First, let's look at the insect as a whole. Remember that unlike humans, insects have their skeletons on the outside of their body, which we call an exoskeleton since the prefix "exo" means outside or external. The exoskeleton covers the entire outside of the insect, including all three body, regions or tagmata; the head, the thorax, and the abdomen. Common to all adult insects, these three tagmata are an important identifying feature of the hexapoda. The ventral side or underside of the insects body is the sternum, while the dorsal side or top side is called the tergum. Note that tergum on the thoracic region is also called the notum. Finally, the lateral region of the body is referred to as the pleuron. Let's take a closer look at the most anterior tagma of the insect's body, the head, and identify some of it's commonly found structures. In insect's head is the result of fusion of the six or seven anterior most ancestral segments of an insect's body. The head supports the mouthparts as well as important sensory organs such as the paired antennae and compound eyes. The head also houses the insect's brain, and we will discuss how this organ interacts with the rest of the nervous system in a later lesson. Antennae are a pair of appendages on the insect's head that come in a variety of shapes and sizes, and can be used to identify certain groups of insects. Insect antennae accommodate a variety of Sensilla, which if you remember, are sensory organs with neurons that respond to various cues in the environment. While the antennae are best understood as olfactory organs that register smells, they can also detect tastes, movement, moisture, air currents, vibrations, and even heat. Incredibly, antennae even allow insects to estimate their flight speed. They do this by perceiving air currents and detecting how much the antennae distort at various speeds. Antennae may also be modified for non-sensory purposes, such as holding onto partners during mating. The head of an insect contains different kinds of visual organs, also called photoreceptors. Photoreceptors can include a pair of compound eyes, which are usually accompanied by up to three simple eyes known as ocelli. Compound eyes get their name because they are composed of multiple units known as ommatidia. Images captured by individual ommatidium combine to provide the insect with one large comprehensive image. In fact, each ommatidium can be considered as an individual eye that contains a lens and crystalline cone which focus light onto photoreceptor and pigment cells. Pigment cells adapt the photosensitivity of the eye to changing light conditions, and help to adjust the amount of light entering each individual ommatidium. In some fast flying visual predators like dragonflies, each compound eye can be made up of tens of thousands of ommatidia. Insects that rely on vision to find mates like male honeybees have many ommatidia, unlike compound eyes, ocelli or simple eyes contain a single facet covered in transparent cuticle which can be curved into a lens. The lens focuses light onto an extended retina made up of many sensory cells. The ocelli appear as small swellings on an insect's head, and most are not sophisticated enough to pick up focus images. Instead, they are usually used to detect polarized light, and changing light intensities like day and night or the horizon. In addition to the sensory organs, an insect's head also supports the mouthparts. Mouthparts are appendages found on the third to sixth segments of the insect's head, and include four main structures. The labrum is essentially a cover that protects the other mouthparts structures. Think of it as an insect upper lip. The mandibles are found immediately posterior to the labrum. They are usually the largest mouthparts structures and are armed with powerful muscles. Insects mostly use their mandibles to cut, tear, and crush their food, but they can sometimes be used as tools. For example, worker ants use their mandibles to carry the young. Mandibles can also be used as weapons by insects like the trap-jaw ant soldiers who lock their mandibles in an open position and quickly snap them closed for defense or for prey capture. The mandibles can even propel the ants into the air to escape predators, or knock invaders away from the nest. Posterior to the mandibles are the maxillae. Although the maxillae may assist in cutting and crushing food, they are less powerful than the mandibles. Instead, their greater mobility makes them more suitable for positioning food close to the mandibles. Ventral to the maxillae is the labium, what you may think of as the floor of an insect's mouth. The labium is often used together with the maxillae to manipulate food. It can be modified dramatically in some insects. For example, the labium of Dragonfly Nymphs has evolved into a powerful pair of extendable pincers that thrust out, and rapidly pierce and capture prey, and can sometimes be strong enough to capture fish. Some mouthparts also have structures known as palps. Paired palps can be found on both the maxillae and labium, and are known as maxillary palps and labial palps respectively. Sensory receptors on these structures allow insects to smell and taste their food. The structures we have covered so far make up the basic biting and chewing mouthparts that are thought to have evolved in the earliest insects. These mouthparts are found in a wide variety of adult insects today such as dragonflies, beetles, grasshoppers, cockroaches, and in the vast majority of juvenile insects too. The basic biting and chewing mouthparts can be used to break down and ingest many different kinds of solid foods. Although biting and chewing mouthparts are considered to be ancestral, these structures have been substantially modified over evolutionary time in many insect groups in accordance with changes in diets specificity. There are four main types of modified mouthparts used to exploit liquid food sources; piercing-sucking mouthparts, siphoning mouthparts, sponging mouthparts, and lapping mouthparts. Piercing-sucking mouthparts are used by insects to pierce animal or plant tissues to access nutritious fluids within. These mouthparts are often associated with salivary glands that produce secretions to assist with feeding, and chemically digesting food. In order to efficiently ingest liquid food, insects with piercing-sucking mouthparts usually have muscular sucking pumps located in their heads. Depending on the insect, the structures that form the piercing-sucking mouthparts evolve from different components of the basic chewing mouthparts. This means that piercing-sucking mouthparts in different insect orders do not share a common evolutionary history. Instead, they evolved independently multiple times. These types of mouthparts are present in all hemiptera insects, in fleas, and are also found in many diptera like the infamous mosquitoes. Siphoning mouthparts, as the name suggests, allow insects to siphon liquids such as nectar and water through a long proboscis. The proboscis is made up of highly modified maxillae, and these mouthparts are only found in the order diptera, which includes the butterflies and moths. Sponging mouthparts are present in some derived diptera species such as horseflies. The sponging structure is known as the labellum, and it is covered in many tiny grooves that allow the uptake of liquid food by capillary action. While some diptera only have sponging mouthparts, others have both cutting and sponging mouthparts. This occurs in blood feeding flies such as the horseflies that need to cut through a host skin first before they can access the blood. The final type of modified feeding structures we'll look at are the chewing-lapping mouthparts. These mouthparts are unique to bees, and are sometimes referred to as the bee's tongue, as they allow the bees to feed on liquid nectar. The lapping tongue in social bees is extremely important for pollen feeding, as the small particles they lap up with the nectar are crucial part of their diet. Bees also have mandibles that can be used in nest building, hive defense, and cutting through the basis of flowers to access nectar. That's it for the external features of the insect head. Next, we will move further back along the body to the thorax and the abdomen.
