Insects are a major component of Earth's biodiversity, and are among the most diverse organisms on the planet. To date over one million species of insects have been described. However, since many insect species remain undiscovered, mostly in the tropical regions of the world, there may actually be more like 5 to 15 million species on Earth. All insects belong to the class Insecta which is a rank within the Hexapoda. Currently there are 29 recognized insect orders in the class Insecta. We will start with insect orders that go through the simple form of metamorphosis that we learned about in our last lesson, incomplete metamorphosis. Some of the most ancient orders like the Odonata, the dragonflies and damselflies as well as the order Blattodea, the cockroaches and termites undergo this type of development. Other more diverse orders that undergo incomplete metamorphosis include the grasshoppers and crickets in the order Orthoptera as well as the true bugs and their relatives in the order Hemiptera. We will then examine a few insect orders in the Holometabola, all of which have the complex form of development known as complete metamorphosis. These insects include the beetles in the order Coleoptera, the ants, bees, and wasps in the order Hymenoptera, and true flies in the order Diptera. Finally, the visually spectacular moths and butterflies in the order Lepidoptera. Let's start with the order Odonata, a group of predatory insects that includes dragonflies and damselflies. The Odonata is one of the oldest orders in the class Insecta, with the earliest specimens known from over 300 million years ago in the carboniferous period. There are approximately 6,000 species of odonates in the world today. Adults have long abdomens, small antennae, large eyes, and four powerful membranous wings that cannot be folded flat over the abdomen. Adult odonates are commonly found near bodies of water, as the juveniles also known as naiads are aquatic or semi-aquatic. Both adults and juveniles are voracious predators, and some naiads can even consume small fish. The Blattodea is another ancient order stemming from early in the carboniferous period. Today this order comprises approximately 4,500 species of cockroaches, and 3,000 species of the eusocial termites. Eusociality is the highest level of organization in colonial animals, and will be covered in detail in a later module. Cockroaches have long, thin legs for running in our dorsoventrally flattened which allows them to hide in small spaces to escape danger. The adults have leathery, protective forewings and unlike the Odonata, they can fold their wings flat over their body when at rest. Cockroaches are among the most familiar insects and can be found in a wide variety of habitats all over the world except in the polar regions. Many species have close symbiotic relationships with microbes in their guts, that allowed them to exploit a wide variety of food sources including unappetizing dishes like wood. Most cockroaches hide in the day and emerge at night to feed. Termites are thought to be the descendants of wood eating cockroaches, and light cockroaches are dependent on extremely complex interactions with symbiotic microbes. The earliest known fossil termites date back to the cretaceous period more than 66 million years ago, but are thought to have originated around a 100 to 200 million years earlier in the Jurassic or even the Triassic. All termite species are eusocial, and live in large colonies headed by a termite queen and king. Many termites are important for nutrient cycling and terrestrial environments. However, when termites build their nests in human-made structures, they are often considered pests. The third group we will be looking at, the order Orthoptera, contains herbivores, omnivores, and predators. It is made up of over 27,000 species of grasshoppers crickets and katydids. They are present all around the world, but our most diverse in the tropics. Although there is variation in the ecology of orthoptins, many members of this order feed on the leaves of plants, and some can be significant agricultural pests. All juvenile insects in this order closely resemble the adults except in size, in the presence of fully developed wings, and genitalia. Like the cockroaches, adult orthoptrins have leathery four wings. They have well-developed thorax that houses the flight muscles, and most have hind legs modified for jumping. Insects in this order also have elaborate courtship behaviors. In many species, males will produce audible courtship songs by rubbing their wings together or against their legs. The most diverse order of insects that go through incomplete metamorphosis is the order Hemiptera. The term Hemiptera translate fluently as half wing in Greek and refers to the halfhearted four wings of some of the species in this order. The order Hemiptera contains just over 80,000 species, and includes organisms with diverse life history strategies. Some species are herbivores like cicada and aphids, while others are predators or blood feeders like assassin bugs. The order Hemiptera includes insects known as the true bugs. All hemipterans have piercing sucking mouth parts that allow them to specialize on a variety of liquid diets. Some hemipteran species are considered important agricultural and medical pests. Herbivorous species can cause substantial feeding damage to crops and transmit plant diseases, while blood feeders can transport pathogens and cause disease in humans and animals. For example, Charles Darwin, the English naturalists best known for his contributions to the field of evolution, may have contracted chagas disease on his famous Beagle voyage when he was bitten by a blood feeding hemipteran. The remaining insect orders that we will introduce in this lesson, all belong to the Holometabola, and undergo complete metamorphosis. This allows for ecological specialization by the different life stages of the insect. The order Coleoptera, the beetles, is one of the earliest diversifying groups within the Holometabola and is one of the most diverse insect orders with over 390,000 known species. Adult beetles have characteristics scleratised forewings known as elytra, which helped protect the hind wings that predominantly power flight. Beetles occupied many different habitats on the planet including aquatic and some intertidal zones. The earliest beetles are thought to have fed on plant tissues, fungi, as well as decaying organic matter. Many herbivorous beetle species alive today are closely associated with their host plants, living within various plant tissues, or on the outside of the plant. Beetles that feed on decaying material play important nutrients cycling roles in ecosystems. Many beetles feed on fungi, and some have symbiotic relationships in which the beetle transports the fungus between plant hosts. Though few beetles are parasitic, predation is common and widespread within the order. Some beetles species are important agricultural pests and pests to forest resources, while others can actually help control both weed and insect pest populations. The order Hymenoptera is another example of a mega diverse insect order in the Holometabola. This order includes over 150,000 described species of ants, wasps, bees, and sawflies, but it is thought that there may be over 800,000 species. This would make the Hymenoptera even more diverse than the Coleoptera, and the most species rich order of animal life on the planet. One characteristic of the Hymenoptera, is their sex determination mechanism as all display haplodiploidy. In this system, the sex of each insect is determined by its ploidy, the number of sets of chromosomes it receives. In Hymenopterans, males are haploid because they receive only one set of chromosomes from their mother. Females on the other hand are deploid because they received the one set of chromosomes available from their father, and one set from their mother. The consequence of haplodiploidy is that daughters that share a common father are on average more related to each other than they would be to their own offspring. This is theorized to be one reason that social behaviors have evolved multiple times in the Hymenoptera. Well-developed social behavior occurs in many members of this order, and in some groups like the ants all species are eusocial. Social groups are well-defended. Some Hymenopterans have stingers, modified from their ovipositor, while other species spray defensive chemicals. The order Diptera is another large group of insects with over 160,000 recorded species. Dipteran insects are commonly referred to as true flies and have only one single pair of membranous wings in the adult stage. Their hind wings are modified into halteres which act like sensitive gyroscopes that helped guide their rapid flight. Members of this diverse order also occupy a wide range of habitats and exhibit diverse life histories. As juveniles, some species may have a parasitic lifestyle, while others are herbivorous and can be important agricultural pests. Flies with carrion feeding larvae are important for nutrients cycling, and can be exploited by humans in forensic investigations and in medical treatments. Adult flies mainly feed on liquids including nectar, honeydew, and blood. They therefore impact humans in a number of ways ranging from important pollinators, to vectors of human diseases. The final order of insects that we will introduce in this lesson is one of the most popular, the Lepidoptera. The latest fossil evidence suggests that the Lepidoptera evolved almost 200 million years ago during the Triassic Period. The order now consists of over 180,000 described moths and butterflies. Much of this diversity is attributed to the massive radiation of this group alongside flowering plants or angiosperms during the Cretaceous period. All adult lepidopterans have wings that are covered by scales, which are modified flattened hairs that can be very colorful. Most also have a specialized feeding structure called a proboscis, that allows them to pump up liquid food. When not feeding, the proboscis is coiled up under the head, as nectar feeders adult moths and butterflies can be important pollinators. The majority of species in this group are herbivores as larvae, but how those larvae get their food varies widely, from leaf mining to wood-burning. Larval lepidopterans commonly referred to as Caterpillars are sometimes considered pests in agricultural, horticultural, and forestry settings. This has been a brief introduction to eight of the 29 insect orders that together represent tens of thousands of insect species. This is only the tip of the iceberg. The best way to see a wide variety of insects in a single sitting is to visit a museum. In the next video, we'll introduce you with the help of a few professional entomologists, to insect collections in museums. They'll show us how they're assembled, maintained, and what value they have to entomologists. Stay tuned to learn more.
