Animals > Arthropods > Insects
Insects: class Insecta
The rest of this chapter is devoted to a closer look at insects. The most important goal of this part of the lab is for you to look closely at some insects and understand their features. We’ll approach this goal by studying the features that differentiate the various insect orders from one another.
Almost all insects molt, or shed their cuticle, periodically as they grow. At each molt, an insect’s body may go through a morphological change, until it finally reaches its adult form, in which it is sexually mature. After this, molting generally stops. There are two styles of insect development: hemimetabolous and holometabolous. Hemimetabolous insects go through a series of nymph stages, each stage looking a little more like the adult than the previous stage. This is probably the original type of life history for insects, and is similar to some other arthropods, such as crustaceans.
Holometabolous insects go through a dramatic metamorphosis, passing though a pupal stage during which they break down almost all the structures in the larval body and use the materials to construct a new adult body. Holometabolism probably evolved only once in insects; the various orders of holometabolous insects form a single clade, called Endopterygota. Holometabolous insects are an extremely diverse group, including beetles (Coleoptera), flies (Diptera), and bees (Hymenoptera), and making up the big majority of insect species. Part of the reason for the remarkable evolutionary success of this group may be that the holometabolous life cycle allows a single species to have larval and adult phases that are very different from one another, often living in different habitats.
- Hemimetabolism and Holometabolism (Wikipedia)
- The earliest known holometabolous insects (abstract in Nature)
Insects have complicated mouths, formed from multiple parts. The various orders of insects have their own particular styles of mouthparts; within an order, mouthparts may also differ according to diet.
- Insect mouthparts (Wikipedia)
Insects are the only invertebrates that can fly. Most kinds of insects have two pairs of wings. The wings normally appear only when an insect reaches the adult (reproductive) stage; larval or nymph stages don’t have wings. The form and function of the wings varies from one insect order to another. Many have membranous wings, which are thin and usually more or less transparent, and are functional for flying. In some insect orders, the wings may be thick and opaque (not membranous), or greatly reduced in size so that they don’t function for flying, or one or both pairs of wings may be missing entirely. See the insect order descriptions below for more specific descriptions.
- Insect wing (Wikipedia)
- Insect wings (Brisbane Insects)
- Insect wings shred bacteria to pieces (Nature). While not directly relevant to Bio 6A, this article describes the surprising way that insect wings can destroy bacteria.
- Insights into insect wing origin provided by functional analysis of vestigial in the red flour beetle, Tribolium castaneum (PNAS) Well beyond the scope of Bio 6A, this article gives the advanced reader a look at how modern biologists study evolution.
Adult insects normally have one pair of antennae, which function as chemoreceptor and mechanoreceptor organs. The antennae vary widely, from tiny (e.g., dragonflies, which primarily locate their prey by vision) to huge and elaborate (as found in some beetles and moths, which locate their mates by following pheromone trails). Antenna morphology is important for identifying insects.
- Antennae (Wikipedia)
Insects normally have a pair of compound eyes, which are large and obvious. Many insects also have two or three ocelli, which are smaller, non-compound eyes.
- Ocelli (BugGuide)
Appendages on the abdomen:
Many insects have some sort of appendages on the posterior end of the abdomen. One example that comes up in today's lab is the large pincers (cerci) on earwigs (Dermaptera).
- Cerci (Wikipedia)
Identifying insect orders:
There is a lot of diversity in the class Insecta. In this lab, you'll get an overview of some of that diversity by learning to identify a few orders of insects. To do that, you'll need to learn to recognize some of the characteristics that distinguish one insect order from another. When you see a new insect, start by looking closely at the characteristics listed above, especially the wings, antennae, and mouthparts.
Learn to look at these three things, and you're well on your way to identifying many kinds of insects. The images below are intended to help you learn to recognize these characteristics. Later, on a test, you will be asked to look at some insect specimens, recognize what order they're in, and understand what characteristics identify them as belonging to that order. To get started, look at the following pages on the insect orders you'll see in lab:
- Coleoptera: beetles.
- Dermaptera: earwigs.
- Diptera: flies.
- Hemiptera: true bugs.
- Hymenoptera: ants and wasps.
- Lepidoptera: butterfies and moths.
- Odonata: dragonflies.
- Orthoptera: Grasshoppers and crickets.
The traditional way to identify insects is to use a dichotomous key. To use such a key, you make a series of choices describing the insect you're looking at -- for example, it has wings, or it doesn't. Each choice leads you to a new dichotomy, narrowing down the possibilties until you have identified the insect. See InsectIdentification.org for an example, which will help you identify the insects in today's lab.
Objectives for this lab:
On this site, read the pages on Arthropoda, Insecta, and the various orders of insects listed above. On the next lab exam, you will see some insect questions such as this:
- What order does this insect belong to?
- What features does insect specimen A have that tell you that it's not in the same order as specimen B?
- Contrast the mouthparts of these two insect specimens, and relate the mouthparts to the insect's way of feeding.
- Compare & contrast holometabolous and hemibetabolous life cycles in insects.
BugGuide, a good reference for identifying insects.
Insect Morphology in Wikipedia.
Insect Mouthparts in Wikipedia.
InsectIdentification.org Dichotomous key and descriptions of orders.
P.J. Gullan and P.S. Cranston, 2010. The Insects: An Outline of Entomology, Wiley-Blackwell. An excellent entomology textbook; if you want to learn about insects, you'll do far better reading this book than searching online for information.
This page updated November 12, 2013