- Describe arthropod characteristics and life cycles.
- Identify special traits of insects, and explain why they have been so successful.
- incomplete metamorphosis
The animal pictured in Figure below is an insect called a weevil. Insects belong to the phylum of invertebrates called Phylum Arthropoda. This phylum is not only the largest phylum of invertebrates. It’s the largest phylum in the entire Animal Kingdom. Obviously, animals in this phylum have been very successful.
Weevil: an insect in Phylum Arthropoda
What Are Arthropods?
Arthropods are invertebrates in Phylum Arthropoda. There are more than a million known species of arthropods. However, scientists estimate that only about a tenth of all arthropod species have been identified. In addition to insects, arthropods include animals such as spiders, centipedes, and lobsters. You can see why arthropods were successful both in the water and on land, by watching these excellent videos:
There are several traits shared by all arthropods. Arthropods have a complete digestive system. They also have a circulatory system and a nervous system. In addition, they have special organs for breathing and excreting wastes. Other traits of arthropods include:
- segmented body;
- hard exoskeleton; and
- jointed appendages.
Body Segments of Arthropods
Most arthropods have three body segments. The segments are the head, thorax, and abdomen. You can see the three segments in a range of arthropods in Figure below. In some arthropods, the head and thorax are joined together.
Arthropods have three body segments.
The exoskeleton (or external skeleton) of an arthropod consists of several layers of cuticle. The exoskeleton prevents water loss. It also protects and supports the body. In addition, it acts as a counterforce for the contraction of muscles.
The exoskeleton doesn’t grow larger as the animal grows. Eventually, it must be shed and replaced with a new one. This happens periodically throughout an arthropod’s life. The shedding of the exoskeleton is called molting. You can see a time-lapse video of an insect molting at this link: http://commons.wikimedia.org/wiki/File:Cicada_molting_animated-2.gif?fastcci_from=9630
Because arthropod appendages are jointed, they can bend. This makes them flexible. Jointed appendages on the body are usually used as legs for walking or jumping. Jointed appendages on the head may be modified for other purposes. Head appendages often include upper and lower jaws. Jaws are used for eating and may also be used for defense. Sensory organs such as eyes and antennae are also found on the head. You can see some of these head appendages on the bee in Figure below.
Bee head appendages include jaws, eyes, and antennae.
Arthropod Life Cycles
Arthropods reproduce sexually. Male and female adults produce gametes. If fertilization occurs, eggs hatch into offspring.
After hatching, most arthropods go through one or more larval stages before reaching adulthood. The larvae may look very different from the adults. They change into the adult form in a process called metamorphosis. During metamorphosis, the arthropod is called a pupa. It may or may not spend this stage inside a special container called a cocoon. A familiar example of arthropod metamorphosis is the transformation of a caterpillar (larva) into a butterfly (adult) (see Figure below). Distinctive life stages and metamorphosis are highly adaptive. They allow functions to be divided among different life stages. Each life stage can evolve adaptations to suit it for its specific functions without affecting the adaptations of the other stages.
Metamorphosis of a caterpillar into a butterfly
In some arthropods, newly hatched offspring look like small adults. These arthropods don’t go through larval stages. They just grow larger until they reach adult size. This type of life cycle is called incomplete metamorphosis. You can see incomplete metamorphosis in a grasshopper in Figure below.
Incomplete metamorphosis in a grasshopper
The majority of arthropods are insects (Class Insecta). In fact, more than half of all known organisms are insects. There may be more than 10 million insect species in the world, although most of them have not yet been identified. In terms of their numbers and diversity, insects clearly are the dominant animals in the world.
Like other arthropods, insects have three body segments and many jointed appendages. The abdomen contains most of the internal organs. Six legs are attached to the thorax. There are several appendages on the insect’s head:
- The head has a pair of antennae. Insects use their antennae to smell and taste chemicals. Some insects can also use their antennae to hear sounds.
- The head generally has several simple eyes and a pair of compound eyes. Simple eyes have a single lens, like the human eye. Compound eyes have many lenses.
- For feeding, the insect head contains one pair of lower jaws and two pairs of upper jaws. Insects have also evolved a wide range of specialized mouthparts for eating certain foods. You can see some examples in Figure below.
Specialized mouthparts for eating in insects
The main reason that insects have been so successful is their ability to fly. Insects are the only invertebrates that can fly. They were also the first animals to evolve flight. The ability to fly is highly adaptive. It’s a guaranteed means of escape from nonflying predators. It’s also useful for finding food and mates.
Insects that fly have wings, like the dragonfly in Figure below. Insects generally have two pairs of wings. They are attached to the thorax. The wings form from the exoskeleton. You can learn how insects fly—and how scientists study insect flight—by watching this short video: http://www.nytimes.com/video/science/100000002475937/the-flight-of-the-fly.html
Insects and People
Most humans interact with insects every day. Many of these interactions are harmless and often go unnoticed. However, insects can also cause humans a lot of harm. Some insects are vectors for human diseases. The mosquito in Figure below is a vector for malaria. Malaria kills millions of people each year. Many other insects feed on food crops. Growers may need to apply chemical pesticides to control them. On the other hand, without insects to pollinate them, many flowering plants, including important food crops, could not reproduce.
Anopheles mosquitos like this one are vectors for malaria.
- Arthropods are invertebrates in the Phylum Arthropoda. They include insects, spiders, centipedes, and lobsters. Traits of arthropods include three body segments, a hard exoskeleton, and jointed appendages. The arthropod life cycle may include larva and pupa stages and the process of metamorphosis.
- The majority of arthropods are insects. All insects have six legs and multiple head appendages and sensory organs. The main reason for the success of insects is the ability of many insects to fly. Insects both help and harm human beings.
Lesson Review Questions
- List five traits of arthropods.
- Describe the arthropod exoskeleton and its functions.
- Identify types of appendages you might see on an insect.
- It might be said of insects that we can’t live with them or without them. Do you agree or disagree? Apply lesson concepts to explain why.
- Why are distinctive life stages and metamorphosis adaptive?
- Explain the significance of flight in insects.
Points to Consider
One species in a mystery phylum is called a starfish, but it’s not a fish. Another species in the same phylum is called a sea cucumber, but it’s not a plant.
- Which phylum includes both starfish and sea cucumbers?
- What are some traits of organisms in this phylum?