With nearly 1 million species recorded, insects are the most diverse group of organisms on the planet. They account for approximately 75 percent of all described animal species, and the vast majority of the species on Earth that are yet to be identified are likely insects and other arthropods. In fact, scientists conservatively estimate that the total number of insect species could exceed 8 million when all are eventually discovered. While the majority of that amazing diversity exists in the tropics, there are well over 150,000 insect species found within the United States and Canada alone and many more if other arthropods, the larger group to which insects, spiders, centipedes, and scorpions all belong.
Insect Anatomy
Insects share several common characteristics. Unlike mammals, birds, reptiles, and amphibians, insects lack an internal skeleton. Instead, insects have a hard exoskeleton on the outside of their bodies; this provides both protection and support. Their body is divided into three distinct regions: the head, thorax, and abdomen.
The Head
An insect’s head has two rather prominent compound eyes, two antennae, and mouthparts. The rounded compound eyes are composed of hundreds of tiny individually lensed eyes. Together, they render a single, somewhat pixilated image and afford insects rather good vision, especially for both distance and motion. Above the eyes are two antennae. They bear sensory structures that help with orientation, smell, and taste. The head also bears mouthparts, which vary considerably across insect groups; some insects, such as grasshoppers, beetles, and caterpillars, have chewing mouthparts. Others, such as true bugs and mosquitoes, have piercing-sucking mouthparts, but there are many other options, and they include mouthparts adapted for sponging (flies), rasping-sucking (thrips, biting flies), and siphoning (butterflies and moths). Some insects have reduced or vestigial mouthparts or may even lack them altogether.
The Thorax
The thorax is an insect’s second body section. It bears the appendages and muscles that enable an insect to move. All insects have three pairs of jointed legs, one pair on each segment. Many insects also have one or two pairs of wings. Besides flight, wings may serve a variety of other functions, including assisting in thermoregulation, sex recognition, sound production, camouflage, mimicry, and self-defense.
The Abdomen
The last section of an insect’s body is its abdomen, which contains the reproductive, digestive, and excretory systems along with a series of small lateral holes, called spiracles, that enable air exchange. In female insects, the tip of the abdomen may have an added structure called an ovipositor, which is used to insert or place eggs. In some insects, such as bees and wasps, the ovipositor is modified into a stinger that can be used for self-defense.
Stages of Development
All insects pass through a series of developmental stages as they grow. This transition is known as metamorphosis. Most insects, including butterflies and moths, flies, beetles, and bees and wasps, undergo a complete metamorphosis consisting of four developmental stages: egg, larva, pupa, and adult. The immature stages look much different than the adults, eat entirely different foods, and live in separate environments. Butterflies and moths are good examples. The wormlike larvae have chewing mouthparts; they feed primarily on plant material, live on plants, and have a mostly sedentary lifestyle. The adults have siphoning mouthparts, feed primarily on flower nectar or other liquid resources, and are highly mobile thanks to their wings.
Other insect groups, including grasshoppers, dragonflies and damselflies, true bugs, and cockroaches, undergo incomplete metamorphosis. This process consists of three developmental stages: egg, nymph, and adult. The young nymphs often closely resemble the adults in appearance, although they are smaller in size. They also may share the same environment and food resources that adults frequent, and they often behave similarly. True bugs are a good example. The nymphs are essentially smaller versions of the adults. They have piercing-sucking mouthparts, feed on plant sap, live primarily on plant material, and typically move by walking. The resulting adults differ in that they are larger, reproductively active, and have wings, enabling flight.
What You Might Find
Arthropods
Arthropods belong to the phylum Arthropoda and represent the largest group in the animal kingdom. Insects, spiders, millipedes, centipedes, and crustaceans are all arthropods; together, the arthropods are the most successful group of organisms on the planet! Arthropods share many common features, including a protective external skeleton made of chitin, a segmented body, and paired and jointed appendages. Insects are often the most noticeable and commonly encountered terrestrial group. They can be divided into more than 30 different orders (subgroups). The members of each have certain basic characteristics and behaviors that can be particularly useful for identification. The following include some of the most charismatic, distinctive, and commonly encountered orders covered in this book.
Coleoptera (Beetles)
This is the largest and most diverse group of animals on the planet. Beetles represent about 40 percent of all known insects with over 350,000 described species. Adults tend to be rather large, robust, and conspicuous organisms with a hard exoskeleton and two pairs of wings. The first pair is modified into protective covers, called elytra, which cover the larger, membranous wings beneath. Beetles occur in both terrestrial and aquatic freshwater environments. They have chewing mouthparts; some feed as predators on a variety of other invertebrates, others are herbivores and consume various plant parts, and still others are scavengers on animal dung, carrion, or decaying plant material. Beetles undergo complete metamorphosis. Many beetles are also attracted to artificial lights at night.
Lepidoptera (Butterflies and Moths)
Butterflies and moths are some of the most well-known and charismatic of all insects. Adults tend to be rather large and showy organisms with two pairs of transparent wings that are covered with numerous tiny scales. They have two large compound eyes, two elongated antennae, and siphoning mouthparts (although in some species they are significantly reduced and nonfunctioning), which enable them to drink flower nectar or other fluids. Butterflies are active by day, whereas most moths are nocturnal, although some species are seen during the day. Butterflies and moths undergo complete metamorphosis. Their larvae, known as caterpillars, are primarily plant feeders and have chewing mouthparts.
Hymenoptera (Bees, Wasps, and Ants)
This is a large and diverse group of insects known for their complex social systems that even include division of labor. Adults typically have two pairs of transparent wings, although some groups or individuals may be wingless. Most have chewing mouthparts with the exception of bees, which have a tongue for feeding on flower nectar or other fluids. The majority of adults also have a noticeable constriction between the abdomen and thorax that resembles a narrow waist. Many adults feed on nectar and are common flower visitors. Ants are typically predators, omnivores, or scavengers. All bees, wasps, and ants undergo complete metamorphosis. Their larvae feed on a variety of resources, including prey or pollen provisioned by the adults and plant material. Some are parasites of other insects.
Diptera (Flies)
Flies are a large and cosmopolitan group of insects that get their name for their most obvious behavior—flying. The rear wings are modified and reduced to small, club-shaped structures called halters, which help stabilize the insect during flight. Adults have well-developed compound eyes, short antennae, and diverse mouthparts designed for piercing, sucking, or sponging up liquid foods. Flies undergo complete metamorphosis. The larvae lack legs and live in terrestrial, freshwater, or