Key points from the key
- The key leads to 17 common urban ants, named to species or genus, through yes or no questions about body features.
- The first question is the number of petiole nodes, the small segments between the thorax and the abdomen: two nodes lead to the Myrmicinae, such as pavement, acrobat and fire ants.
- With one node or none, the next tests are a boxy petiole (Asian needle ant) and an acidopore, a ring of hairs at the tip of the abdomen found in the Formicinae.
- Body size is printed where it decides the answer, from 0.5 to 2 mm for the thief ant to 5 to 16 mm for large ants such as the carpenter ant.
All points are from the key, created by D. M. Sorger and adapted from L. M. Nichols.
The key in text
Title: Urban Ant Identification Key. Start at “Start here” and follow the arrows. Step numbers are ours; questions are as printed (US spelling, including “aprox.”), with size ranges written out with “to”.
| Step (our numbering) | Question on the key | Answer and next step |
|---|---|---|
| 1 | Number of petiole nodes | Two: go to 2 (Myrmicinae). One or none visible: go to 7 |
| 2 | Spines present on the posterior part of propodeum? | Yes: go to 3. No: go to 6 |
| 3 | Gaster heart-shaped in dorsal view? | Yes: Acrobat Ant. No: go to 4 |
| 4 | Petiole & postpetiole square & approx. same size? | Yes: Pavement Ant. No: go to 5 |
| 5 | Antennal club present? | No: Winnow Ant. Yes: Big-Headed Ant |
| 6 | Segments of antennal club, color and body size? | 3-segmented club, black, body size approx. 1.5 mm: Little Black Ant. 2-segmented club and yellow & tiny (0.5 to 2 mm): Thief Ant. 2-segmented club and reddish-brown & larger (3 to 6 mm): Fire Ant |
| 7 | Petiole prominent & boxy (aprox. twice as tall as wide)? | Yes: Asian Needle Ant (Ponerinae). No: go to 8 |
| 8 | Rear end with acidopore? (circular opening surrounded by fringe of hairs) | Yes: go to 9 (Formicinae). No: go to 13 (Dolichoderinae) |
| 9 | Antenna 9-segmented and body size 1 to 2 mm? (Remember to count the scape, first, long segment!) | Yes: Rover Ant. No: go to 10 |
| 10 | Body size? | Smaller 1.8 to 4 mm: go to 11. Large 5 to 16 mm: go to 12 |
| 11 | Head & body with many isolated thick hairs & mesosoma with row of such hairs arranged in pairs? | Yes: Crazy Ant. No, and body smooth & shiny, mesosoma in dorsal view hourglass-shaped, body size 2.5 to 3.5 mm: Winter Ant. Otherwise, usually densely covered in short appressed hairs giving it dull appearance: Garden Ant |
| 12 | Very large body size & mesosoma in side view rounded? | Yes: Carpenter Ant. No: Field Ant |
| 13 | Petiole flattened, barely visible? | Yes: go to 14. No: go to 15 |
| 14 | Body size 1 to 2 mm? | Yes: back to Rover Ant (You missed the acidopore!). No: Odorous House Ant |
| 15 | Anterior margin of clypeus without indentation & color of gaster yellow? | Yes: High Noon Ant. No: Argentine Ant |
The 17 ants on the key
Scientific names are colour coded by subfamily on the graphic. Each ant has a photograph and a black silhouette at actual size next to a 10 mm scale bar.
| Common name | Scientific name | Subfamily | Fact on the key |
|---|---|---|---|
| Acrobat Ant | Crematogaster spp. | Myrmicinae | …can raise its abdomen (gaster) above its head |
| Pavement Ant | Tetramorium spp. | Myrmicinae | …has turf wars on sidewalks |
| Winnow Ant | Aphaenogaster spp. | Myrmicinae | …has a slender body and long legs |
| Big-Headed Ant | Pheidole spp. | Myrmicinae | …has two worker sizes, big-headed “majors” and regular-headed “minors” |
| Little Black Ant | Monomorium minimum | Myrmicinae | …has colonies with multiple queens |
| Thief Ant | Solenopsis molesta | Myrmicinae | …steals food from other ants |
| Fire Ant | Solenopsis invicta | Myrmicinae | …varies in size from small to very large workers and everything in between |
| Asian Needle Ant | Brachyponera chinensis | Ponerinae | …is an invasive species |
| Rover Ant | Brachymyrmex spp. | Formicinae | …lives underground |
| Crazy Ant | Nylanderia spp. & Paratrechina longicornis | Formicinae | …runs around like crazy when foraging |
| Winter Ant | Prenolepis imparis | Formicinae | …is mainly active during spring & winter months |
| Garden Ant | Lasius spp. | Formicinae | …milks aphids for honeydew |
| Carpenter Ant | Camponotus spp. | Formicinae | …nests in wood |
| Field Ant | Formica spp. | Formicinae | …smells like vinegar |
| Odorous House Ant | Tapinoma sessile | Dolichoderinae | …smells like blue cheese |
| Argentine Ant | Linepithema humile | Dolichoderinae | …is an invasive ant that forms supercolonies |
| High Noon Ant | Forelius spp. | Dolichoderinae | …is active during high temperatures |
Anatomy diagram
A side view drawing labels the antenna (scape and antennal club), head (eye, clypeus, mandibles), thorax or mesosoma (pronotum, mesonotum, propodeum), petiole and postpetiole, and abdomen or gaster with stinger, plus the leg parts coxa, femur, tibia and tarsus. Footnote: structure varies depending on subfamily/genus.
Credits printed on the side: studentsdiscover.org. Created by D.M. Sorger (adapted from L.M. Nichols). Photos by D.M. Sorger, round close-ups from antweb.org.
Identifying black ants with the key
Only one ant on the key is identified by being black: the little black ant, Monomorium minimum. To reach it you need two petiole nodes, no spines on the propodeum, and a three segmented antennal club on a black body about 1.5 mm long. A larger dark ant with one node, an acidopore and a body of 5 to 16 mm leads instead to the carpenter ant or the field ant, which the key separates by the shape of the mesosoma in side view.
Invasive ants on the key
Two of the ants are flagged as invasive. The Argentine ant is native to northern Argentina, Uruguay, Paraguay, Bolivia and southern Brazil, and has been spread by people to many regions with Mediterranean climates, including the continental United States, where it is a significant pest in farms and towns [3]. The Asian needle ant, a stinging ant native to Japan and mainland Asia, was introduced to the United States in the 1930s; its sting can cause anaphylaxis, and it competes with the Argentine ant for territory [4].
Notes on this infographic
- The graphic does not name the region it covers. Sorger’s current site offers ant keys for the south-eastern and Midwest United States and for central and northern Europe [1].
- Our submission records credited Roberto M. Gibert and Grzegorz Buczkowski of Purdue University. The graphic itself credits D. M. Sorger, and her former site says her identification keys have moved to her new site [1][2].
- The credit line “studentsdiscover.org” is printed on the graphic and kept as written. Students Discover, a free science curriculum project, published Sorger’s ant artwork in its Ant Picnic lesson [5].
- The Odorous House Ant branch asks whether the body is 1 to 2 mm; if so, the key sends you back to the rover ant, noting “You missed the acidopore!”
Questions about ant identification
How do you identify ants?
With a key like this one, look at the ant from the side under magnification. Count the petiole nodes first, then check for spines on the propodeum, an acidopore at the tip of the abdomen, the number of antennal segments and the body length.
How many ant species are there in the world?
About 15,000, according to D. Magdalena Sorger’s site [1].
Sources
- D. Magdalena Sorger, Discover Ants, Identification tools (in German). The creator’s current page for her identification keys.
- D. Magdalena Sorger, the ant life, Identification keys.
- Wikipedia, Argentine ant.
- Wikipedia, Brachyponera chinensis.
- Students Discover, Ant Picnic lesson.
Last reviewed 30 October 2026
