Insects

Insects are the most diverse group of animals that are found in most environments. In the Animal kingdom, Insects are in the Phylum Arthropoda; Arthropods have an exoskeleton of chitin that they shed as they grow; they also have segmented bodies and jointed appendages. In addition to the Class Insecta, the Arthropoda also includes the arachnids (spiders and mites), myriapods (ex. centipedes), and crustaceans (crabs, lobsters, etc.). Insects are distinguished from the other Arthropod classes by the following features:

As adults and in some species in the juvenile stages, insects have three body parts: the head, thorax, and abdomen. Although in some insect species, some of the three body parts are fused together and may be difficult to distinguish. See this website for images and more discussion of insect anatomy: Purdue University, College of Agriculture, Department of Entomology, 4-H and Youth: Insect Anatomy
The adults have antennae on their heads that they use to sense their environment, and they have three pairs of legs or six legs.

Figure 8.1.1: Honeybees are important pollinator insects. Note the three body parts (head, thorax, and abdomen) two antennae, and three pairs of legs.

Credit: MaryAnn Frazier, Department of Entomology, Penn State University
Most insects undergo a morphological change that occurs between the time they hatch from eggs and develop into adults. The morphological change is called either complete metamorphosis or incomplete metamorphosis referring to how significantly the insect's appearance changes from the early stage of development to the adult stage. Go to these links to see images of the types of metamorphosis and read more about insect metamorphosis: ASU School of Life Sciences: Metamorphosis

Check Your Understanding

Question - Multiple Select

Browse the following websites for two major agricultural crop pests. What kind of organisms are they? In what stage of their lifecycle do they cause the most damage to the crop plants?

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ANSWER:
Both are insects that undergo complete metamorphosis. They do the most crop feeding and damage in the larval stages when they resemble worms, that are sometimes also called caterpillars. Thus, their common names include the name: worm.

Feeding Types

Insects may be herbivores or omnivores. Herbivorous insects may eat plants by directly feeding on plant tissues such as leaves or roots. Herbivorous insects include caterpillars, beetles, grasshoppers, and ants. Some insects pierce plants and suck plant nutrients from the plant vascular system, typically the phloem, (the cells that transport plant carbohydrates and amino acids); although some insects feed on the xylem, the vascular cells that transport water and nutrients. Examples of piercing-sucking insects include aphids and mosquitoes. By contrast, butterflies and moths have siphoning mouthparts for drinking nectar. Omnivore insects consume multiple kinds of food including other insect prey and plant tissues such as leaves and/or nectar and pollen.

Beneficial insects

Although insect pests are major agronomic pests, only about 1% of insect species are agricultural pests. Insects also contribute to important ecosystem processes, including: i. pollination, ii. predation and parasitism (ex. lady beetles, lacewings, praying mantis, parasitic wasps); iii. decomposition of organic materials such as crop residues and manure (Ex. dung beetles) iv. providing food for other organisms, such as fish and birds. Review the photos below for some categories of beneficial insects, and some of their characteristics here: National Pesticide Information Center

Figure 8.1.2: Dung Beetles contribute to decomposing dung or manure. This photo shows a Large Copper Dung Beetle (Kheper nigroaeneus) on top of its dung ball.

Credit: Bernard DUPONT from FRANCE (CC BY-SA 2.0), via Wikimedia Commons

Figure 8.1.3: A beetle (Chlaneius sp.), predating on insect larvae

Credit: Heidi Myer

Figure 8.1.4: Parasitic wasp laying eggs on an alfalfa weevil

Credit: Arthur Hower, PSU

Figure 8.1.5: Caddisflies: An important food source for fish.

Credit: Jason Neuswanger, Troutnut

Activate Your Learning

Read the following website: Omnivorous Insects: Evolution and Ecology in Natural Agriculture Ecosystems.

Then answer the following questions:

What did scientists observe happened to cotton plants and insect herbivores after cotton plants were injured by herbivorous insects?

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ANSWER: The cotton plants produced defense compounds that reduced the plant quality for herbivorous or omnivore insects. The compounds reduced insect feeding on the cotton plants (as indicated by fewer plant scars) and increased insect predation of the eggs of the insects that had been feeding on the cotton.

To conserve or maintain predatory insects, what is required? What can farmers do to attract and conserve predatory insects?

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ANSWER: Habitat that provides alternative food sources and protection from predators is typically needed to attract and maintain predatory and other beneficial insects. Farmers can maintain or plant alternative and diverse plants in fields or around field edges to provide alternative plant food and habitat for beneficial insects and their alternative prey. These plants typically include flowering plants that can provide pollen and nectar.