Do you love catching butterflies? Have you read the story of The Very Hungry Caterpillar? Are you scared of spiders and cockroaches? Well, butterflies, caterpillars, spiders, cockroaches, and their many other friends belong to an animal group called Insects. Insects are some of the most common and amazing creatures on Earth. Spring, summer, and fall are filled with buzzing sounds and beautiful fluttering wings.
In this lesson, we are going to learn about insects - their body structure, basic internal anatomy, life cycles, and their strategies to survive in the winter months.
Insects are animals that have the following characteristics: no backbone, a three-part body, six legs, and antennae. As insects don't have a backbone, we can also call them invertebrates. Insects are a class of invertebrate animals that lies within a phylum called the Arthropods. Bees, butterflies, cockroaches, flies, dragonflies, mosquitos and ants are all insects. They have segmented bodies and legs, three pairs of legs, and usually have two pairs of wings.
Let's briefly explain an Anthropod. An "anthropod" is an invertebrate animal that has an exoskeleton, a segmented body, and jointed appendages. It includes the following families of organisms:
Insects can be distinguished from spiders and crustaceans by the number of pairs of antennae - insects have one pair of antennae whereas crustaceans have two pairs and spiders have no antennae. With respect to invertebrates, insects have a unique characteristic - the evolution of wings that allow flight, and this is believed to be a primary reason for the amazing success of the insect species on land.
The body is divided into three distinct regions - head, thorax, and abdomen. Each region is further divided into segments.
In general,
Insects are a diverse group and have evolved in many different forms. In the more advanced insects, segments may become fused together, particularly in the abdomen.
Below illustration shows the body structure of the insect:
There are three pairs of walking legs on the thorax, one pair to each segment. The legs are often modified to carry out a variety of tasks e.g. swimming or holding prey.
Below illustration shows a generalized structure of an insect leg:
Most adult insects have two pairs of wings, one on each of segments two and three. The wings are supported by a series of veins, the pattern of veins is an important way to classify insects.
Vision
The head bears a pair of compound eyes. These consist of a number of 'individual eyes' each of which produces a separate image. Hence the overall picture that the insect sees is made up of a series of dots. This is rather like a television picture, but with much poorer sharpness. This type of eye is very good at judging distance and movement. Hence insects that are active predators such as dragonflies have very well-developed eyes.
No. Spiders belong to the family Arachnids and Insects belong to the family Insecta.
Because of their shared ancestry, both spiders and insects have certain common characteristics. But, the two groups branched off many millions of years ago and developed many unique characteristics which make them different.
| Characteristic | Insects | Spiders |
| Number of legs | 6 | 8 |
| Body parts | Three main body parts: head, thorax and abdomen |
Two main body parts: cephalothorax and abdomen; head and thorax are fused to form 'cephalothorax' |
| Number of eyes | Compound eyes | Have several pairs of simple eyes with each pair adapted for a specific task |
| Antennae | Have two antennae | No antennae |
| Wings | Have wings | No wings |
Do you know a full-grown caterpillar has more muscles than a human being?
The internal anatomy of insects differs from vertebrates (including human beings) in many ways:
Digestive/excretory system
Like vertebrates, insects also have a complete digestive system consisting of a tube from the mouth to the anus, but it differs in a very important way. The insect's digestive system has three major regions - foregut, midgut, and hindgut.
The foregut and hindgut are lined with chitin, a polysaccharide that makes the exoskeleton of the insect. When an insect sheds its skin, it also sheds the internal lining of the foregut and hindgut. The gut fauna often lies in the hindgut (for example in termites). If the insect relies on gut microorganisms to aid digestion, the loss of the internal lining of the gut may become a problem. Therefore, gut fauna is replenished with every molt (shedding of skin).
Insects do not have kidneys. Instead, metabolic wastes are removed with the Malpighian tubules - which, like the posterior intestine, forms the primary system in insects for ionic, osmotic, and excretory regulation by which excretion products and toxic compounds are transported.
Respiratory (ventilation) system
Insects don't breathe as we do. They don't use blood to transport oxygen. They do not have lungs. Insects take in oxygen and expel carbon dioxide through holes in their bodies called spiracles. These holes connect to branching and interconnecting tubes, called tracheae. Insects can limit oxygen flow by closing their spiracles. In fact, one reason insects are so hardy is that they can close their spiracles and live off the oxygen they already have in their tracheae.
Whereas humans have one trachea, insects have a whole tracheal system that transports oxygen to all areas of their bodies and removes carbon dioxide. As the insect grows, tracheal tubes get longer to reach central tissue, and get wider or increase in number to meet the additional oxygen demands of a larger body.
Have you ever thought why can't insects grow as big as an elephant?
Because they wouldn't be able to get enough oxygen. Air penetrates the trachea by diffusion. Air can travel only up to the length of 1cm in such tiny tubes. So that is why insects cannot grow larger than a few centimeters across. Above this size, diffusion of oxygen into the body tissues becomes too inefficient for the insect to live. If insects were to become very large, they would have to develop lungs, gills, or something else. However, that hasn't yet happened.
Circulatory system
Like all arthropods, insects have an open circulatory system as opposed to our closed circulatory system. Whereas our blood is confined within blood vessels, insect blood called hemolymph flows freely throughout the body. They do not have veins or arteries. Inside their exoskeletons lies a fluid-filled body cavity known as the hemocoel. Inside this body cavity are the organs all suspended in the fluid hemolymph, which is synonymous with the blood of higher organisms.
Do insects have hearts?
Yes, insects have hearts. The heart is the organ known to pump blood. Unlike humans, they have a slightly different structure that does the pumping of blood all over their bodies. They have a long heart-like organ known as the 'dorsal vessel' in the abdomen that helps circulate the hemolymph through the body. The dorsal vessel is suspended in the hemocoel by muscular ligaments. Each chamber of the dorsal vessel comprises of the alary muscles that either contract or expand to control the flow of hemolymph. Meanwhile, the anterior portion of the dorsal vessel with no such muscles attached is called the aorta. The insect heart wall has various perforations known as ostia that functions as the passageways of hemolymph to enter from the hemocoel. Hydrostatic pressure created by muscle contractions helps push the hemolymph from one location to the next, helping it move to the head and the thorax.
The main drawback of the exoskeleton is that it can't expand with growth. In order to grow, the exoskeleton must be shed and a new one formed. The new one will be soft at first, so the body can expand before this new one hardens. The organism grows to fill the space created before molting becomes necessary again.
The process of molting is called 'ecdysis', and the stage between successive molts is called 'instar'. Once adulthood has been reached, growth stops and the adult insect doesn't molt again. This means that the stages occurring before the adult are the ones in which growth occurs.
There are two different types of an insect life cycle - incomplete metamorphosis and complete metamorphosis. Metamorphosis is a biological process that involves sudden and abrupt physical changes in an organism after birth.
Also known as hemimetabolism, this is shown by the less highly developed insects. The life cycle shows only three stages: EGG - NYMPH - ADULT
These insects start as eggs, which are usually very small. When the egg hatches, a larva or nymph comes out. Nymphs are just baby insects. Most of the time, the nymph looks similar to the adult, but it is smaller, may have different colorations, and does not have wings. The nymph grows through stages called instars, shedding its skin (epicuticle) at each stage (ecdysis). The wings develop during the nymph stages as wing buds. These grow larger at each successive instar. They are fully formed at the final molt into adulthood. Finally, it changes into a mature adult with wings. The wings, therefore, develop outside the body and the young resemble adults but have externally developing wings, and they undergo modest change between immature and adult, without going through a pupal stage.
Some insects nymphs are aquatic, which means they live in water. These nymphs usually have gills and look very different from the adults they will turn into. Nymphs that live in water are called naiads.
This life cycle has the disadvantage that both nymphs and adults often share the same food source. Therefore, they can be in direct competition with one another for food. The advantage is that the vulnerable pupal (chrysalis) phase is avoided.
Some insects that have a life cycle of egg-nymph-adult are cockroaches, dragonflies, and grasshoppers.
Also known as holometabolism, this is shown by the more highly developed insects. The life cycle shows four stages: EGG - LARVA - PUPA - ADULT
These insects start as eggs, which are very small. The egg hatches and a larva comes out. The larva looks like a worm and is in the growth phase. It eats to grow much bigger. It is generally very different from the adult. Usually the larva and adult use different food sources. Therefore, they are not in direct competition. This is a distinct advantage as more individuals of the species can be fed.
When the larva has grown it changes into a pupa. The pupa usually cannot move or eat. It is a stage of internal reorganization. There are no visible signs on the outside of the body as to the activity within. Because of this, the pupal phase is called the 'resting' stage. It is a special time when the insect is changing into an adult that will look very different from the larva or the pupa. During the pupal phase, the internal organs are broken down, forming a 'soup'. This 'soup' then acts as food for special growth buds to develop. These form the adult body. When the reorganization is complete, the adult is ready to emerge. When outside conditions are suitable, the final molt occurs and the adult insect emerges. Moth pupa stays inside cocoons. When the cocoon opens, the adult insect comes out. Wings develop internally during the immature stage, just before the final molt occurs.
All butterflies have "complete metamorphosis." To grow into an adult they go through 4 stages: egg, larva, pupa, and adult. Each stage has a different goal - for instance, caterpillars need to eat a lot, and adults need to reproduce.
Below illustration shows the complete metamorphosis of a butterfly:
Other insects that show complete metamorphosis are beetles, bees, wasps, ants, moths, and flies.
Class Insecta is divided into 2 subclasses, specifically, Apterygota and Pterygota.
Apterygota - They are insects that never had wings at any time in their evolutionary history. While some other insects, such as fleas, also lack wings, they descended from winged insects but have lost them during the course of evolution. Examples: silverfish, firebrat, jumping bristletails.
Pterygota - They are a subclass of insects that includes winged insects. It also includes orders that are secondarily wingless (that is, insect groups whose ancestors once had wings but that have lost them as a result of subsequent evolution).
Within the Pterygota the Subclass is divided into two further divisions depending on the type of metamorphosis exhibited by insects in each group:
Kingdom - Animals
Phylum - Arthropoda
Class - Insecta
Orders - Below are the 9 orders of Insects
1. Beetle Order - Coleoptera
2. Mantid & Cockroach Order - Dictyoptera
3. True Fly Order - Diptera
4. Mayfly Order - Ephemeroptera
5. Butterfly & Moth Order - Lepidoptera
6. Ant, Bee & Wasp Order - Hymenoptera
7. Dragonfly Order - Odonata
8. Grasshopper & Relatives Order - Orthoptera
9. Stick & Leaf Insect Order - Phasmida
Come winter, and we don't see any flies buzzing around, spiders spinning their webs, or ants foraging for food. Do you ever wonder where do all these bugs disappear in the wintertime?
Being cold-blooded creatures, insects are vulnerable to the cold temperatures of winter. Not only does the cold slow them down making them easier prey for hungry birds, but temperatures below zero can kill. To survive the winter months, insects have different strategies. The process by which an insect passes the winter season is called overwintering.
Migration - One way to avoid cold weather is to migrate to a warmer climate and return after winter. The best example is the Monarch butterfly in North America. Monarch butterflies migrate south from North America every year and overwinter in Mexico or California. In spring, these migrate back again.
Hibernate - Many species of insects hibernate through the winter months. But, only adult insects can hibernate. Some hibernating bugs burrow down into the soil or leaf litter. This helps them avoid not only the cold, but also the chilly winds and the beaks of hungry birds. Examples of hibernating bugs include ladybirds, outdoor cockroaches, certain species of wasps, and beetles. Honey bees also hibernate in their hives during the winter, forming heat-generating clusters when temperatures fall.
Overwinter in different lifecycle stages - For many insects, certain stages of their lifecycle allow them to overwinter through the cold months. For instance, they may overwinter as larvae, nymphs, pupae, or even eggs.
Overwintering as larvae. Many insects successfully pass the winter as immature larvae. The protection of heavy covers of leaf litters or similar shelters protect the wooly bear caterpillar, while other insects replace the water in their bodies with glycerol, a type of antifreeze. Some grubs simply burrow deeper into the soil to escape the cold.
Overwintering as nymphs. Not many insects are active in the winter, but the nymphs of dragonflies, mayflies, and stoneflies live in waters of ponds and streams, often beneath the ice. They feed actively and grow all winter to emerge as adults in early spring.
Overwintering as eggs. Lesser numbers of insects lay eggs that survive the winter. The most prominent insects in this category are Praying Mantids, and the destructive Corn Rootworms.
Overwintering as pupae. Some insects overwinter in the pupal stage, then emerge as adults in the spring. Moths in the Silkworm family, Saturniidae may be found attached to food plant branches as pupae in the winter.
Freeze tolerance
Some insects can survive ice formation within their tissues.
- Freeze tolerant insects are those that can survive by being frozen solid. They can control where ice crystals are formed within their bodies, so that ice crystals do not damage the cells and organs. When the weather gets warmer the crystals melt and the insect becomes active again. This is used in really cold areas.
- Freeze intolerant insects are those that use special "anti-freeze" chemicals to stop themselves from freezing. These anti-freeze chemicals work with other components of the insect's body fluids to prevent ice formation inside the body. This is found in cool to mildly cold climates.
Insects exhibit two types of behavior - innate and learned.
Many insects exhibit “social” behaviors (e.g. feeding aggregations, parental care of the young, and communal nest sites). All termites, ants, and various bees and wasps are the insects that best exhibit social behavior. Eusociality is an extreme form of social behavior found in just a few types of insects and is characterized by the following:
Insects can communicate in various ways. For example, ants release hormones called 'pheromones' that are sensed and responded to by other ants. Have you noticed how a group of ants is walking in a straight line? This is because the first ant that discovers the food leaves a trail of pheromone which is sensed by other ants who then follow it to reach the food. Another interesting communication method is the honeybees' waggle dance. When a worker bee discovers a good source of nectar or pollen (note the pollen spores dusting this bee's back), she will return to the hive to perform a waggle dance to let her nestmates know where it lies.
Usually, warm weather signals insects that cause allergies from stinging and biting. There are some other bugs that cause an allergic reaction like asthma without biting or stinging you.
Here are some different kinds of insects that can cause an allergic reaction:
1. Stinging insects - When they sting you, they inject a toxic substance called venom. In some people, this venom can cause a mild reaction that wears off within a few hours or days; in some other people, this may cause a life-threatening reaction. Examples include wasps, yellow-jackets, bees, and hornets.
2. Household pests - This includes cockroaches and dust mites that are responsible for allergy and asthma. Unlike cockroaches, dust mites aren't visible to the naked eye.
3. Biting insects - Most common examples of biting insects are mosquitoes, bedbugs, fleas, and flies. Insect bites may cause pain, itching, redness and minor swelling in the area around the bite. Insect bites are rarely life-threatening.
Signs of an allergic reaction to insects
The normal reaction to an insect sting or bite is pain, redness, itching, and minor swelling in the area around the bite or sting. This subsides within a few hours or days. Some insects like cockroaches or dust mites that don't sting or bite cause a different kind of allergic reaction. The person may cough, sneeze, or get itchy eyes, mouth, throat, nose, or stuffy, runny nose. These symptoms are similar to that of a common cold. In case the person is asthmatic, it may trigger an asthma attack.
In some people, insect bites or stings may cause a life-threatening allergic reaction (anaphylaxis). If these symptoms are not treated immediately, it may lead to death. Some symptoms of life-threatening allergic reaction are:
A person may react to insect venom causing a toxic reaction. The symptoms of a toxic reaction are similar to those of an allergic reaction. These include nausea, fever, seizures, dizziness, fainting, shock, and death.
Poisonous Insects
The order Hymenoptera includes families of venomous insects, known as honeybees, bumblebees, wasps, hornets, yellow jackets, and ants. Female insects have venom located in their posterior abdomen. Bites and stings from this group may cause allergic reactions and sometimes rapid death from anaphylactic reactions.
