How do we identify an insect?

The world of insects is rich and varied. By carefully observing the external structure of insects, we can get to know them better.

The world of insects is rich and varied. By carefully observing the external structure of insects, we can get to know them better. Biologists have thoroughly described the structural features of the insect body through centuries of observations, and they divided the body into several different parts according to different functions and structures.

▲Legend: Anatomy of a Dragonfly

The body of every adult insect consists of three parts: head, thorax, and abdomen. The head contains the majority of the sensory organs such as the antennae and eyes, and of course the mouthparts as well. The legs and wings are on the thorax, so the thorax assumes all of the creature’s locomotive functions. The abdomen is the center of metabolism and reproduction; almost all of the insect’s internal organs are concentrated in the abdominal cavity. The excretory pores and external genitalia are located at the end of the abdomen.

While insects of the same class often have structural similarities, the differences between taxa can be significant, making identification easier. For example, the texture of the wings, the shape of the legs, the type of antennae, etc., can all be used to aid in identification.

▲Legend: A close-up of a butterfly

Wings are very important to insect taxonomy. Most winged insects can be classified into the correct order based on the appearance of their wings (see Appendix for details). Then we can further narrow it down by looking at other features to determine the category (order, suborder, family, etc.) to which it belongs.

Identifying butterflies and moths

As you can see, the two insects pictured above have wings that are covered with finely colored scales, so they both belong to the order Lepidoptera (this order includes all butterflies and moths).

The insect on the left is fat, has short wings, and looks a lot like a moth, right? But as we can see, the end of its antennae are expanded into a knob shape – a characteristic of butterflies – so we can conclude that it is actually a butterfly (it is a large skipper (Ochlodes sylvanus), which belongs to the family Hesperiidae).

The insect on the right looks like a butterfly, but its antennae are feathery. This is a characteristic of moths, so we can conclude that it’s a moth (it’s a Luna moth (Actias luna), which belongs to the family Saturniidae).

▲Legend: Types of insect antennae

Identifying different beetles

In addition to Lepidoptera, Coleoptera is another large and well-known order in the insect world. Collectively called beetles, they have forewings that are highly ossified, hard, and opaque. Based on their other characteristics, they can be subdivided into several groups such as weevils, scarabs, longhorn beetles, rove beetles, etc.

▲Weevils have a head that extends forward into a snout-like rostrum.

▲Scarabs have lamellate antennae which look like folding fans.

▲Longhorn Beetles possess long setaceous antennae which often make up over half of their total length.

▲Rove Beetles have extremely short sheath-wings, leaving a long section of the abdomen exposed.

Once you figure out these regular patterns, you will be able to correctly group insects under the proper category.

Same taxon, similar appearance

The greater challenge, however, comes when trying to distinguish between similar species that belong to the same family. Insects of the same taxon always have a similar structure, meaning we must look at the insect more closely and discover other, more minor details, such as the presence, number, and size of markings.

The most common and infamous of the mosquitoes include the two above – the yellow fever mosquito (Aedes aegypti) and the Asian tiger mosquito (Aedes albopictus). We can quickly distinguish them by the patterns on their backs (pronotum): the yellow fever mosquito has two straight white lines and two white curves that form the shape of a violin; the Asian tiger mosquito, meanwhile, has a lone white line on its back.

In addition to these two, there are many other species of black-and-white mosquitoes, all of which can be distinguished by the patterns or other details of their backs. In general, the smaller the insect and the greater the number of congener species (species under the same genus), the more difficult it is to identify them.

▲Legend: A close-up of Ants

There are also insects that have evolved into multiple variants, making it more difficult to identify them. For example, different individuals of the famous Asian lady beetles (Harmonia axyridis) mature with very different shell colors and varying numbers of spots. Distinguishing their variants from other species of lady beetles can thus be tricky.

Confusing species, closer observation

In some cases, insects of completely different genera can have a similar appearance. In these cases, they have usually evolved through mimicry in an attempt to protect themselves from predators. This requires that we focus on some detailed characteristics to distinguish them.

Do the two bugs pictured above look and act like bees?

A detailed comparison will help us to distinguish them.

The insect on the left has short antennae – barely more than nubs. Its eyes are large, round, and brownish-red. If you look closely, you will notice that it has only one pair of membranous forewings, and in place of hindwings, it has yellow halteres – tiny club-shaped balance rods. Its three pairs of legs are very slender and delicate looking.

The insect on the right, on the other hand, has thick, prehensile antennae with elbow-like joints. Its compound eyes are long, black, and almond-shaped. It has two pairs of membranous wings and no halteres. Its legs are relatively thick, and its hind legs are, in fact, specially adapted to carry pollen in little pollen baskets. It is common to see a yellow patch hanging from the hind leg of a bee, but this is actually a pollen clump that it has collected.

The insect on the left is actually a hoverfly that doesn’t make honey and cannot sting, while the one on the right is the real bee.

Insects are so diverse, and have so many different characteristics that it is often impossible to identify them by a single physical feature. Observation and awareness of their many special body structures can help you to distinguish them from one another.

In addition to physical characteristics, we sometimes need to look at the juvenile stages, specific behaviors, feeding habits, host plants, and geographic distribution of insects to identify species that appear very similar.

Appendix: Common insect taxa (Each taxon will be followed by a picture)

Coleoptera

Characteristics: The forewings are ossified (hardened), and the wing veins are not visible. This order contains almost all beetles.

Common examples: ground beetles, stag beetles, scarabs, click beetles, fireflies, leaf beetles, longhorn beetles, and lady beetles

▲Examples of various types of beetles

Diptera

Characteristics: The forewings are plainly membranous and the hindwings are little more than rods, known as balance rods or halteres.

Common examples: blow flies, flesh flies, hoverflies, mosquitoes, crane flies, gnats, gadflies, and midges

Lepidoptera

Characteristics: The dual pairs of large membranous wings have dense, soft scales on their surfaces. Their mouthparts consist mainly of a straw-like proboscis.

Common examples: swallowtail butterflies, brush-footed butterflies, skippers, cup moths, geometer moths, tiger moths, and owlet moths

Hymenoptera

Characteristics: In members with wings, all four wings are membranous, with large forewings and small hindwings. This order contains all the bees and ants.

Common examples: ants, bees, spider wasps, parasitoid wasps, velvet ants, and leaf-cutting bees

Orthoptera

Characteristics: The forewings are leathery and translucent, with visible veins. The hind legs are very long and used for jumping.

Common examples: katydids, crickets, mole crickets, grasshoppers, and locusts

Homoptera

Characteristics: The wings are of uniform texture and are often closed and ridged when resting. Homoptera is now considered by most biologists to be a suborder of Hemiptera thanks to extensive genetic studies.

Common examples: cicadas, planthoppers, leafhoppers, whiteflies, scale bugs, and aphids

Hemiptera

Characteristics: Leathery anterior wings cover and ordinary membranous posterior wings. All the insects that were originally in the order Homoptera are also included in this order.

Common examples: stinkbugs, jewel bugs, leaf-footed bugs, shield bugs, and bedbugs

Neuroptera

Characteristics: The difference between the front and rear wings is not obvious. The wings are transparent and have distinct reticulated veins that resemble dense blood vessels.

Common examples: lacewings, owlflies, dusty wings, and antlions

Isoptera

Characteristics: The texture, size, and shape of the fore and hind wings are almost indistinguishable. The wings fall off easily (and some individuals do not have wings). This order has been incorporated into the order Blattodea following extensive genetic studies.

Common examples: termites

Dermaptera

Characteristics: These species may be winged or wingless. The forewings of winged individuals are leathery, short, and cover only the thorax, while the hindwings are membranous and folded under the forewings. The tail has pincer-like appendages.

Common examples: earwigs

Trichoptera

Characteristics: These insects are identified by the presence of hairs on the surface of the body and membranous wings.

Common examples: caddisflies and sedge-flies

Thysanoptera

Characteristics: These tiny species are either wingless or have narrow wings with coma-like hairs on the edges of the wings. They are unique for their asymmetrical mouthparts.

Common examples: thrips

Zygentoma

Characteristics: These insects prefer dark areas and have a flattened body without wings. The antennae and tail whiskers are long.

Common examples: silverfishes and firebrats

Archaeognatha

Characteristics: Their spindle-shaped body has a raised back and lacks wings. It has one pair of tail whiskers and one mid-tail wire.

Common Examples: bristletails

Phthiraptera

Characteristics: The insects’ bodies are small and translucent, with reduced, nonfunctional wings. Their climbing feet are capable of holding hairs but have difficulty on flat terrain. All of these species are parasitic.

Common examples: lice

Siphonaptera

Characteristics: These small bugs have laterally flattened bodies and hopping hind legs that give them the ability to leap large distances.

Common examples: fleas

Mecoptera

Characteristics: The wings are membranous, narrow, and often patterned. The underside of the head extends into a downward-pointed rostrum. Males’ genitalia look similar to scorpion tails.

Common examples: scorpionflies and hangingflies

Plecoptera

Characteristics: The body is flattened. All four wings have a similar veiny texture, and get neatly stacked when resting, with only the outermost wing visible.

Common examples: stoneflies

Megaloptera

Characteristics: The forewings and hindwings are similar in texture and exhibit pronounced veins. The bodies are large, with well-developed pincer-like jaws (some species’ jaws are very long).

Common examples: fishflies, alderflies, and dobsonflies

Phasmatodea

Characteristics: These insects evolved effective camouflage to make them appear like twigs or leaves. Some species have wings while others do not.

Common examples: walking sticks and leaf-insects

Odonata

Characteristics: These carnivorous insects possess very long abdomens and four long, thin, veiny wings. They have large compound eyes for hunting other insects.

Common examples: dragonflies and damselflies

Ephemeroptera

Characteristics: These insects sport large compound eyes and thin, veiny wings. Hairlike appendages extend from the rear of their delicate abdomens.

Common examples: mayflies

Mantodea

Characteristics: These are predatory insects with long, thin bodies and an upright posture. They typically use only four legs for locomotion because the large forelegs are specifically adapted for snatching and holding prey. Some species have wings while others do not.

Common examples: praying mantises

Blattodea

Characteristics: These are low-crawling bugs with flattened or semi-flattened bodies and powerful mouthparts capable of chewing. Some species possess wings while others do not. Termites have recently been reclassified under this order.

Common examples: cockroaches and termites