Entomology for Master Gardeners

This web resource is designed as an introduction to basic entomology for Kentucky gardeners. Specifically, it is a supplement for those who are enrolled in the Kentucky Master Gardener Program, but it may be useful for anyone who wants to learn about insects in gardens and other "backyard" habitats. This material is adapted from the "Insects" chapter written by Dr. Lee Townsend for the Kentucky Master Gardener Handbook. These written materials are the main study materials for master gardeners, and they are available to download at no cost:

To learn more about becoming a master gardener in Kentucky, visit your local county extension office.

This guide is presented in the following sections:

Introduction (this page)
Insect Basics
Insect Orders I: Gradual Metamorphosis
Insect Orders II: Complete Metamorphosis
Understanding Insects in the Garden


Insects are very small, but they make a big impact. Out of the aproximately 1.8 million known species on earth, about 1 million of them are insects. That means that there are more kinds of insects on earth than all other kinds of animals, plants, fungi, and even viruses and bacteria, combined. If you add in the other arthropods—that is, the other creatures that are closely related to insects, like spiders, ticks, and crustaceans—the number grows to about 1.2 million known species. Insects and there relatives are found in almost every habitat on earth. In Kentucky, they are found in essentially every habitat, from treetops to roots, the wilderness to the home, and from the sky to the bottoms of rivers. Of course, gardeners know that insects are very common on tomatoes, roses, melons, and other garden plants. Kentucky has about 15,000 known species of insects. Fortunately, only a few of those species are garden pests. With a little bit of knowledge, gardeners can learn to tell the "bad" bugs from the beneficial ones. Proper identification is the most important step in any pest control attempt.


The circles above show that there are more known species of insects and their relatives than, for instance, all known plants, animals with bones (mammals, birds, reptiles, amphibians, and fish), and viruses. 

What is an Arthropod?

To understand what an insect is, it is a good idea to know a little bit about arthropods. Arthropoda is simply a very big group (or "PHYLUM") of animals that includes the insects plus all of their relatives, including spiders, ticks, crabs, and other small invertebrates (that is, creatures without internal bones) with multiple pairs of legs. The definition of an arthropod is any creature that has:

  • no internal skeleton
  • multiple pairs of jointed, segmented legs

Some examples of creatures that are NOT arthropods include: slugs, snails, earthworms, snakes. and starfish.  

What are the Five Classes of Arthropods?

There are five groups, or CLASSES, of arthropods found in Kentucky. Learning the differences between them is the first step in identification. The chart below shows the classes along with some of their characteristics:


Crustacea Crayfish, lobsters, and crabs are the biggest and best-known members of this class. Most live in the water and breathe by gills or through the exoskeleton. Pillbugs and sowbugs are among the few crustacteans that live on land, but they have gills and must stay in humid areas to survive. Pillbugs and sowbugs are the only crustaceans that are common in Kentucky gardens and they are beneficial decomposers in most cases. (Optional: Learn more about Kentucky crustaceans: Sowbugs | Crayfish)

Chilopoda Centipedes have at least 15 pairs of legs, with one pair of legs attached to each body segment. These fast-moving predators use a pair of fangs (modified legs) on the segment behind the head to bite and paralyze their prey. They are common in mulch and leaf litter in gardens and are beneficial predators. (Optional: Read more about Kentucky Centipedes)

Diplopoda Millipedes have typically 40 or more pairs of short legs, with two pairs attached to each body segment. They generally feed on decaying organic matter, helping to break it down into smaller pieces. Millipedes can be difficult to distinguish from centipedes, but millipedes generally move very slowly, while centipedes in general can move very fast. Occasionally, millipedes feed on plant roots or leaves in contact with the soil. They are common in gardens but are generally beneficial. (Optional: Learn more about Kentucky Millipedes)

Arachinda Arachnida is a large class that includes many different types of organisms, including all spiders, mites, ticks, daddy longlegs, and scorpions. Arachnida have four pairs of walking legs and two body regions: the head and thorax are one region and the abdomen is the other. These two regions are clear in spiders. Mites, ticks, and daddy longlegs, however, look as if they have only one region, and a scorpion, because of its segmented tail, appears to have more than two. Other than some species of mites, arachnids found in Kentucky gardens are not plant pests. One arachnid order that is important for gardeners to know is Acarina (mites). Plant-feeding mites are small, round-bodied arthropods with eight pairs of legs that use needlelike mouthparts to pierce plant cells and remove sap. The empty cells appear on leaves as very small white-to-yellow specks. The amount of spotting increases as mite numbers build. Heavy infestations may cause brown leaves or needles and premature leaf drop. Foliage infested by spider mites becomes covered with distinctive fine silk webbing. (Optional: To learn more about common Kentucky arachnids, visit these pages: Spiders | Daddy Longlegs | Scorpions | Mites & Ticks)

common arachnids

Common archnids in gardens and yards: Wolf Spider (Karan A. Rawlins, University of Georgia, Bugwood.org), Sheetweb Spider (Sturgis McKeever, Georgia Southern University, Bugwood.org), Boxwood Spider Mite (Rayanne Lehman, Pennsylvania Department of Agriculture, Bugwood.org)

Insecta. Insects are the main focus of this guide and there are more kinds of insects than all of the other arthropod classes combined. With some exceptions (most notably, mites) most of the important garden pests are insects, but there are many beneficial species too. Insects can be plant-feeders, predators, or decomposers, depending on the species. Insects have 3 pairs of legs, 3 body regions (head, thorax, abdomen), and 1 pair of antennae.

NEXT: Learn about insects in more detail, including basic biology and identification.