Often we are asked for an easy way to identify ants using colour and size. Unfortunately, there are so many small brown ants (for example) that it is very difficult to provide a simple key. With over 12,000 ant species, identifying them can be a tricky business. Some species are easy to identify and some are more difficult. Correctly identifying your target species is the first step in planning an appropriate management response.

The PIAT gives a general description of the 5 worst threat ants of the Pacific, and another 13 common invasive ants in the Pacific to help you narrow down your search.

You can find out what other ant species may be in your area by looking on our Ants of the Pacific database. Knowing the ants already in your country can make identification much simpler.

We have also developed resin blocks, with the 5 worst invasive ants in the Pacific embedded in them as a reference. These are available for biosecurity / quarantine officers from SPREP / SPC / Pacific Biosecurity.

If you need help in identification, find out how to capture images and send samples to experts. As well as contacting individual experts, the Pacific Island Network for Taxonomy may be able to help.

A basic guide to identifying ants : How to use a binocular or dissecting microscope : Additional resources for identification : Chemical identification of fire ants

The Pacific Invasive Ant Key

The home page of the Pacific Invasive Ant Key (© Eli Sarnat)
  Arguably the best and most comprehensive online resource to help identify invasive ants in the Pacific is the Pacific Invasive Ant Key (PIAkey), which provides pictures and diagnostic characteristics for dozens of different invasive ants that have invaded the region (in the factsheets section).

Identifying ants from pictures can be difficult without a microscope. The PIAkey includes baiting videos of ant behaviour, which can really help in identifying them.

Behaviour can be an easy way to identify ants. For example, a small yellow ant falling from trees and stinging people is most certainly a little fire ant

Note that the key to species section in the PIAkey only works for old Java / Firefox versions and will not work on Chrome browsers. However, the factsheets are very useful.

Although the PIAkey is an online tool, free CD copies can be ordered. Please contact Eli Sarnat regarding ordering.

By the same team that made the PIAKey, is also an excellent resource with a key (that works with all browers and versions of Java) to over 100 species of exotic ants found worldwide.

Antkey is also available as a mobile app for Android and iOS.

Back to top

A basic guide to identifying ants

By dividing animals into smaller and smaller groups based on physical characteristics that are exclusive to each group, we are able to identify them more efficiently than looking at an individual and asking "what is this animal?". This is based on viewing all living things within a hierarchy like the image on the right.

The physical characteristics we use to identify animals (and all other living things) are known as morphological characters. For example, we can separate ants from every other type of animal on Earth in just five steps:

Kingdom: Animalia – all the animals on earth

Phylum: Arthropoda – are animals with an exoskeleton (external skeleton) and a segmented body, with paired jointed appendages (this group includes crabs, lobsters, spiders, centipedes, woodlice and insects)

Class: Insecta – all insects have three body segments (head, thorax and abdomen) and 3 pairs of legs attached to the thorax (includes beetles, butterflies, grasshoppers, wasps, bees, ants etc.)

Order: Hymenoptera - have two pairs of wings joined by wing hooks. Only this order has this characteristic wing morphology (includes sawflies, woodwasps, wasps, bees and ants)

Sub-Order (not shown on the right): Apocrita - all have a constricted joint between the thorax and abdomen known as a “wasp waist”. Only ants, wasps and bees are the animals to have this characteristic

Modern classification hierarchy (Pengo [Public domain], via Wikimedia Commons)

Family: Formicidae - all have a well-defined segment (the petiole) between the thorax (mesoma) and abdomen (also called the gaster in ants) and have “elbowed” antennae. Only ants have these characters.

From there we can use special features to identify the sub-family, then the genus and finally the exact species of ant. The genus and species identifiers are used to give the ant its scientific name. For example, the red imported fire ant's scientific name is Solenopsis invicta. Solenopsis is the genus and invicta is the species part of the name. This way we can also tell that the tropical fire ant (Solenopsis geminata) is closely related to the red imported fire ant (they are in the same genus). And these ants look very similar because they are closely related.

Back to top

Key features used to identify ants

Ants are identified by experts using taxonomic keys that are based on characteristics (characters or features of those ants).

Identifying ants can be a complex problem as there are thousands of species of ants and sometimes only minor, microscopic features set them apart.

There are more than 12,000 species of ants worldwide. By grouping these species into sub-families it becomes easier to identify them. There are 16 sub-families of ants, but only four of these sub-families are a concern in the Pacific:

  • Myrmicinae
  • Ponerinae
  • Formicinae
  • Dolichoderinae

Identifying a sub-family in 5 easy steps

The four ant sub-families found in the Pacific (Myrmicinae, Ponerinae, Formicinae, Dolichoderinae) can easily identified in a few short steps by looking at just a couple of characters (or body parts).

First it is important to know the names of these body parts or characters, which are shown in the diagram below. They key characters used to separate the sub-families of ants are: the petiole and the gaster (particularly the pointy end, which has a sting, acidopore [hole to spray acid], or slit, depending on the sub-family). The petiole is the joint that separates the mesoma - which the legs are attached to - from the gaster (the abdomen). Ants have either one or two of these petiole segments. 

You will need to use a microscope to see these features.


Left: Diagram showing the four main sections of an ant's body and anatomical features. Above: Diagram showing main features of the head. (© Eli Sarnat, Creative Commons Attribution, Share Alike CC BY-SA Licence)

Back to top

Step 1.

Does the ant have one or two waist / petiole segments? Or is the waist / petiole flattened or hidden? See below for examples.

Line drawings © Eli Sarnat, Creative Commons Attribution, Share Alike CC BY-SA License; Photos © Alex Wild

If the Pacific Island ant you are looking at has two petiole segments, you know it is in the Myrmicinae sub-family (a myrmicine ant). Problem myrmicines in the Pacific include African big-headed ant (Pheidole megacephala), bicoloured pennant ant (Tetramorium bicarinatum), bicoloured trailing ant (Monomorium floricola), little fire ant (Wasmannia auropunctata), pharaoh ant (Monomorium pharaonis), red imported fire ant (Solenopsis invicta), similar groove-headed ant (Tetramorium simillimum), Singapore ant (Trichomyrmex destructor) and tropical fire ant (Solenopsis geminata).

Step 2.

If the ant only has only one waist / petiole segment is it raised or hidden/flattened? See the image on the right for examples.

If the petiole is flattened or hidden it most likely belongs to the Dolichoderinae sub-family (a dolichoderine ant).

 Back to top

Line drawings © Eli Sarnat, Creative Commons Attribution, Share Alike CC BY-SA License; Photos © Alex Wild

Step 3.

Check to see that the end of the abdomen has no stinger or acidopore (hole) to confirm if it is a dolichoderine ant. Problem dolichoderine ants in the Pacific include Argentine ant (Linepithema humile), ghost ant (Tapinoma melanocephalum) and white-footed ants (Technomyrmex albipes, Technomyrmex difficilis, Technomyrmex vitiensis).

© Eli Sarnat, Creative Commons Attribution, Line drawings Attribution Non-Commercial CC-BY NC License; Photos Share Alike CC BY-SA License

Step 4.

If there is a single waist segment (petiole) and it is raised check to see if there is a pinched in constriction between the first and second segment of the abdomen.

If this constriction is present check the end of the abdomen for a pronounced stinger.

If the ant has a stinger and pinched abdomen it is a ponerine ant (sub-family Ponerinae).


Left photo © Alex Wild; Right photo and line drawings © Eli Sarnat, Creative Commons Attribution, Share Alike CC BY-SA License 

Ants from the Ponerinae sub-family are not a serious invasive threat in the Pacific.

However, some species, such as the trap-jaw ant, Odontomachus simillimus (left), can give a painful sting.


© Eli Sarnat, Creative Commons Attribution, Share Alike CC BY-SA License

Step 5.

If the waist / petiole is raised and there is no pinched abdomen check for the presence of an acidopore (left).

The acidopore is used to spray formic acid, and gives the sub-family it's name: Formicinae.

Problem formicine ants for the Pacific include yellow crazy ant (Anoplolepis gracilipes), black crazy ant (Paratrechina longicornis), browsing ant (Lepisiota frauenfeldi) and tawny crazy ant (Nylanderia fulva).


 © Eli Sarnat, Creative Commons Attribution, Attribution Non-Commercial CC-BY NC License

Back to top

Further identification

Once the sub-family has been identified other characters are used to identify species. We haven't included identification down to species level here, as the best resources for this are the PIAkey, Antkey and other resources are available.

Key characters to help determine the species include:

1. Presence or absence of hairs

2. Length of the first antennal segment (called the scape) relative to the head

Drawings © Eli Sarnat. Photographs © Alex Wild

3. Presence or absence of propodeal spines on the rear end of the mesosoma

© Eli Sarnat

4. Presence and number of antennal clubs

Drawings © Eli Sarnat. Photographs © Alex Wild

Back to top

How to use a binocular or dissecting microscope

Place the microscope on a level bench and plug it into the nearest power supply.

Place the specimen you want to identify into a Petri dish that is partially filled with preservative and place it under the microscope.

There should be two light sources on the microscope. The top light, shines down on the specimen. The bottom light shines from below the specimen. 

Turn on the light(s).

You should be able to adjust the light to allow you to view the specimen lit only from above, only from below, or lit from both above and below. Decide which offers you the best view of the specimen. 

Make sure the magnification control is set to the lowest setting. Close your left eye and look through the right eyepiece. Frame the specimen in the centre of the field of view.

The distance between the two eyepieces should be adjustable. Open your other eye and adjust the eyepieces until you can see a single image.

A standard binocular microscope with the main features labeled (© Allan Burne, Pacific Biosecurity)

One of the eyepieces may have a fine focus on it. If none is present there will be a fine focus dial on the coarse focus knob.

Close the eye that that is looking through the eyepiece with the fine focus (or if no fine focus is present on the eyepiece, close whichever eye you prefer) and use the coarse focus knob to bring the image into sharp focus for the other eye.

Once the image is in focus close the eye you just had open and look through the other eye. Use the fine focus adjustment to bring the image into sharp focus for that eye. The microscope should now be adjusted for your eyes.

You may now adjust the magnification control to increase the magnification enough to observe fine details on the insect.

Back to top

Additional resources for identification 

There are a  number of resources available online to help you identify ants. This workshop manual (download 4 MB) from the Pacific Invasive Ants Taxonomy Workshop is based on the PIAkey. 

For a more technical identification key for those that are familiar with ant identification, E.O. Wilson and R.W. Taylor published The Ants of Polynesia (download 10 MB), which provides a taxonomic key for identifying many native and introduced ant species in the Pacific.

This key to the ants of Micronesia (download 3 MB) may also be helpful, as could be the key to common pest ants of Malaysia (note that the identification of red imported fire ant being present in Malaysia is incorrect). For those trying to identify invasive ants from North America, identification and habits of major ant pests in the Pacific Northwest (download 4 MB) may be useful.

If you are having trouble with identification, consider contacting someone with expertise in identification. 

Chemical identification of fire ants

USDA scientist Steven Valles and APHIS colleagues developed a quick test to identify red imported fire ants (© Sanford Porter, USDA D3712-1)

Red imported fire ants and tropical fire ants are just two species of ants in a larger group of fire ants.

It is important to know the difference as red imported fire ants cause such major problems, while most of the other fire ant species do not.

Different fire ant species are very difficult to tell apart using their physical features. However, fire ants sting venom (a chemical poison) differs between species so that is one way to tell them apart.

Until recently this been a very specialised method only done by scientists.

USDA scientists have now developed a kit that can identify the red imported fire ant by detecting the specific venom of the ant. The kit uses an on-the-spot test that requires no specialized training and takes just 10 minutes to complete.

The kit is now being further developed so that it can be mass produced.

Content reviewed by Eli Sarnat, Antwork Consulting, LLC, October 2016

Previous page: Sugar devils Next page: Resin blocks