Identifying insect orders

In this lab you’ll be introduced to the diversity and identification of insects.

Objectives:

After completing this lab, you should be able to explain the following concepts:

  • Special features of insects
  • How to identify insect orders
  • Compare & contrast holometabolous and hemibetabolous life cycles in insects.

Specimens:

  • Insects. We have a collection of preserved insects, including specimens mounted on pins and specimens embedded in plastic.
  • Other arthropods. Specimens embedded in plastic.

Class Insecta: Introduction

The phylum Arthropoda is  the most diverse of all the animal phyla, and the class Insecta is the most diverse group of arthropods. In fact, most animal species are insects.

Insects have some distinctive characteristics that set them apart from other arthropods.

  • 3 main tagmata. Insect bodies are divided into three main parts: head, thorax, and abdomen. The thorax is where all the legs and wings are attached.
  • Legs: Three pairs (hence the name Hexapoda, meaning "six legs").
  • Antennae: one pair.
  • Wings and flight. Most insects can fly at some point in the life cycle; flight is thought to be one of the characteristics of the common ancestor of all insects. On the other hand, many insects have long, flightless larval or nymphal stages, so you'll find plenty of insects that lack wings. Other arthropods don't fly.
  • Gas exchange organs: Tracheae. The tracheal breathing system is one of the key adaptations enabling insects to live on land. Some insects have aquatic larval forms, which have gills.

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.

Life history:

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. The adult is the only stage that has wings. 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 have a larval stage that looks very different from the adult. They 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.

Mouthparts:

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. The diversification of mouthpart structures in insects is one of the essential things that has allowed the amazing evolutionary success of this group of animals.

Wings:

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.

Antennae:

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.

Eyes:

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.

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).

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 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, but the most important thing is for you to recognize them on the actual specimens in lab. 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. You'll see the following orders 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.

We're only going to look at adult insects; in many cases, the nymph or larval stages look very different and must be identified using different characteristics.

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.

Coleoptera: Beetles

Coleoptera are the most diverse order of insects, with hundreds of thousands of species.

Key features

  • Holometabolous
  • Chewing mouthparts
  • Wings: Two pairs of wings. The forewings (called elytra) are thick and hard, and often function as a protective shell. The hindwings are thin and membranous, and function for flying. At rest, the hindwings are usually completely hidden beneath the forewings. At first glance, beetles may appear to lack wings. The most distinctive and recognizable feature of the Coleoptera is that their hardened forewings meet in a straight line down the back. Some other insect orders also have hardened or leathery forewings, but they don't meet in a straight line and cover the entire back.
  • Antennae: Widely variable from one beetle family to another.

Examples

Beetle

Spotted Cucumber Beetle, Diabrotica undecimpunctata, Sonoma County, California. The forewings look like a hard shell on the beetle's back, with a seam straight down the middle.

Tiger beetle, Scotland

Tiger beetle, Scotland. Note the wing covers, which are opened slightly to reveal the membranous wings hidden beneath. These wing covers are actually the beetle's forewings, modified to become hard and non-functional for flight. This is typical for beetles, and it often confuses beginners trying to use an identification key to the orders of insects. If you didn't look closely, you might be tempted to think that beetles don't have wings at all. In fact, most beetles can fly, using only their hind wings.

Also note the long antennae and the large chewing mouthparts, typical for beetles.

Beetle larva

Beetle larva: I think this is a larval form of a predaceous water beetle, which I photographed in Henry Coe State Park. Beetles go through a complete metamorphosis (they are holometabolous), so their larval forms look completely different from the adults. Many insect live in water during their larval phase and then live in the air for their adult phase, but the beetle pictured here spends its entire life in the water.

If you saw this specimen on a lab exam, you might have a hard time distinguishing it from the other orders, but don't worry -- you won't be asked to identify larval insects on the exam.

References

Beetle on Wikipedia.

Dermaptera: Earwigs

Earwigs are very common in environments with damp, dark places to hide and organic matter to eat. If you have a backyard, you can probably find earwigs there. The extremely common earwig, Forficula auricularia, was introduced to California from Europe.

Earwig females provide considerable parental care to their offspring, guarding them even after hatching.

Key characteristics

  • Life cycle: hemimetabolous; earwigs go through four or more instars (developmental stages) on their way to adulthood.
  • Mouthparts: Chewing.
  • Wings: Very small, so earwigs appear to lack wings. The fore wings are usually hard and very short, leaving abdomen uncovered. Membranous hind wings fold up under the short fore wings. Some earwig species do lack wings entirely.
  • Antennae: Simple antennae, shorter than body.
  • Cerci (a pair of appendages on the posterior end of the abdomen) enlarged to form pincers, like a pair of forceps. This is the most distinctive feature of Dermaptera.

Examples

I don't have any earwig pictures; here's a nice one on flickr.

Reference

Dermaptera at Tree of Life Web Project.

Earwig at Wikipedia.

Diptera: Flies

The order Diptera includes mosquitoes as well as houseflies and a great many other species.

Key characteristics

  • Life cycle: Holometabolous.
  • Mouthparts: Diptera are adapted to a liquid diet, but their mouthparts are variable, according their feeding habits. Houseflies have "sponging" mouthparts for soaking up liquids; biting flies and mosquitoes have piercing-sucking mouthparts.
  • Wings: The name Diptera is given to this group because they have one pair of functional wings (Diptera = "two wings"). This makes them unique; other orders of winged insects have two pairs. In fact, the Diptera do have the vestigial remains of a second pair of wings. These tiny structures, called halteres, flap along with the wings and apparently help stabilize the insect in flight.
  • Antennae: In many flies, the antennae are very short and aristate (see below); in mosquitoes the antennae are longer and simple.
  • Eyes: Usually large in relation to body size, compared with beetles, earwigs, or other insects that don't fly very much.

Examples

Housefly

Housefly, my back porch. Note the unusual antennae, which are typical of flies. These short, branched antennae are called aristate antennae (see arista on BugGuide or "antenna types" for a diagram).

This picture shows a good example of "membranous" wings: they are thin and nearly transparent, with visible veins. Most flying insects have membranous wings, but you'll also see other kinds of wings (for example, in beetles, or Coleoptera).

Biting fly

Biting fly, drawing blood from a biology professor's leg. Unlike the housefly, this biting fly has a piercing proboscis. In this photo, you can clearly see that there is a single pair of wings. The halteres (modified hind wings) are hidden beneath the functional wings. Also note the short antennae and large eyes.

References

Diptera, Tree of Life Web Project

The Diptera site

Drone Flies and Rat-tailed maggots from the University of Florida... in case you ever wondered about rat-tailed maggots.

Fly on Wikipedia.

Morphology and anatomy: Mouthparts. More than you want to know about dipteran mouthparts; also see the Diptera Introduction on the same site.

More fly (diptera) mouthparts by Anthony Thomas

Hemiptera: "True bugs"

Like beetles, the Hemiptera often have non-membranous forewings, and they might appear at first to lack wings. However, the Hemiptera differ from Coleoptera in two important respects: the wing morphology (described below) and their piercing-sucking proboscis, which is completely different from the chewing jaws of beetles.

Key characteristics

  • Life cycle: Hemimetabolous.
  • Mouthparts: Piercing-sucking proboscis, forming a straight tube that points toward the posterior when not in use.
  • Wings: Two pairs of wings. The fore wings of many Hemiptera are thickened and leathery at the anterior end but membranous at the posterior end.
  • Antennae: Variable.

Examples

Hemiptera

The wings of many Hemiptera, like this "true bug," are hard to see at first glance. The membraous hind wings are visible at this insect's posterior end, but the leathery fore wings don't look like wings. The hind wings are hidden beneath the fore wings. Look closely, and you can see the membranous tips of both the forewings and hindwings over the tip of the abdomen.

Note that the hard, leathery forewings don't meet in a straight line down the back, as the do in beetles (Coleoptera). Also, Hemiptera have piercing-sucking mouthparts, unlike the chewing mouthparts of Coleoptera.

Cicada

This cicada is another example of Hemiptera. Cicadas use their piercing-sucking mouthparts to penetrate plant tissue and suck the sap. Cicadas don't have the leathery forewings of true bugs, but they do share the piercing-sucking mouthparts. You'll need to turn the insect over to look for the long tube of its mouth (proboscis) running from the head along the ventral side of the thorax.

References

Hemiptera: bugs, aphids and cicadas from CSIRO Australia.

Hemiptera on BugGuide.

Hemiptera on Wikipedia.

Hemiptera from Orange County, CA.

Hymenoptera: Wasps, ants, etc.

Key characteristics

  • Life cycle: Holometabolous.
  • Mouthparts: Chewing, but in bees the mouthparts are modified to form a long tubelike structure for drinking nectar.
  • Wings: Two pairs of membranous wings, but in ants the wings may not be present.
  • Antennae: Long and multisegmented.
  • Eyes: large, with three ocelli on top of head.

Examples

WaspWasp, Wyoming. Look closely and you can see that there are two pairs of wings. Often this is not easy to see, because the forewings lie on top of the hind wings. This is an important characteristic, because some flies (Diptera) look a lot like bees or wasps, but the number of wings will identify the order.

Another important distinguishing feature is the antennae; flies generally have very short antennae, while wasps and bees have longer ones.

Reference

Hymenoptera at CSIRO.

Hymenoptera at Wikipedia.

Lepidoptera: Butterflies & moths

Butterflies are easy to recognize due to their colored wings. It's worth taking a close look at those wings under the dissecting microscope. At first, they may appear to be simply opaque, but if you look closer you will see that the wings are thin and transparent, but covered with tiny colored scales.

Key characteristics

  • Life cycle: Holometabolous.
  • Mouthparts: A long tube, extended for feeding on nectar and coiled when not in use.
  • Wings: Two pairs, large and membranous, but covered with colored scales.
  • Antennae: Long; the shape differs between butterflies and moths.

Examples

Butterfly

Satyr comma butterfly, Polygonia satyrus, Yosemite. Note the long, coiled proboscis hanging down in front of this butterfly's face; this style of mouthparts is one of the most distinctive features of the lepidoptera.

Also note the long, "clubbed" antennae, with their wider ends; this is a characteristic of butterflies, but not moths.

Moth

Moth: This picture shows a moth with plumose (feather-like) antennae; the shape of the antennae is one of the important distinguishing characteristics between butterflies and moths.

Reference

Lepidoptera on Wikipedia.

Odonata: Dragonflies & damselflies

Dragonflies and damselflies are aerial predators, with large eyes and powerful, maneuverable wings that help them chase down other insects.

Key characteristics

  • Life cycle: Holometabolous. The larvae are typically aquatic and predaceous.
  • Mouthparts: Chewing.
  • Wings: Two pairs of elongated, membranous wings. The forewings and hindwings are close to the same size.
  • Antennae: Very short, bristle-like.
  • Eyes: Very large.
  • Body shape: Very long, thin abdomen with 10 segments.

Examples

DragonflyDragonfly, Waddell Creek, CA. Note the huge eyes and extremely short antennae; this is a visual predator.

Reference

Odonata Central: Fantastic rotatable Quicktime 3-D images, plus a picture gallery.

Orthoptera: Grasshoppers, Crickets, etc.

Key characteristics

  • Life cycle: Hemimetabolous. The nymph stages resemble the adults, and often live in the same habitat.
  • Mouthparts: Chewing.
  • Wings: Usually two pairs of wings, but some (e.g., Jerusalem Crickets) are wingless. The forewings are often thickened and colored. The hindwings are often broad and folded like a fan.
  • Antennae: Often long and thin, with many segments.
  • Eyes: Variable.
  • Body shape: Large hind legs. Cerci present near tip of abdomen.

Examples

GrasshopperCalifornia Rose-Winged Grasshopper, Henry Coe state Park. Grasshoppers are known for their jumping ability, but adults can also fly; some perform loud and conspicuous display flights when searching for mates.

Sample test questions

On a test, you should be able to answer questions such as:

  • 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.
  • Does this specimen show a holometabolous or hemimetabolous life cycle? (You would be provided with both adult and larval or nymph stages.)
  • How many pairs of wings does this specimen have? (It's not always obvious.)

References & Further Reading:

Insect Identification

BugGuide, a good reference for identifying insects.

Insect Morphology in Wikipedia.

Insect Mouthparts in Wikipedia.

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.

Insect photo galleries

Alex Wild

Adrian Thysse

Additional topics

Insect wings shred bacteria to pieces (Nature). "Antibacterial 'nanopillars' on cicada wings pull bacterial membranes apart." 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.

The earliest known holometabolous insects (abstract in Nature)

Phylogenomics resolves the timing and pattern of insect evolution (Science). This technical article gives a detailed explanation of insect phylogeny, based on comparative genomic studies. The article has a great cladogram.

 

 

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