Here is a home made key to Lasius based on information found in the links below. Bare in mind this is my interpretation of the info and I've never seen half of these in person. This is a very common genus of ant that I've decided to focus on because to often people were identifying the ant to subgroup level and accepting that as the species. I feel we can do better then that and chances are one of you may have found something other than the common L. neoniger, L. alienus, L. flavus, L. claviger, or L. umbratus.
Before we begin, I would to offer a special thanks to AntWeb.org and Bugguide.net as I link to both for pictures. Antweb is favored heavily though.
Identifying Lasius is surprisingly difficult. Good magnification is key. Species are broken up into four groups. Niger tend to be dark black brown workers and sometimes amber-orange in color. Flavus, Umbratus, and Claviger all have orange workers, and are almost completely subterranean. Within each group there seems to be a lot of hybridization but this doesn't seem to spread beyond the group they're in.
To determine which group the species is found two traits are vital. The first is the size of the eye compared to the size of the head! The second is the length of the maxillary palp which is located under the head and looks like an antenna. Both need to be viewed with a side view of the head.
This needs to be done to the worker caste. Queens are almost completely different and vary in appearance in some species.
Niger group all have large eyes compared to the head. They also have the longest maxillary palp.
Claviger group have small eyes compared to the head. They also have the shortest maxillary palp. It's said to be 3 segments long which is another way of saying it can not be seen with the naked eye!
Flavus group have small eyes compared to the head (on par with Claviger). They have a short maxillary palp but unlike Claviger it's viable though tiny.
Umbratus group have large eyes similar to Niger group. The maxillary palp is said to be somewhere between Niger and Flavus in length. They are probably the easiest to confuse with the Flavus group.
Hopefully you're still with me. As said before species hybridization tends to be common. What's more some species have subspecies or varieties where the Lasius you thought you knew has something crazy going on like no hair what so ever. You may be wondering if species can simply breed with one anther interchangeably what in the world keeps them pure? I don't have the answers for all the species listed but can offer an example. Lasius neoniger is a common species that nests in fields, prairie, and open grassland areas. Step into the woods though and you start finding Lasius alienus which prefers to nest in logs, tree stumps, or mixed nests under dead wood. Hybrids occur in both forest and field but for the most part they hold their nuptial flights above areas where they would normally nest, thus swarms are mostly separate and they keep their own genetics reasonably pure.
Before moving on it's important to know what a Seta Count is as well a few other things.
Seta Count - The number of Standing Hairs beyond the outline of the following: Anterior Scape Surface as viewed in line with the plane of funicular flexion, and the Outer Surface of the Fore Tibia as viewed in line with the plane of tibial flexion.
Standing Hairs - A hair with subdecumbent, suberect, or erect, i.e. forming an angle with cuticular surface of 45degrees or more.
To determine which group the species is found two traits are vital. The first is the size of the eye compared to the size of the head! The second is the length of the maxillary palp which is located under the head and looks like an antenna. Both need to be viewed with a side view of the head.
This needs to be done to the worker caste. Queens are almost completely different and vary in appearance in some species.
Niger group all have large eyes compared to the head. They also have the longest maxillary palp.
Claviger group have small eyes compared to the head. They also have the shortest maxillary palp. It's said to be 3 segments long which is another way of saying it can not be seen with the naked eye!
Flavus group have small eyes compared to the head (on par with Claviger). They have a short maxillary palp but unlike Claviger it's viable though tiny.
Umbratus group have large eyes similar to Niger group. The maxillary palp is said to be somewhere between Niger and Flavus in length. They are probably the easiest to confuse with the Flavus group.
Hopefully you're still with me. As said before species hybridization tends to be common. What's more some species have subspecies or varieties where the Lasius you thought you knew has something crazy going on like no hair what so ever. You may be wondering if species can simply breed with one anther interchangeably what in the world keeps them pure? I don't have the answers for all the species listed but can offer an example. Lasius neoniger is a common species that nests in fields, prairie, and open grassland areas. Step into the woods though and you start finding Lasius alienus which prefers to nest in logs, tree stumps, or mixed nests under dead wood. Hybrids occur in both forest and field but for the most part they hold their nuptial flights above areas where they would normally nest, thus swarms are mostly separate and they keep their own genetics reasonably pure.
Before moving on it's important to know what a Seta Count is as well a few other things.
Seta Count - The number of Standing Hairs beyond the outline of the following: Anterior Scape Surface as viewed in line with the plane of funicular flexion, and the Outer Surface of the Fore Tibia as viewed in line with the plane of tibial flexion.
Standing Hairs - A hair with subdecumbent, suberect, or erect, i.e. forming an angle with cuticular surface of 45degrees or more.
Scape - Long antenna segment that touches the connects with the head.
Tibia and Femur see here.
Please note this map and the initials for each region. After the species name the initials for the regions the ant is found is listed. If you live right on the boarder then consider including neighboring regions but otherwise only include ants that are found in your region. Ants found in SW, and SC may also be found in Mexico. Ants found in NW, NC, and NE may be found in parts of Canada that touch those regions. Individual states are mentioned in the event an ant has really limited distribution in a region.
Lasius Niger Group
Lasius neoniger SW, SC, SE, NW, NC, NE
This is the most common niger species found in the US. Found in fields nesting in open soil or under stones. Can hybridize with Lasius alienus. Workers tend to be brown in color with dark legs that are evenly colored. Scape and Tibiae are furry. Queens tend to have a velvet like sheen to the abdomen when viewed at certain angles, but this is also found in other Lasius.
Lasius alienus SW, SC, SE, NW, NC, NE
The most common niger species found in wooded areas. Found in forests, nesting in soil, rotten logs, and stumps. Can hybridize with Lasius neoniger. Workers tend to be black with lighter legs, usually yellow in color. Scape is lacking hair. Seta Count is less than 40, usually less than 20 standing hairs. Queens seta count is never more than 10 and is usually 0.
Lasius niger SW, NW, and Eastern Canada
It's unknown how the SW, and NW populations arrived, or did they originated there and spread to Europe and Asia where they're more common? Who knows. The Eastern Canada population only recently arrived from Europe within the last 50 years. (I know this by memory and am still looking for the source.)
Lasius alienus, and L. neoniger are subspecies of Lasius niger. It is found in both forests and fields, they nest understones and in open soil. Workers tend to be black like L. alienus. Scape and Tibiae are furry like L. neoniger. Legs tend to be dark but yellow at joints, however I don't know how consistent this is. Queens are described as being similar to L. alineus but are covered in hair like L. neoniger. (One wonders if the SW, and NW population isn't just a hybrid between L. neoniger and L. alienus in need of it's own species name.)
Lasius crypticus SW, NW, NC
Found in fields mostly but sometimes woodland. Nest under stones. Similar to Lasius neoniger but darker in color. Scape and Tibiae are mostly free of hair.
Lasius pallitarsis SW, NW, NC, Alaska, Most of Canada.
Found in forests and fields, nesting under stones. Amber Brown in color out west but to similar to L. umbratus in the east to rely on color alone. Has an extra tooth on the mandibles if you have a microscope. Scape tends to be free of hair. Legs tend to be lighter in color. Supposedly this species produces an odor but that hasn't been confirmed. What's really odd is the source mentioned the odor and also talked about umbratus species using them as a host. Niger group species aren't known to produce an odor and one would think for such a widely distributed species this would be an easy confirm or deny. They were being called by their former name though Lasius sitkaensis. (Assuming there is a population of them in Canada that produces the citronella odor unlike in the rest of it's range perhaps L. sitkanensis, or another name, should be reinstated for that group?)
Lasius sitiens SW
Found in forests and fields, usually at high elevations between 7000-8000 feet, give or take. Juniper shrubs and dry open places also describes their environment. Nest under stones. These are a subterranean forager (which is odd for the niger group but that's where they are) Eye's can be smaller and L. flavus like (also very odd for the niger group), but the maxillary palp doesn't lie. Scape and Tibiae and for that matter The Entire Body mostly lacking hair.
Lasius xerophilus SW (Only found in New Mexico)
Found nesting in sandy soil with small mounds. Similar to L. neoniger but they have more erect hair on the Tibiae.
Lasius Flavus Group
Queens are niger group-like but are a tiny bit smaller and lighter in color.
Lasius fallax SW, NW, NC
This species isn't as common as L. flavus or L. nearcticus. Found in forest clearings where they nest under stones. Scape and Tibia have hair.
Lasius flavus SW, SC, SE, NW, NC, NE
Found in forests and fields (though I want to say finding them nesting in fields is more typical of Europe where they make mounds too.) nest in soil under stones or fallen logs. Open sparsely covered ground is ideal. Scape and Tibia are reasonably free of erect hair. See paragraph below.
Lasius nearcticus SW, SC, SE, NW, NC, NE
Found in dense woodlands. Nest in moist soil under stones or fallen logs. Thick leaf litter is a must. This is a subspecies of Lasius flavus. Scape and Tibia are reasonably free of erect hair.
Lasius flavus and L. nearcticus are hard to tell apart in the western US. They hybridize with greater frequency out there. This doesn't happen as much in the eastern US. Head to head L. nearcticus tends to have a smaller eye compared to L. flavus who have similar sized heads. Lasius flavus's head also tends to narrow some at the mandibles while L. nearcticus doesn't do as much. Supposidly L. flavus workers are polymorphic and L. nearcticus are monomorphic but I find this to not be true. Perhaps it's more accurate to say L. flavus is MORE polymorphic than L. nearcticus tends to be. This theme of Lasius nearcticus having longer traits (sometimes only by fractions of a millimeter) is consistent, but still makes the two species hard to tell apart.
Lasius Umbratus Group
Queens have a smaller abdomen than Niger and Flavus group. These are social parasites of the Niger group. Some are more select than others.
Lasius umbratus SW, SC, SE, NW, NC, NE
This is the most common and wide spread species in this group. Found in both forests and fields, nests under stones in rotting logs and in rotting stumps. Scape and Tibia are not very hairy. This ant is kind of the default model for four subspecies: minutus, speculiventris, subumbratus, and vestitus. They use Lasius alienus, neoniger, and niger as host species.
Lasius minutus NC, NE
Subspecies of Lasius umbratus. Found in sphagnum bogs, dry forests, or swampy fields. Nests in soil often with mounds or masonry domes. Lots of erect hairs all over the body except for the Tibiae which is mostly free of hair.
Queens supposidly share mimetic coloration with host species Lasius alienus. (Wheeler, W.M. 1917 : 167-176)
Lasius speculiventris SC, SE, NC, NE (New Jersey)
Subspecies of Lasius umbratus. Found in forests and field. Nest under stones and in rotten wood. Abdomen is mostly free of hair. Scape and Legs have lots of erect hairs!
Queens might use Lasius minutus as a host along with other Lasius. This is interesting because L. minutus is itself a social parasite. (Kannowski, 1959b : 138-141 153-154)
Lasius subumbratus SW, NW
Subspecies of Lasius umbratus. Found in forests and fields. Nests under stones and rotting logs. Replaces Lasius umbratus where found? Supposedly Scape and Legs have dense pubescence (thin layer of fuzz) and occasionally standing hairs. Ant Web specimens don't seem to have this though, at least not on the legs. The hairs must be very short if they do.
Lasius vestitus SW, NW
Subspecies of Lasius umbratus. Found in forests. Nests in rotting logs and stumps. Replaces L. umbratus where found. Scape and Tibia have lots of hairs. Abdomen is especially covered with lots of erect hairs!
Lasius nevadensis SW (Nevada) Only found in Nevada.
Found in open forests. Nests either under stones or in exposed soil. Scape and Tibiae have longer hair than L. umbratus. Not as shiny as L. vestitus. Has more hair than L. vestitus but less than L. subumbratus.
Lasius atopus SW (Only found in California)
Nest in soil under stones. The Scape is very long for a Lasius extending well above the head. This is unique enough that that's all you need to know to ID this species.
Lasius humilis SW
Found in open woodland and moist fields. Nest under stones. Is found in what has been described as a mountain meadow. Workers are small and pale colored.
Lasius Claviger Group
Queens are very similar to Umbratus group species but most produce a citronella odor. These are also social parasites of Niger group species.
Because Claviger used to be it's own genus (Acanthomyops) the language of identifying them is a little different. The Dorsum side of the Propodeum (top of the last thorax segment), and the Gula (underside of the head, behind the palp) are the main ones. A side view of the ant is best. Some Claviger species have Barbulated or Plumose hair. Barbulated hair means the hair has lots of barbs poking all along the hair. See the top of this ant's head. Plumose hair is similar to barbulated hair but the majority of the barbs at at the tips or top third of the hair, and the effect is greater. see here.
By far the most common two species found are Lasius claviger and Lasius interjectus.
Lasius claviger SW, SC, SE, NC, NE
Found in forests and fields. Nest in soil, and rotting wood. Different from L. interjectus by having hair uniformly along the dorsum of the abdomen. Different from L. latipes by having less hair under it's Gula. Hybridization with Lasius latipes has been noted (Umphrey & Danzmann, 1998 : 431-440)
Lasius subglaber SW, SC, SE, NC, NE
Subspecies of Lasius claviger. Found in forests and fields. Nest in soil, in or under rotting logs and stumps. Mostly short or very little body hair. Gula is usually free of hair.
Lasius interjectus SW, SC, SE, NW, NC, NE
Found in forests and fields. Nest in soil, under stones, rotting logs, and in stumps. Next to walls of buildings is also noted. Propodeum is very convexed. Differs from L. claviger in that hairs on abdomen fall in rows fallowing the tergites (plates that make up the abdomen). Gula has hair.
Lasius arizonicus SW
Subspecies of Lasius interjectus. Found at high altitudes between 5000-8500 feet above sealevel. Nest understones. Easily recognized by almost completely lacking pubescence (thin layer of fuzz) and having a very shiny body. Hairs are very sparse on legs. Propodeum is very convex.
Lasius californicus SW
Subspecies of Lasius interjectus. Found under stones in mountains at mid levels. I couldn't find exact heights. Body hair is somewhat barbulated and is said to be related to L. colei. This is odd because L. colei isn't considered a subspecies of L. interjectus and yet both have the barbulated hair. Gula and Clypeus has 10 or so standing hairs each. Dorsum of gaster with fair amount of standing hairs.
Lasius coloradensis SW, NW, NC (No pictures found, sorry)
Subspecies of Lasius interjectus. Despite being a subspecies of L. interjectus, I read it's easier to confuse this with Lasius claviger. Workers have more body hair but it's shorter in length. L. claviger and L. coloradensis have divided distribution and are rarely found in the same locations.
Lasius mexicanus Mexico
Subspecies of Lasius interjectus. Found in woodlands at high elevations 7900-9000 feet above sea level. Nest under stones. Pubescence on abdomen is dilute to moderate. Hairs are long! Body is shiny. Propodeum is usually convex.
Lasius bureni NC (Wisconsin) Only Found in Wisconsin.
Long standing hairs on Gula. Shorter hairs on dorsum of abdomen. Said to be brown or "yellow-brown" in color. (If that's possible. They might be referring to the uneven color pattern on the head as seen in antweb's pictures.)
Lasius colei SW Only found in the New Mexico and Arizona area. Very uncommon.
Related to Lasius californicus. Body hair is barbulated. Gula and Clypeus have only 4 to 8 standing hairs each. Less hair on the abdomen than L. californicus.
Lasius creightoni SW (Utah) Only found in Utah.
Pubescence somewhat dense and whitish silver in appearance.
*
Lasius latipes SW, SC, SE, NW, NC, NE
Found in open forests, and fields. Nest in soil, under stones, or at the base of stumps. Hair all over the body is long, numerous and evenly distributed. Queens are easily identified by their unusually thick legs and modest amount of hair all over their body. Hybridization between L. latipes and L. murphyi is one of the rare cases where a hybrid has been given a species name, L. pogonogynus. (Trager, "Advances in Myrmecology:" 405-417) Hybrids between claviger and with colordensis also occur but not enough to be given formal names.
Lasius murphyi SW, SE, NW, NC, NE
Found in open forests, and forest edges. nest in sandy soil under or next to stones. Hair all over body is short. Standing Hairs mostly on propodeum. Queens are orange in color and have yellow fuzz on their face.
Lasius pogonogynus SW, SC, NW, NC, NE
Hybrid species between L. murphyi and L. latipes. Workers are exactly like L. latipes. Latipes has longer hair but that's not saying much. Queens have thick legs of L. latipes but lack the hair on the abdomen. Body hair is otherwise distributed similar to L. murphyi but much longer. This may vary some from hybrid to hybrid.
*
Lasius occidentalis SW, SC, NW, NC
Rarely found. Nest in dry soil under stones. Hairs on face has whisker-like effect occurring under the eyes and along the side of the head. Other Lasius have hair here too but the white color seems unique enough to this species.
Lasius plumoplosus SE, NC, NE
Nest under stones and in rotting logs. Lots of standing plumose tipped body hair.
Lasius pubescens NC (Only found in Minnesota)
Found in open forests. Nest in soil. Hair on Gula. Pubescence all over body.
The End
Sources
Cole, A. C., Jr. 1956a. Studies of Nevada ants. II. A new species of Lasius (Chthonolasius) (Hymenoptera: Formicidae). J. Tenn. Acad. Sci. 31: 26-27
Cole, A. C., Jr. 1958a. A remarkable new species of Lasius (Chthonolasius) from California (Hymenoptera: Formicidae). J. Tenn. Acad. Sci. 38: 75-77
Cover, S.P.; Sanwald, R. (1988) Colony Founding in Acanthomyops murphyi, a Temporary Social Parasite of Lasius neoniger. Advances in Myrmecology by James Trager, 405-417
Wheeler,W.M. The Temporary Social Parasitism of Lasius subumbratus Vierect. Psyche, 24, 166-176
MacKay, W. P.; MacKay, E. E. 1994. Lasius xerophilus (Hymenoptera: Formicidae), a new ant species from White Sands National Monument, New Mexico. Psyche (Camb.) 101: 37-43
Wilson, E.O. (955) A Monographic Revision of the Ant Genus Lasius. Bulletin of the Museum of Comparative Zoology, 113, 1-201
Wing M. W. (1968) Taxonomic Revision of the Nearctic Genus Acanthomyops (Hymenoptera:Formicidae). Memoirs of the Cornell University Agricultural Experiment Station, 405, 1-173.