DRAFT: Insect Anatomy: Structure & Function

Insect Anatomy: Structure & Function

Introduction

The insect has evolved to become, arguably, the most successful organism on earth. This could be linked to mainly two attributes, wings and size. The ability to fly has allowed the insect to spread throughout every point on earth and to escape predators easier. Their size also helped with better disguise from predators. It allowed them to hide in places large predators such as birds couldn’t get in.

Insects have evolved in every temperature and every environment. They can survive in any type of weather. Insects live in water and on land. They can fly and they can swim. They eat everything, from the carnivores to the detrivores. They are the number one pollinators and the number one recyclers. They can devastate a human population or save it. This is all due to their diverse anatomies which have adapted to do anything.

Throughout this article I compare human and insect anatomical structures and function. In fact, I know there are some parts where I give human names to insect structures even though they are distantly related. Nevertheless, they are. John and Jane Balaban recently reminded me of anthropomorphism. Yes, I have committed the crime, but only with good intentions. I believe that by giving human names to non human things insects become easier understood and better appreciated.

This article is divided into two sections and is outlined as followed:

1. External Anatomy

Integument
Locomotion
Sensory Organs
Mouthparts

2. Internal Anatomy

Digestion and Excretion
Muscular System
Nervous System
Respiratory System
Reproductive System

External Anatomy

Integument

Exoskeleton

The insect integument like the human integument is the outer layer. In humans it consists of skin glands and the skin and in insects it also consists of glands but differs in the exoskeleton. Unlike humans, insects do not have bones and their only skeleton is the exoskeleton. The exoskeleton is the hard outer shell of the insect that provides support and protection. It also helps to regulate water loss and limit the size of insects. The insect exoskeleton is made up of a compound called chitin. Chitin is a close chemical relative of glycogen (basic human fuel) and cellulose (what wood is made of).

Three basic parts make up the exoskeleton of insects and they are the epidermis (a one celled layer), basement membrane and the cuticle. Spread between the epidermal cells are dermal gland cells, some of which secrete part of the cuticle, defensive substances, pheromones or silk. Figure 1 shows the layering structure of the exoskeleton components.

In the cuticle, using a light microscope, striations can be seen, which in some insects can be used to estimate the age of the insect (Tyndale, Biscoe, 1984). It is in this part of the integument where most chitin concentrations occur. The cuticle consists of three different layers and they are the endocuticle, the exocuticle and the epicuticle. The epicuticle is the outermost of the cuticles and the endocuticle is the innermost. During molting, the endocuticle is digested while the exocuticle is not. Therefore, the exuviae is solely the exocuticle.

Color of an Insect

Like humans, insects have pigments that give them color. In humans the compound is keratin, but in insects there are many different colors that give different colors. They are as follows:

melanin: yellow, brown, black
carotenoids: red, yellow, orange (hence the name carrot)
pterins: red, yellow, white
ommochromes: red, yellow, brown
anthraquinones: red, orange
aphins: red, yellow, orange and only confined to aphids
chlorophyll derivatives: green. Chlorophyll is the compound that gives leaves their green color.

Since many insects can change color, they can control which pigment is expressed more. But not all do it this way. For example, the Hercules Beetle (Fig 3), Dynastes Hercules, can appear yellow green or black depending on the moisture in the atmosphere. Their epicuticle in their elytra is transparent, beneath it is a spongy yellow layer and beneath that is a black cuticle. During dry conditions, the spongy layer is filled with air and reflects back a yellow color. During humid conditions the spongy layer is filled with water, absorbs light and so the black is reflected. This is believed to be a camouflage adaptation since humid conditions are mostly at night. Figure 3

Locomotion

Legs

Each leg pair is attached to a segment of the thorax. The front pair to the prothorax, the second pair to the mesothorax and the third to the metathorax. Most insect legs consist of six segments (Figure 2), and they are as follows in order, from the thorax to the tip of the leg:

coxa: analogous to the human hip
trochanter: the rotating part of the leg, analogous to the ball joint of the hip in humans
femur: analogous to the human femur
tibia: analogous to the human tibia
tarsi: Consist of multiple segments (tarsomeres) depending on the insect and are analogous to the human phalanges
pretarsus: The ‘hooks’ on the end of each leg and are analogous to the human metaphalanges

The leg is the primary source of locomotion in most insects and all adult insects possess them. Many larvae have legs but some don’t such as maggots. The insect leg has adapted to perform almost every function. For example the front legs of a praying mantis or a mantid fly are used to catch prey. The front legs of the assassin bug, Zelus, has ‘sticky ends’ to hold on to prey. The legs of the mole cricket are adapted to digging. Other Orthoptera have hind legs adapted for jumping, which is the same for all fleas. The hind legs of the water boatman or the diving beetle have adapted to swimming. The legs have hairs on them to increase the push during swimming like a row of a boat (Fig 3).

There are other adaptations of the legs that don’t have a locomotory function. The legs of the honey bee, Apis mellifera for example have “pollen baskets”, corbiculum, which consist of two rows of hair that maximize the amount of pollen that could be attached to them at a single stop (Fig 4). The tarsi of male diving beetles have suction cups on them to hold on to the female during mating, which takes place in water (Fig 6). Grasshoppers and other Orthoptera have tympanic organs, or organs for the detection of sound, on their legs. Other Orthoptera have organs to produce sounds on their legs when rubbed on their wings such as some crickets. Many flies have ‘taste hairs’ on their legs called setae and that is why you will see a fly walk around food before sticking its tongue out. Figure 4, Figure 5, Figure 6

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Wings

The wings are actually two layers of epidermis covered by a thin layer of cuticle. Through the wing run blood vessels. Most of the orders of Pterygota have four wings. Some, like the Diptera have only two wings. Each wing pair is attached to a segment of the thorax, more specifically the last two pairs, the mesothorax and the metathorax. Since wings are organs, they need blood (which is not the same as human blood as we will see later on) to function. Therefore, all insect wings have blood vessels running through them. Some orders like the Odonata have very complicated wing venation and some like the Diptera or Hymenoptera have much simpler venation.

Like legs, there have been all types of modifications in the wings over time. The wing, is obviously used for flight but some insects use them for other functions too as we will see. In the Coleoptera the forewings have become very hard to form a structure called an elytron (pl. elytra). The elytra are a covering for the soft, delicate membranous pair of wings beneath them that are used for flight. Hemiptera only have partially hardened elytra called hemelytra. The wing can also be used to make sound as is the case with crickets. Diptera have only one pair of wings but have much smaller, circular structures called halteres in the place of the hindwings. It is believed that they are used to increase the stability of the flight of them.

Sensory Organs

Eyes

All insects have eyes which are used for vision. Some insects have compound eyes only, simple eyes, called ocelli and compound eyes, or only ocelli. The number of ocelli varies with the different types of insects but are usually three. They are usually located between the compound eyes on the dorsal surface.

Compound eyes are present in most adult insects and many immature insects, but are not present in many immatures. In these larvae they instead have structures called stemmata which are simple, simple eyes and function as light receptors. Many primitive insects, soldier termites and some species of fleas lack compound eyes.

Antennae

The antennae are sensory organs located on the dorsal surface of the head in all insects. Antennae sense smell in insects. They are composed of different segments and are moved by muscles in their base. A typical antennae is composed of four main parts:

an antennal socket is where the antennae attaches to the head
an antennal scape is the first segment of the antennae attached to the socket
a pedicel the first true antennal segment and contains Johnston’s organ in most insects
a flagellum is the rest of the sensory part of the antennae.

There are many types of antennae and the different types are mostly seen in the beetles. However some distinctive types are listed:

filiform (Grasshoppers)
setaceous (Dragonflies)
plumose (Male mosquitoes and midges)
stylate (Horseflies & deerflies)
geniculate (some Hymenoptera like bees and ants)

Sound Producing & Perceiving Organs

Hearing is primarily important in detecting predators that use sound to hunt (e.g. bats) and detection of a mate (e.g. cicadas & Orthopterans). Three main external organs function in the perception of sound and the are:

tympanic organs
trichoid sensilla
Johnston’s organ

Tympanic organs are the most basic of the sound organs. They are located in insects that can perceive sound. In the longhorned grasshoppers and crickets (Tettigoniidae and Gryllidae) they are located on the tibia of the front legs and in the short horned grasshoppers (Acrididae), they are located in the first abdominal segment. In noctunid, pyralid moths and cicadas the organ is in the abdomen. In most of these insects, except for the Orthopterans and cicadas, the organs are there to detect the ultrasound waves of bats. The tympanic organs of cicadas and Orthopterans are used for mating. That is when the chirping sound of Orthopterans and the constant loud buzzing noise of cicadas is deected by the female, or you.

It has been demonstrated that both Johnston’s organ and trichoid sensilla have other functions besides the detection of sound. Trichoid sensilla are locate at the pores of some sensitive hairs called setae. Johnston’s organ is located between the pedicel and the third segment of the antennae in most insects. It is most advanced in mosquitos and midges (Culicidae & Chironomidae). The mechanism and function is not known too well, and what is known is too detailed for me to include, so I have chosen to only identify them.

Mouthparts

A generalized insect mouth contains the following structures:

an “upper lip” called the labrum
a “tongue” called the hypopharynx
”teeth” called mandibles and there is only a pair in insects that have them
”jaws” called maxillae that aid in grasping of food
a “lower lip” called the labium

The mouthparts can be split into two divisions depending on the method of feeding of the insect. The chewing type are called mandibulate and the sucking are called haustellate. Like every other structure in insects, the mouthparts have evolved to perform many different functions.

The mandibulate mouthparts are generally for purely chewing and grinding as in grasshoppers. However, in ants and some beetles the mandibles are used to carry objects and to cut. The mandibles of the male Dobson Fly which many people fear are only used to grasp the female during mating.

Many insects that have sucking mouthparts have sharp, needlelike structures called stylets, as in the horseflies and lice. They are used to penetrate the layer the insect is trying to feed beneath. However, other insects that have haustellate mouthparts lack stylets. Insects such as butterflies and moths have a single structure called a proboscis since they do not have to penetrate anything to suck nectar up. The non biting flies such as the common housefly feed by a method called sponging. These flies stick their hypopharynx out and excrete saliva through tiny pores on the bottom of the tongue. Then they suck up the food, which is mostly liquid or partly digested solid, through the tiny pores so next time a fly lands on your burger, you might want to throw it away :).

Internal Anatomy

Digestion & Excretion

After having a meal, which could have been a plant in your backyard, an insect in your backyard, your blood, or maybe your burger :P, the meal enters the digestive system and thus digestion begins.

The purpose of the digestive system is to transport food to the stomach, called the ventriculus in insects and further, to break down food to its basic components (e.g. glucose, amino acids, DNA, etc.), to absorb those nutrients into the insects via the blood, and to excrete what can’t be used as waste or as Chris Wirth first taught me, frass.

The insect digestive system can be divided into three different parts according to the function performed:

Foregut
Midgut
Hindgut

Foregut

The foregut is the first place food passes through in an insect. The first event that happens when an insect (or human for that matter) lands on food is trigger the salivary glands to produce saliva or to increase the amount produced. Of course some insects don’t have salivary glands and therefore don’t produce saliva. Saliva is very important in both humans and insects. First saliva lubricates the food making it easier to chew and swallow. Second, saliva contains enzymes such as amylases that help breakdown food. Third, in blood feeing insects (hematophages) it contains chemicals that prevent clotting. In some insects such as caterpillars, the salivary glands produce silk. In some predatory insects such as robber flies (Asilidae) and assassin bugs (Reduviidae), saliva contains toxins that help in immobilizing the prey.

After saliva is released and food enters the mouth, which is located at the base of the hypopharynx it enters the pharynx. Humans have a pharynx too and its common name is the throat. From the pharynx, muscular contractions move the food down further to the esophagus until it enters the crop. The crop acts as a ‘pre-stomach’ and it stores food in insects and sometimes partially digests it. Whatever is not stored in the crop moves on to the next part of the digestive system, the hindgut.

Midgut

From the crop the food moves on to the stomach, known as the ventriculus in insects. This is where most digestion occurs and where food is broken down into its basic components. Through many chemical processes, and enzymatic reactions food is broken down and then moves through a valve called the pyloric valve to the hindgut to be absorbed.

Hindgut

If humans had a hindgut it would consist of the small intestine, large intestine, the rectum and the anus. Insects have a anus, rectum and an intestine and an extra organ called Malpighian tubules.

After the digested food moves through the pyloric valve, it enters the Malpighian tubules. Malpighian tubules are long, thredlike structures that mainly function for excretion. But they also have an absorbing function. The tubules have tiny fingerlike projections called villi, which humans too have. These structures increase the surface area of the tubules and the intestine absorbing as much as possible (more will be said about the tubules in the excretion section). Food moves through to the intestine where more than 90% of water is reabsorbed into the insect, much like humans. Since insects are very small, any amount of water loss can lead to death. Whatever is not absorbed by now is excreted. Waste is stored in a muscular rectum and excreted as frass through the anus. To sum up, the hindgut performs main functions:

water and ion absorption
absorption of nutrients
pheromone production in some insects located in frass or in specialized rectal pouches
respiration (e.g. dragonfly nymphs)

Excretion

Exctretion in insects is achieved by two ways:

Malpighian tubules
the anus

The Malpighian tubules, named after their discover Marcillo Malpighi, are the primary source of nitrogenous excretion in insects. Through a series of reactions in them, enzymes convert toxic ammonia ions into harmless semisolid uric acid. The uric acid moves to rectum where water is absorbed and then excreted. One interesting thing to note is that cockroaches, unlike any other insect, do not excrete uric acid. Instead, they store it in specialized cells called urocytes. It is believed that, because cockroaches are so primitive, they use the uric acid as an alternative source of nitrogen. Symbiotic bacteria in the cockroaches have the enzyme (uricase) to break uric acid down (Mullins and Cochran, 1987).

The Muscular System

The most commonly known function of muscle is movement. However, this movement is not limited to appendages. Food moves down the esophagus with rhythmic contractions known as peristalsis. Muscle is also what keeps beating in humans and other animals (not insects).

Humans have three types of muscle and they are skeletal (why you move your arms, legs, etc…), cardiac muscle (why your heart beats), and smooth muscle (the lining of most of your internal organs). Insects on the other hand only have two types of muscle and they are skeletal and visceral. Skeletal muscle performs the same function in both humans and insects. Visceral muscle in insects is analogous to smooth muscle in humans. In insects, visceral muscles are associated with the movement of internal organs such as the esophagus, the ovaries and the Malpighian tubules.

Muscle contraction requires that I go into detail about microscopic filaments called actin and myosin and the chemistry of Ca2+ ions to contract a muscle. Again, I choose not to complicate matters and go into these details. It is sufficient to know the types of muscle and their function. But for a muscle to contract it requires two things; energy and a nerve impulse.

The Nervous System