What is a Insect
What is an insect?
Insects are one of five groups that belong to a phylum known as the Arthropoda meaning animals with jointed legs. Arthropoda is a phylum of animals subdivided into five classes. Insects are one of these classes of arthropods.
The arthropods are a vast and diverse phylum. One group, the Trilobita, are now extinct and judging from the fossil record these trilobites were once numerous and large in size. The four remaining groups are the Insecta, Crustacea (lobsters, crabs, shrimps, prawns, and barnacles); Myriapoda (millipedes and centipedes) and Chelicerata (which include the order known as Arachnida – spiders and scorpions).
There are more than a million species of insects although the majority of them are beetles. A list of insects would include silverfish, flies, mosquito, bees, ants, fire ants, aphids, termites, wasps, earwigs, bed bugs, centipedes, roach or cockroach, German cockroach, Japanese beetle, fleas, millipede, fire ants, butterflies, and moths. The list is quite endless. There are so many types of insects and many are still not known to science. We shall see later that termites, wasps, ants, and bees all belong to the same group of insects known as the Hymenoptera.
Without insects, the ecosystems of the world would collapse very quickly although many insects are considered to be pests. Insects such as locust destroy crops, mosquitoes carry the disease of malaria, whilst some insects such as bed bugs suck blood. However, insects are food to many animals such as birds, amphibians and reptiles. Some mammals such as anteaters, hedgehogs, moles, shrews, and desmans eat insects and are known as insectivores. Some insects such as wasps eat other insects so play an important role in natural pest control.
Most importantly of all is the service that insects provide in pollinating flowers. Pollinating insects particularly bees play a crucial part in food production. If flowers are not pollinated there will not be any fruit, nuts, and vegetables to eat for all living creatures. When pesticides are used to control insect pests birds will then die off. Wildflowers that rely upon the insects to be pollinated die too.
Insects sting when they feel threatened or when they want to feed upon blood causing a painful bite wound. Stinging insects can be avoided by keeping calm around them and using natural insect repellents such as citronella, peppermint, and lavender oils. Blood-sucking mosquitoes also avoid these natural flower oils too.
Biological pest control whereby another organism is used to destroy another usually harms the environment and can make matters worse. Another organism moved to a new location will not have their usual predators, pests, and challenges and will breed unabated. The new organism will then become a pest in itself. For example, the Harlequin ladybird is an invasive species. It has spread across the UK in just one decade. The Harlequin ladybird is responsible for the decline of seven native species of ladybirds. The Harlequin ladybird is a cannibal eating other ladybirds. It has no predators of its own so has bred out-of-control. Another dangerous invasive species is the bee-eating Asian hornet that has now become established in France.
Before discussing whether a bee is an insect it is worth considering what actually is an insect. Sometimes people spell insect incorrectly and spell them as insexts Some people ask if insects are bugs. A ‘bug’ is an informal word meaning an insect, bacterium or an infections, e.g. ‘tummy bug’. The word bug has no meaning in science but is a colloquial term. Movies such as ‘Bugs Life’ is fun to watch but it is fictitious.
There are some general features which all insects share. Insects have three pairs of legs so they are also known as ‘hexapods’ which means ‘six legs’. An adult insect’s body is divided into three parts – the head, thorax, and abdomen. An adult insect’s head has a pair of compound eyes and in the case of the bee, has three ordinary eyes on its head called ‘ocelli’. Adult insects also share common features such as a pair of antennae or palps on their heads. Mandibles, maxillae and a lower lip of ‘labium’ make up the mouthparts but these vary greatly in design according to what the insect feeds upon. Bees, for example, have a specialized long tongue used to reach deeply into the nectaries of flowers to drink nectar. Nectaries are found in flowering plants only.
Adult insects have three thorax segments or different regions called tagmata. Each of these three thoracic segments has a pair of legs. Insects usually have two pairs of wings on the two segments furthest away to the insect’s head.
An adult insect abdomen has 9-11 segments. There are no legs or wings joined to their abdomens. At the furthest segment is the insect’s gut opening or anus. Malpighian tubules carry waste to the rectum and expel waste as uric acid which is insoluble in water. This, therefore, enables the insect to conserve water very efficiently.
An insect has what is known as an ‘exoskeleton’ and means that its skeleton is on the outside of its body instead of the inside. Other animals such as mammals have their skeleton within them and are therefore said to have an endoskeleton. The main purpose of the skeleton is to provide a framework for the body, to protect internal organs and to provide something in which appendages can attach to allow for movement. The insect does this very well with its exoskeleton.
However, the only problem for the insect is that as it grows it will need to shed its skeleton and grow a new one leaving it very vulnerable during this phase of ‘molting’ as it is called. It is not unusual to find discarded exoskeletons of flies. An insect’s cuticle is completely waterproof and covered with an impenetrable wax layer. This tough cuticle is made from a hardened protein called chitin. Chitin is a polysaccharide carbohydrate strengthened with nitrogen atoms. Each plate of the cuticle is called sclerites which are linked with flexible membranes to allow movement.
Insects breathe through holes in their cuticles. These holes are called spiracles. Beneath the insect’s cuticle, there is a network of breathing tubes called trachea. This trachea is a continuation of the cuticle and made of the same material. When an insect molts it also needs an enlarged and functioning tracheal system to remain in place.
An insect has a very efficient body in many respects. The tracheal system is designed to take in oxygen, allow gas exchange removing carbon dioxide as the waste product of respiration. The insect is able to breathe and respire with the minimum of water loss. Only some spiracles on the cuticle allow water loss but these are controlled by valves. The insect is so good at conserving water when necessary that it enables them to be able to live in the driest and arid desert conditions surviving in extreme places. However, as we shall see elsewhere the honey and bumble bees prefer more temperate climates and are not found in the tropics or in very cold places.
How are insects able to fly?
All flying animals need wings to fly. Flying insects also need wings that enable them to fly or hover around feeding sources. Isaac Newton Third Law of Motion says that for every action there is an equal and opposite reaction. For every force, there is an equal and opposite force. Wings provide a force called the lift, an upward force that opposes gravity. Wings also provide thrust which is a forward force that opposes the backward drag which always accompanies lift.
For most flying animals the way that they obtain lift and thrust depends upon their size and this will determine the design of their wings. Lift and accompanying thrust is provided in any wing design so long as air moves faster over the upper surface of the wing than the lower surface of the wing.
The appearance of a bird’s wing in cross-section has a shape known as an ‘aerofoil section’ meaning that it has a curved upper surface providing a longer path for the air to flow over it. The air that flows underneath the wing, therefore, reaches the back of the wing first. On reaching the end of the underneath of the wing air is sucked upwards to meet the air-stream over the upper surface. This differential in airflow causes a vortex to form a what is known as the ‘trailing edge’. This vortex causes the air on the upper surface of the bird’s wing to accelerate towards it. This provides lift. Lift as said earlier, is always produced when air moves faster over the upper surface than the underneath surface of the wing. Once a bird is in flight it can take advantage of air thermals and glide when it is not flapping its wings. Marvellous design features of a bird’s wing mean that it can maneuverer easily in the air.
Insects that fly do so quite differently to birds mainly because they are so much smaller. An insect has a problem in obtaining lift because there is little difference in air flow over wing surfaces. Insects overcome the problem of lift by flapping their wings very fast. Insects also are able to turn their wings on the upstroke as well as turning their wings on the down stroke. This flying motion provides a series of vortexes that cause air flow to accelerate over the upper wing surfaces and create lift. In addition, hovering which is something bees do frequently is skilled and strenuous because there is little air movement provided by moving the whole body forward.
A bumblebee flies rather like a helicopter performing a complex figure-eight motion with its wing tips twisting through the air using a complex stroke. A bumble bee will use up to 200 wing beats per second proving sufficient lift to forage and hover over flowers.
Most adult insects can fly but some insects seldom fly. The honey bee queen, for example, will only fly a few times in her lifetime – once on her nuptial flight when she mates for the one and only time. She may fly again in her lifetime if she moves the colony causing a swarm. Other insects such as ants even remove their wings once they have made their nuptial flight.
Insects flap their wings by using muscles in the thorax segment of their bodies. There are three types of muscles used in insect flight: direct muscles, indirect muscles, and asynchronous (myogenic) muscles. Only the dragonflies use direct muscle motion. Indirect, asynchronous muscles are used by Diptera (flies) and Hymenoptera plus a few others. These muscles are the fastest and most flexible known insect muscles which enable insects to beat their wings as much as a 1000 times per second. The Hymenoptera as we shall see later is the family of insects which include bees. A bumblebee beats its wings at a rate of 130-240 beats per second.
Insects that beat their wings so fast such as the bees use an enormous amount of energy. Insects can absorb all the oxygen that they need to create energy through the spiracles (holes) in the cuticles of their bodies. Unlike most other animals, oxygen is delivered straight to where it is needed without being carried dissolved in blood supply. Insects do not need lungs to ventilate oxygen around their bodies either.
Insects may be limited in the amount of fuel that they have to power their flight muscles. Insects use a sugar called trehalose as fuel. Trehalose is a disaccharide sugar. Vertebrates rely on glucose as fuel which is a monosaccharide. The advantage of trehalose is that being a disaccharide it is able to be stored more efficiently than glucose. The insect can, therefore, carry more fuel in its small body.
Insect life cycles
Insects start their life as an egg which hatches into larvae. The larvae stage of an insect will then enters the pupae stage and then undergo metamorphosis before becoming an adult. Different types of insects vary in how these different stages occur. Broadly speaking there is two main divisions in which all winged insects fall. There is the Exopterygota which is an incomplete metamorphosis – the young can look much like the adults. There is the Endopterygota which means a complete metamorphosis. Bees, for example, will be this latter group since the queen lays eggs which develop into helpless larvae which then enter a pupae stage where the adult structures develop. This division is also important to remember because it helps entomologists, those who study insects, to classify insects into orders, families, genera, and species.