Insect Anatomy: Parts of an Insect

The criteria for an organism to be classified as an insect is that in its adult form, the organism must have three distinct body regions and six legs.  The three body regions include the head, thorax, and abdomen.  Most adult insects also have one or two pairs of wings though this is not a requirement for being an insect. Most have complex eyes and antennae as well.

Insect Legs

Adult insects have three pairs of jointed legs for a total of six legs which are typically (but not always) present in other stages of growth as well.  They are useful for a variety of things including walking, running, jumping, digging, grasping, carrying, swimming, cleaning other body parts, building nests, and more.  The various sizes and shapes of insect legs are used in classifying them.

The Three Regions of an Insect Body

Bodies of adult insects have three distinct regions: head, thorax, and abdomen.  An insect’s body is supported by a tough body wall known as an exoskeleton.  The outer portion of an insect’s exoskeleton is called the cuticle.  

The cuticle has two non-living layers.  The thin, waxy outermost layer of the cuticle on the outside of the insect is called the epicuticle.  This epicuticle layer prevents outside water from getting into the insect and internal water from getting out.  The inner layer of the cuticle is called the procuticle.  It contains protein strands woven together with a very tough material called chiton, which makes their exoskeleton strong.

Beneath the cuticle is the living layer, called the epidermis.  It is the epidermis which secretes the non-living cuticle that covers it.  Sensory hairs originate from the epidermis and protrude through the cuticle.  These hairs transmit information to the nerves at their base.

The thorax is made of three segments.  From the front (closest to head) to rear (closest to the abdomen), they are the prothorax, the mesothorax, and the metathorax.  The cuticle of each of the three segments is formed into four groups of hardened plates called sclerites.  The group of plates on top are called notum.  The two groups of plates on the sides are called pleura. And the group of plates underneath are called sternum.  Thoracic sclerites on the three segments are named by using a prefix to indicate the segment (pro-, meso-, and meta) and the name of the sclerite group.  So, the sclerites on the underside of the mesothorax would be called mesosternum.

A pair of jointed legs is attached to each of the three thoracic sections.  For those insects with wings, the wings occur in one or two pairs attached to the mesothorax and metathorax, but they are never attached to the prothorax.  Wings come in lots of forms with lots of venation patterns.

The abdomen of an adult insect typically consists of 11 or 12 segments.  In most insects, clear segmentation can be seen in the abdomen though segments are fused together in some.  It contains the heart, reproductive organs, mid gut, and other digestive organs.

Features of an Insect’s Head

An insect’s head is made up of a series of segments specialized for gathering and manipulating food, sensory perception, and neural integration.  The head has three main features:  eyes, antennae, and mouthparts.

Types of Insect Eyes

Most adult insects have a pair of compound eyes, for which insects are famous.  A compound eye is made of hundreds, sometimes thousands, of tiny independent photoreceptor units called ommatidia.  Each photoreceptor unit has a cornea, lens, and photoreceptor cells used to detect color and brightness. 

The second type of eyes found in insects are called ocelli (singular form is ocellus).  These are simple eyes, meaning they have a single lens like a human eye.   “Ocellus” means “little eye” in Latin.  Insects can have one to three ocelli located in various places on their head, often between their compound eyes.  Ocelli have developed as a secondary visual system for insects and are used to detect color and brightness like their compound eyes.

Insect Antennae

Insects have a pair of sensory organs on their head called antennae (singular form is antenna).  Sometimes they are called “feelers” because they wave the antennae around, but this is not an accurate term because they are used for more than just touch.  These organs are sensitive to smell, touch, and, in some cases, sound.  They can even be used to detect changes in humidity.  They are located on the adult insect’s head, typically between or in front of the eyes.  They come in many forms with different levels of complexity and sophistication.

Insect Mouthparts

While the mouthparts of insects exist with many variations in form and function, insects are generally divided into to two categories based on their mouthparts:  those with mouths for chewing and those with mouths for sucking.  The one exception to this is the mouthparts of certain flies that act more like sponges.

Insects with mouths for chewing have strong jaws to open and close their mandibles that contain teeth. Those insect larvae and adults that chew create holes and tunnels in various plant parts.  Chewing insects are often grouped based on how they go about this. 

  • Borers are a group of insects that feed on the inside of roots, stems, and branches during their larval or adult life stage.  Sometimes they tunnel beneath the bark or into the heartwood of trees and shrubs. 
  • Miners are a group of insects whose larva live inside of and feed on leaves and sometimes the surface of fruit. 
  • Defoliators or skeletonizers are larva or adult insects that feed on and destroy the leaves and needles of plants.  They typically devour the entire green leafy portion of a plant leaving only the inner structure of the plant resembling a skeleton.  Some examples are grasshoppers, Japanese beetles, and potato beetles.

The second group of insects have mouthparts designed for piercing and sucking.  These pests damage plants by piercing the plant tissue with slender, needle-like mouthparts called stylets and sucking sugars and other complex carbohydrates out of the plant tissue.  When this happens, the affected parts of the infested plants become spotted (white, brown, or red spots), yellow, wilted, deformed, or stunted and may eventually die.  Some examples of these types of pests are whiteflies, aphids, and true bugs.

Some insects transmit diseases, especially viruses, and are referred to as vectors.  When they feed on a diseased plant and then move on to another plant, they can transmit the disease from the previous plant to the current plant with their saliva.

The main difference between damage done by chewing insects vs. sucking insects is that chewing insects remove plant tissue where sucking insects typically only damage the tissue and rarely result in tissue loss.  Knowing this distinction is useful in identifying pests.

It should be noted that the mouthparts of larvae are often different from those of their adult counterparts because they feed on different materials.  Caterpillars are chewers because they feed on leaves while moths and butterflies are suckers feeding on liquids like nectars.