An insect is an arthropod. Its segmented body parts are fused forming its three major regions: the head located at the forefront of the body; the thorax, found in the insect's midsection; and the abdomen, found at its lower extremity. Insects too have a strong, external skeleton. The paragraphs that follow expand on the aforementioned, simplified descriptions.
The Head
One pair of compound eyes, antennae, mouthparts, and a brain comprise the basic components of an insect's head. The head, however, consists of numerous additional sensory organs. These organs allow the insect to masterfully experience its surrounding. The head is actually a rigid cranial capsule divided into symmetrical, segmented regions that are indistinguishable with the naked eye; microscopic observation reveals that its divvied sections are evidenced in the way its facial body parts are situated.
The Brain
. This body part constitutes an integral part of an insect's nervous system. Its size varies within different insect species. Insects that engage in complex behaviors tend to have larger brains. The nervous system is additionally composed of sensory, internuncial, and motor nerve cells. Sensory nerve cells tend to lie close to the insects body surface. Internuncial cells are association cells. Nerve impulses are channeled through this region to motor cells via synapses. This suggests that similarly to the brain and nervous system found in humans, these neurological components serve as an information highway within the insect's body.
Compound Eyes.
Insects view the world through a mosaic of hexagonal portals known as ommatadia. Each ommatadium provides the insect with a narrow field of vision in the form of a dot of light shining on each lens. The dots collectively help an insect form an image or picture. That image, however, is unlike the view we experience with our eyes. They see the world through colored images and are capable of detecting motion.
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Speaking of which, have you ever wondered why a fly seems to escape so swiftly when you try to swat it? That's because in addition to its compound eyes, it has three simple eyes arranged in triangular fashion on the front of its head between the compound eyes. That insect can see what's coming towards it in multiple directions! Bottom line: an insect's eyes can be considered to be extraordinary ocular apparatus.
Antennae.
Feelers, as they are alternately called, generally serve as a sensory tool. Their form and usage vary from insect to insect. Some antennae, like those found in butterflies, are capitate in structure, that is they have a club-and knob-tipped appearance. Some are filiform; they are thread-like in structure like antennae found on a cockroach. Some are setaceous or bristle-like, similar to antennae found in the order odonata. Some antennae are plumose or feather-like, as found noted in moths and remarkably mosquitoes. Some are comb-like and are referred to as pectinate in form. Others are serrate in composition; they are notched and angled on specific sides, somewhat like the blades on a saw. Both of these characteristics are evidenced in myriad types of beetles. Entomologists believe antennae are sensitive to odors, taste, touch, heat, moisture, and wind currents.
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Mouthparts.
Like the teeth, mouth, and tongue found in humans, the mouthparts of an insect are used to ingest food. Insect mouthparts vary from one insect species to the next: some are used for masticating, sucking, siphoning, or rasping. Some insects have mouthparts that are not fully developed, suggesting that they undergo change during their growth stages. Whatever its use, insect mouthparts are quite complex. Five basic components constitute this portion of an insect's body: the labrum and labium constituting the upper and lower lip (yes, insects have lips!); the hypopharynx—a tongue-like structure connected with the insect's salivary glands and/or salivary ducts to help spark digestion; and a jaw-like apparatus referred to mandibles. Mandibles tend to be found in omnivorous insect species like cockroaches and crickets; they are used for capturing, snipping and cutting prey into smaller pieces, and for masticating food. A pair of secondary jaws, called maxillae, is located behind the mandible. They serve as sensory organs that move masticated food into the insect's pre-oral cavity, like humans about to swallow a hamburger post chewing it into smaller pieces. Connecting muscles help these mouthparts to work together as an operational masterpiece.
Insects that have sucking mouthparts draw in liquid food similar to humans taking in liquid sustenance by sipping liquid food through a straw. Insects that have this type of mouth structure also have salivary pumps. This is used to inject saliva into the pre-oral cavity and is used to begin the process of digestion. For example, the curling mouthparts found in lepidoptera are referred to as proboscis; butterflies and moths use their proboscis to siphon nectar from flowers. Diptera, e.g., houseflies and blowflies, have tube-like sucking mouthparts that lap and suck up liquids. This type of lapping-sucking apparatus is also found honey bees. Arthropods like fleas, thrips, and sucking lice have stylets—needle-type mouthparts that pierce animal or plant tissue through which it feeds.
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Remarkably, unlike humans, the teeth-like components of an insect are found outside of the mouth opening. Insects like locusts, grasshoppers, beetles, and caterpillars have mandibles that snip and masticate food. Others use their hypopharnyx to digest and ingest liquid food. Ultimately, the basic design of the mouthpiece depends upon the insect and the food sources that sustain it.
The Thorax
The thorax is the central structure of an insect, analogous to the torso found in human beings. To it are affixed the legs and where applicable, one or two sets of wings. Depending upon the insect species, the thorax is divided into three additional sections: the prothorax, the mesothorax, and the metathorax. Spiracles—part of the insect's respiratory system used to help it breathe and exchange gases—and the insect's first pair of legs are present on the prothorax. The larger mesothorax bears the second pair of legs, the forewings where applicable, and an additional set of spiracles. The last set of legs and, where applicable, an additional pair of wings is situated on the metathorax.
Wings
. The forewings and hind wings of many insects are coupled together. This helps to improve its aerodynamic ability, permitting them to fly swiftly oftentimes in multiple directions. All winged insects have the same basic wing structure.
Wings are each comprised of a major vein that runs longitudinally from the segmented thorax to the wing-tip; other veins branch out from them. The major veins contain blood vessels, nerve fibers, and trachea. They too are affixed to powerful muscles, attached between the wing and the thoracic segments. The thoracic walls to which the wings are affixed can move at different speeds in key directions. The movement of those muscles controls the movement and attitude of the wing in flight.
Insect wings are divided into grouped cells. Some wings—as found in butterflies—are multi-colored; others—as found in houseflies—are membranous and somewhat transparent. Still others—as found in white cabbage moths—are dull in color. Some insects, like beetles, appear to have lost a set of wings. The truth is their outer wings are hard and armor-like, suggesting an adaptation for survival. Diptera have but two wings. Affixed to their hind wings are a body part known as halteres. These body parts vibrate upward and downward in a synchronized way with the wings and act as gyroscopes in flight. If the fly deviates from a straight course, flying in a zig-zag motion, rolls, or pitches during flight, the insect's halteres help the insect maintain its balance.
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Legs
. Insect legs are generally used for walking or running and are divided into segmented parts to help facilitate those functions. They include the femur, tibia, tarsus, and other components. The femur, connected to the thoracic region, can be equated to the human thigh bone connected to the lower torso. The tibia can be equated to what constitutes the shin bone in humans; in adult insects, it is the fourth leg segment from the body following the femur. The tarsus comprises what would be considered feet. Insect "feet", however, can have specialized endings in the form of claws (called ungues) or sticky, adhesive- type pads that help to facilitate clinging and climbing.
Based on shape and size, an insect's leg functions can vary. Some insects, like crickets and grasshoppers, have strong hind legs used for leaping. These animals are able to jump great distances when attempting to escape from danger. Some insects have legs that help them balance and/or perch, as holds true with dragonflies and damselflies. These odonata additionally use their legs to form a basket-type stance, used to overtake and trap small flying insects when in flight. Many insects that live in ponds or rippling streams have paddle-like legs that help them navigate effortlessly across the water. Some insects, like camel crickets, have forelegs that look like miniature shovels; they use them to dig burrows. Insects, like mantids, have front legs that are designed for grasping and tenaciously holding their prey.
Remarkably, insect legs differ during the larval stage in that in addition to developing legs, prolegs are often affixed to their abdomen. Prolegs are actually unsegmented legs used by larvae to adhere to and/or navigate through its environment.
Spiracles.
Like humans, insects breathe in oxygen. Unlike humans, they do so via a complex pattern of air-filled openings and accompanying tubes found along the sides of the thorax. Those openings are called spiracles. Insects have can have as many as eight pairs of spiracles on its thorax. Spiracles connect with the insect's tracheal system, similarly to how our trachea connects with the opening in our mouth and nose. Equipped with valves that open and close according to the insects' need for oxygen, spiracles help to move oxygen to the cells. Inhaling and exhaling is a bit dissimilar from the breathing process experienced in humans. Versus making use of lungs, insects use flexible air sacs that expand and contract, working in synch with the insect's spiracles.
The Abdomen
The abdominal area of an insect is also a segmented structure. Found on its lower extremity, the abdomen is primarily used for reproduction. It additionally assists in several other bodily functions, i.e., the insect's digestion, respiratory, and circulatory processes. The abdomen generally consists of 11 segmented sections. The number of segments varies between differing insect orders. The size and shape of an insect's abdomen also varies by species. Some are elongated; others are narrow.
Multiple Functions
. Many adult insects have functioning spiracles along this section of the body; some, however, have spiracles that are permanently closed and no longer in use. On the anterior portion of the abdomen is the anus through which waste is excreted. In some insects, such as earwigs, a pair of sensory organs known as the cerci, is located on the anterior section as well. Similar to humans, the insect's genitalia lie a bit beneath the anus. In females, at about the eighth or ninth abdominal segment, is the ovipositor, an appendix through which eggs are laid. The ovipositor is comprised of valvulae, a tube-like opening through which eggs traverse. It too consists of appendages that are used for reproduction. Ovipositors that are no longer in use for reproductive purposes evolve into stingers, suggesting that insects that sting are probably female.
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The Exoskeleton
The exoskeleton is more than an outer skeletal framework. It is comprised of intricate layers that include a tough, outer layer called the cuticle, internal supports, and muscle attachments. This body structure is similar to bones, ligaments, and muscle attachments found in humans. Chitin, a chemical element, gives the exoskeleton its hard quality. Like human skin and bone, the exoskeleton covers, holds in place, and protects the insects' internal organs. Its strong cuticle additionally helps to protect insects from predatory attacks. The composition of the exoskeleton can range from armor-like and rigid (as found in the exoskeleton of a dung beetle) to pliable and thin (as observed on Painted Lady butterflies when they undergo the molting stage). Microscopic observation further reveals that the exoskeleton consists of additional layers: an epidermis, a single layer of cells that lies beneath the cuticle; an inner and outer epicuticle, i.e., a waxy, waterproof-type covering that helps to keep water
within
its body. Amazingly, the epicuticle is found primarily in insects that have limited access to water, suggesting that this is an adaptation to help them survive.
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An Added Note
Insects have an open circulatory system. This differs significantly from the circulatory system found in humans in that a dorsal vessel—a tube-like structure—serves as the insect's heart. Divided into segmented chambers that run longitudinally through the thorax and abdomen, the dorsal vessel helps hemolymph (insect blood) flow throughout the insect's body cavities from the head to its dorsal body parts. Hemolymph—comprised primarily of plasma—is a clear, watery, greenish or yellowy fluid. Pulsated through the dorsal vessel, capillaries, veins, and arteries, this fluid also serves as an insect's water reserve. The circulatory system carries out additional functions: it transports nutrients throughout the insect's body, assists as a defense mechanism, sealing off wounds with a clotting reaction, and in some insects, serves as a source of thermoregulation, helping to strategically cool off or warm the insect's body.
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Classified Characteristics
Approximately 30 orders of insects are recognized in the 21
st
century. They are scientifically named and categorized based on their physical features. Insects to be studied within this unit belong to the following orders:
