues of the body directly through the tracheae and the trachioles.

Oxygen diffuses from the trachioles into the tissues.

Carbon dioxide diffuses out of the tissue to the trachioles.

To take out air (expiration), the muscles at the entrance to the spiracles contract and the spiracle may close.

Carbon dioxide follows the same path out of the insect to the atmosphere but some of it can diffuse through the body surface.

When the insect is at rest, the spiracles are closed since little oxygen is required. Closing the spiracle conserves water and prevents entry of bacteria.

The tracheal system of insects is different from the respiratory system of vertebrates because in vertebrates oxygen is absorbed by gills or lungs and is then conveyed to the tissues by blood whereas in insects oxygen diffuses through the tracheal system directly to the tissues concerned.

The difference between ventilation in insects and ventilation in mammals is that in insects contraction of muscles at the entrance to the spiracle leads to expiration whereas in mammals contraction of the major intercoastal and diaphragmatic muscles leads to inspiration.

BLOOD IN INSECTS:

In insects blood is carried in a dorsal vessel (a heart of 13 chambers) and is emptied in the body cavity. Such a circulatory system where blood is not confined to the vessels but is instead poured out to the tissues is called an open blood system.

The blood of insects does not transport oxygen and in fact it has no red blood cells nor does it have haemoglobin. But it contains white blood cells or leucocytes.

Functions of blood in insects:

It distributes digested food.

It collects excretory products.

It has white blood cells for preventing infection.

It has a hydraulic function of pushing the cuticle off during ecdysis.

It also has a hydraulic function of pumping up crumpled wings of a newly emerged adult after ecdysis.

Response in insects:

Insects have fine bristles which respond to touch, vibration or chemicals.

The feelers or antennae are sensitive to chemicals and so they function in smelling.

Sight:

The compound eyes of insects are made of thousands of identical units called ommatidia each of which has its own lens system. Insects can thus not form an accurate image; instead a mosaic image is formed. However, this construction of compound eyes makes insects particularly sensitive to moving objects. For example bees are readily attracted to flowers being blown by wind.

Bees are sensitive to blue and violet but cannot distinguish red and green from black and grey respectively.

A butterfly can distinguish red, green and yellow.

In addition to compound eyes, some insects have simple eyes called ocelli. The house fly has three simple eyes, the grasshopper has one while the butterfly has three simple eyes on each side of the head.

Locomotion in insects:

In insects there is an exoskeleton so the muscles are inside the limbs.

Locomotion in insects is achieved by muscles contracting and pulling the limbs.

Most joints in insects permit movement in one plane i.e. hinge joints.

The legs and walking:

In walking, the insect moves three legs at ago while the other three are fixed to support the insect.

In the illustration below, the legs marked R1, R3 and L2 move first while L1, L3 and R2 move next

The legs are attached to the thorax by the coxa which is joined to the femur by the trochanter which could be comparatively small for example in cockroaches or long for example in houseflies.

In many insects the femur and the tibia have sharp spiky spines for defence.

The claws and pads or arolium of insects enable the insect to climb smooth surfaces like glass.

The pads secrete a sticky substance which enables the insect to walk upside down on the ceiling.

Movement of the leg at a joint:

- When the flexor muscle contracts, the extensor muscle relaxes and so the leg is bent at the joint.

- When the extensor muscle contracts, the flexor muscle relaxes and so the leg is straightened.

Wings and flight:

Some insects like the dragonflies have four wings which are not interlocked.

Other insects like the butterflies, moths and bees have four wings with the fore and the hind wings interlocked.

Yet others in the order Diptera like the houseflies and mosquitoes have one pair of wings with the second one reduced to mere halteres which help in balancing the insect during flight.

In the cockroach the fore outer wings are hard, opaque and narrow and are called tegmina while the inner hind wings are soft, transparent, broad and membranous.

There are veins in all the wings.

Flight is brought about by:

The direct flight muscles attached to the wings i.e. the elevator muscles attached to the tergum (roof of thorax) and the depressor muscles attached to the pleuron (side of thorax). These are mostly used by large insects like dragonflies.

The indirect flight muscles which are not attached to the wings i.e. the dorso-ventral muscles and the longitudinal muscles. These are mostly used by small insects.

When the elevator muscles contract, the depressor muscles relax.

At the same time the dorso-ventral indirect muscles are contracting while the longitudinal indirect muscles are relaxing.

So then wing be raised.

When the elevator muscles and the dorso-ventral indirect muscles relax, the depressor muscles and the longitudinal muscles will contract.

So the wing will be lowered.

Some insects can leap for example the grasshopper while others can swim for example the water beetle.

Feeding in insects:

Insects have three pairs of mouth parts.

Grasshoppers, locusts and cockroaches have the first pair of jaws called mandibles that cut pieces off vegetation which, with the help of the labium, are sent to the gullet by the help of the maxillae.

Other processes like the labial palps and the maxillary palps ensure that food is sent to the gullet.

A labrum is a lip that covers the top of the mouth parts.

The mandibles and maxillae of mosquitoes are shaped in such a way that they can pierce through the skin of mammals and plant tissues.

A mosquito pierces till it gets to a capillary for blood.

There are two tubes in the maxilla of a mosquito; one of which injects saliva into the host before the second tube sucks up blood. This prevents blood from clotting in the mosquito's proboscis.

Head of a mosquito Head of a housefly

The housefly sucks liquid but its mouth parts can not penetrate tissues. Instead the labium forms an enlarged proboscis.

The housefly puts its proboscis on the food and pumps saliva on it.

The saliva dissolves the soluble parts of the food.

The liquid formed is then sucked up and absorbed in the alimentary canal.

The butterfly uses the maxilla only in getting nectar from flowers. The maxillae form the coiled proboscis.

METAMORPHOSIS:

In most animals, the young resembles the parents but in some the young are completely unlike the parents until after they have undergone several changes in form. Such a developmental change in form from young to adult through stages that do not resemble the parent is called metamorphosis for example in insects and amphibians.

Complete metamorphosis:

Insects like butterflies, moths, bees, wasps, beetles and flies are called holometabolites because they undergo complete metamorphosis i.e. an egglarva which then changes into a dormant pupa; all stages quite different from the final adult stage. hatches into an active

Life Cycle:

Eggs may be laid in a damp place for example in houseflies; in sand e.g in locusts; underneath a leaf for example in butterflies; or in a dark crevice e.g in cockroaches.

The larva is the feeding and growing stage of an insect and in fact the larva eats voraciously. It is also the growing stage and it moults several times.

The pupa is usually in a dry place and is inactive but with tissue reorganization going on inside it.

The adult female lays eggs after mating with an adult male which deposits sperms inside the female (internal fertilisation).

The larva of flies has no legs and no head and is called a maggot.

The larva of bees and wasps have legs and a head and is called a grub.

The larva of a butterfly has a head, has legs and has prolegs and is called a caterpillar.

The larva of a mosquito has a large head and no legs but has a spiracle for breathing and is called a wriggler.

Incomplete metamorphosis:

Insects like grasshoppers, cockroaches, termites, weevils, dragon flies and cotton stainer bugs undergo incomplete metamorphosis (hemimetabolites) where there is no pupa stage. In fact the larva is only different from the adult due to lack of wings and is thus called a nymph.

The nymph goes through instar stages of moulting with wing buds developing before mature wings finally appear to make it an adult.



Importance of insects:

They pollinate flowers for example bees.

They act as predators of pests for biological control for example the ladybird which eats cotton stainer bugs.

They are eaten for protein for example white ants.

They are beautiful for example butterflies

They are pests on crops for example cotton stainer bugs.

They are vectors i.e. animals which carry disease-causing organisms to other animals or plants for example the mosquito which carries the malaria parasite.

They are food store destroyers for example the bean weevils.

They destroy cloth, furniture, books and buildings for example termites.

Controlling Insect Pests

Though insects can be important, some are also considered pests. Common insect pests include:

a. Parasitic insects (mosquitoes, lice, bed bugs).

b. Insects that transmit diseases (mosquitoes, flies).

c. Insects that damage structures (termites).

d. Insects that destroy crops (locusts, weevils).

Many scientists who study insects are involved in various forms of pest control, often using insect-killing chemicals, but more and more rely on other methods. Ways to control insect pests are described below.

  • Biological control of pests in farming is a method of controlling pests by using other insects. Insect predators, such as lady beetles and lacewings, consume a large number of other insects during their lifetime. If you add ladybugs to your farm or garden, then they will act like a pesticide, or insect-killing chemical.
  • Insecticides are most often used to kill insects. Insecticides are chemicals that kill insects. The U.S. spends $9billion each year on pesticides. Disadvantages to using pesticides include human poisonings, killing of fish, honeybee poisonings, and the contamination of meat and dairy products.

Lesson Summary

  • Insects are the most diverse group of animals on Earth.
  • They have segmented bodies with an exoskeleton, and relatively simple nervous, respiratory, and circulatory systems.
  • Insects are the only invertebrates to have developed flight.
  • Some insects, like termites, ants, and many bees and wasps, are social and live together in large well-organized colonies.
  • Insect movement includes flight, walking, and swimming.
  • There are two major groups of insects, the wingless and the winged, and these are subdivided into various orders.
  • Insects obtain food with the use of specialized appendages for capturing and eating.
  • Most insects can rapidly reproduce within a short period of time.
  • An insect can have one of three types of metamorphosis and life cycle.
  • Insects are beneficial both environmentally and economically.
  • Insect pests can be controlled with chemical or biological means.

Review Questions

Recall

  1. What are the three main parts of the insect body?
  2. Why is the insect's circulatory system said to be "simple"?
  3. What does metamorphosis mean?

Apply Concepts

  1. Describe the difference between the life cycle of a silverfish and the life cycle of a butterfly.
  2. What makes parasitoids especially effective at killing other insect pests?
  3. Describe what it means when an insect is said to be "social."

Critical Thinking

  1. Why do you think an exoskeleton allowed insects to better adapt to their environments than some other invertebrates?