Strumigenys

Full-face view 29 Strumigenys emmae
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Subfamily MYRMICINAE

GENUS

Strumigenys

Smith, 1860

Miniature trap-jaw ants

Described species: 859

Worker size: 1–4 mm

Colony size: 15–400

Foraging: Subterranean / Leaf litter / Arboreal

Diets: Specialist predator (soil arthropods, mainly Collembola)

Genus distribution map 17 Strumigenys

Distribution.

A very successful ant genus with 859 species described. This genus is currently the third-most species-rich genus of ants. The genus has a cosmopolitan distribution, with the exception of northern parts of the northern hemisphere.

Biology & Ecology.

Individuals of the genus are rarely found above ground, with nests usually located within the soil or decaying wood, but some species are known to be arboreal. Nests are small, contain a few dozen workers to several hundreds of workers. Due to their cryptic lifestyle and low abundance of workers, the biology of most species remains unknown.

Strumigenys species are slow-moving specialized hunters after soil arthropods, in particular springtails (Collembola). Workers’ foraging activity is limited to within the soil or leaf litter of forest habitats.

Many species have highly derived mandibles with spring-load mechanism for hunting. Mandibles are held in an open position, and slap close when trigger hairs contact a prey. Some are known for having thanatotic behavior (playing dead) when disturbed.

Identification.

Strumigenys species are small to very small in size (1–4 mm), mostly with light brown to brown coloration, but there are some species that have darker coloration. They can be easily recognized by their characteristic mandibles and the presence of sponge-like tissue on their waist segments.

Genus figure ENG 17 Strumigenys

Species in this genus

Full-face view 29 Strumigenys emmae
Strumigenys emmae
Full-face view 30 Strumigenys membranifera
Strumigenys membranifera
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Basic morphology of ants

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A. Subfamily.

B. Scientific name.

A unique, italicized taxonomical name for each described species, the first word is the genus name, the second word is the species name, the third word (when present) is the subspecies name.

‘cf.’ (=confer) and ‘nr.’ (=near) indicate uncertain identification. Undescribed morphospecies only have genus name, which is followed by an abbreviated species code (e.g., Monomorium sp. psw-cn01).

C. Authority.

Name of the person who first described the species (or genus) and the year of publication. Parentheses indicate the species is now placed in a different genus, different from when it was first described.

D. Rarity.

How common the species is in Hong Kong based on existing records, from ‘common’, ‘uncommon’ to ‘rare’.

E. Common name.

F. Basic information.

For species, information is in the following order:
　1) whether the species is native to Hong Kong
　2) if it is a known tramp species
　3) if it is a known pest or invasive species
　4) where it usually nests
　5) where it usually forage for food
　6) in what habitats the species can usually be found in Hong Kong
　7) colony size range of the species
　8) mode of reproduction
　9) caste variation of workers
　10) active foraging time of the species
　11) known diets of the species

For genus, only the following are presented:
　1) number of valid extant species
　2) worker size range
　3) colony size range
　4) where species from the genus usually forage for food
　5) known diets of species from the genus

Postulated information is denoted by an asterisk (*).

G. Distribution map.

Known distribution of the taxon.

For species, native range is colored in green, indoor introduction (e.g., household, greenhouse) is colored in orange, outdoor introduction is colored in red. When the species is recorded in Hong Kong but not in adjacent regions (e.g., Guangdong), Hong Kong is highlighted using a dot on the map.

For genus, only the native range is presented; darker color indicated the region has a higher diversity of species.

H. Detailed information.

Information is presented in the following sections:
　1) a short summary of its distribution
　2) taxonomical notes (for some species and genera only)
　3) known biology and ecology of the taxon
　4) how to identified the taxon

I. Scale of size.

Typical size of workers of the species. When the species is dimorphic or polymorphic, the typical lower and upper bound will be presented.

J. Specimen pictures.

For species, the full-face view, profile view and dorsal view of worker(s) collected in Saiyuen. Size scale of the pictures is at the corner. When more than one subcaste is collected, the subcaste will be denoted at the upper left corner (minor, major or supermajor workers).

For genus, selected pictures of species form the genus is presented.

Key morphological characters of the species or genus are highlighted.

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Glossary

A･B･C･D･E･F･G･H･I･J･K･L･M･N･O･P･Q･R･S･T･U･V･W･X･Y･Z

A

Alate queens / alate males. Queens or males that are born with wings and are capable of flying. After mating, queens would shed their wings and are now known as dealate queens.

Antennal club. Apical segments of the antenna that are distinctly enlarged.

Antennal scape. The basal-most segment of the antenna that is elongated.

Arboreal. Nesting and/or active on trees.

Army ants. Ants that do not build permanent nest and are constantly on the move. They often forage collectively in mass.

B

Brood. Non-adults, including eggs, larvae and pupae.

Budding. New queens leaving with some workers and brood from the original colony and found a new colony nearby. In contrast with alate (winged) queens founding a new colony alone after mating.

C

Castes. Roles of ants in the colony, usually predetermined before they become adults. Defined by sex, morphology and reproduction capability of the ants. Further subdivisions based on minor differences in morphology and role are known as subcastes.

Clypeus / clypeal margin. The anterior part of the head, in-between the insertion site of antennae and the mandibles of the ant. Clypeal margin refers to the anterior margin of the clypeus.

Coccids. A family of insects (Coccidae) belonging to the infraorder Coccomorpha (scale insects) of order Hemiptera, also known as soft scales or wax scales. They may form mutualistic relationship with ants, offering honeydew in return for protection.

Colony. The social structure of ant, constituted by workers and reproductive individuals that cooperate with one another, as well as their brood. Members of the same colony are able to recognize and are not aggressive with each other. Number of individuals can range from a few to several millions.

Cosmopolitan distribution. Global or near global distribution.

Cryptic. Obscure lifestyle, often refers to subterranean species and species that are active within leaf litter.

D

Dimorphic. There are two distinct subcastes for workers. There is no intermediate form in-between the two subcastes.

Disturbance / disturbed environment. Human activities that change the environment and impact ecosystem, which can be momentary or prolong. Agriculture and environmental management (e.g., cutting or trimming trees, use of pesticides) are two typical examples of the disturbance that may affect ants.

Dorsal view. Top-down view on an ant.

E

Epigaeic. Nesting and/or active above the ground and on lower vegetations.

Ergatoids. Queens or males that are born wingless. Ergatoid queens may look very similar to workers.

Exotic species. Species that has been introduced to place outside of where it is originated from (=introduced species).

F

Forage. Hunting or gathering food.

Full-face view. Frontal view on the head of an ant.

G

Gamergate. Workers that have mated with males and are able to lay fertilized eggs for reproduction. Depending on the ant species, gamergates may coexist with queens, or entirely replace queens.

Gaster. The posterior part of an ant’s body, connected with mesosoma through waist segment(s). In some ant species a sting is present at the end of the gaster for hunting and/or self-defense.

Gastral tergites. The dorsal part of the gaster, can be subdivided into segments.

H

Habitat. Where the organism live, characterize by the landscape and dominant plants (e.g., tree, shrub, grass, mangrove tree) present.

Hemiptera. An order of insects that includes aphids, cicadas, leafhoppers, scale insects and shield bugs. Most of them feed on plant sap, and sometimes form mutualistic relationship with ants, offering honeydew in return for protection.

Hypogaeic. Nesting and/or active within or below leaf litter, inside rotten woods, beneath rocks and within the soil.

I

Intraspecific variation. Variation between individuals of the same species, in contrast with interspecific variation, which describe variation between species.

Introduced species. Species that has been introduced to place outside of where it is originated from (=exotic species).

Invasive species. Species that has been introduced to place outside of where it is originated from and has a significant impact on the local ecosystem.

L

Leaf litter. The layer of plant debris above the soil, which is made up of fallen leaves, branches, fruits, seeds and flowers.

M

Mandibles. The pair of hard mouthparts of insects for biting, cutting and holding food. It is a part of the exoskeleton.

Mesosoma. The anterior part of an ant’s body, connected with gaster through waist segment(s). It is also where the legs and wings (when present) of the ant attached.

Microhabitat. Where the organism live, at a finer scale than habitat. It is further characterized by the local temperature, humidity of the environment. For example, beneath a rock and on the surface of tree branches are two different microhabitats of the same habitat (forest).

Monogyny. Colony with only one egg-laying reproductive individual.

Monomorphic. There is only one size and form for workers, and cannot be further subdivided into subcastes.

Morphology. The form and structure of an organism, both internal and external. For external morphology, it includes the shape, size, color, pattern and external structure of the various parts of the organism.

Morphospecies. A species defined solely base on its morphological differences with respect to other species. It may not have been formally described and named.

Mutualism. Mutually-beneficial relationship between organisms.

Myrmecophily plants. Plants that are positively associated with ants.

Myrmecophyte plants. Plants that have formed mutualistic relationship with ants, and possess adaptations for such relationship, such as food bodies and extrafloral nectaries dedicated to ants and/or specialized structure for ants to nest (e.g., hollowed structures in tree branches).

N

Native species. Species that that naturally live in a particular place without human intervention. ‘Native fauna’ further specifies animal species, while ‘native flora’ specifies plant species.

Nest. The physical structure that ant colony lives in. It may be permanent and extensively built, or temporary and has only been minimally modified by its residents.

Niche. It can broadly refer to the role an organism plays within its habitat, or specifically refer to where an organism lives within its habitat. Here niche means the latter.

O

Oligogyny. A special case of polygyny, where there is more than one egg-laying reproductive individual in an ant colony, but these individuals do not tolerate one another and stay far apart from each other in the nest. Workers still cooperate with one another like in a normal polygynous colony.

Open habitats. Habitats such as open shrubland and grassland, in oppose to habitats with dense vegetation cover, such as forest.

P

Parthenogenesis. Asexual reproduction through unfertilized eggs. For ants, it means reproductive females are able to reproduce partial or complete clones of themselves without mating (i.e., theylotoky).

Pest. Species that has been a source of nuisance to people. Common categories include household pest and agricultural pest (which causes damage to crops).

Petiole. The first segment of waist segments (or the only segment, when there is only one waist segment) of an ant, connecting mesosoma and gaster. It may be reduced and difficult to observe.

Polygyny. Colony with, on average, more than one egg-laying reproductive individual. Facultative polygyny (=secondary polygyny) means that colonies were originally monogynous when founded, but would later adopt other queens (usually daughter queens of the colony) and become polygynous.

Polymorphic. There is more than one size and/or form for workers, hence, can be further subdivided into subcastes. When there are two distinct subcastes but no intermediate form, it is known as dimorphic.

Postpetiole. The second segment of waist segments, connecting mesosoma and gaster. Some species of ants do not have a postpetiole.

Profile view. Side view of the ant, usually with its head facing left.

S

Scale Insects. An infraorder of insects (Coccomorpha) belonging to the order Hemiptera, also includes coccids. They may form mutualistic relationship with ants, offering honeydew in return for protection.

Scavenger. Ants that predominantly gather food instead of through other means, e.g., hunting. Depends on the ants’ preference, it may include fruit, seed and other food bodies of plants, carcasses of invertebrates and small vertebrates, honeydew produced by other insects, etc.

Sculpture. Pit- or grove-like structure on the surface of the exoskeleton of the ant, in contrast with smooth and shining surface. Its pattern can be used to differentiate between species of ants.

Secondary forest. A forest that has been regrown after timber harvest or agricultural activities, in contrast with primary forest, which has not been deforested by human. There are only secondary forests in Hong Kong.

Soil arthropods. Small invertebrates from the phylum Arthropoda that are active within and above the soil, as well as within leaf litter.

Spine. Hard and pointed structure on the back of the ant. It may be long and sharp like a thorn, or low and inconspicuous. It most often appears at the posterior part of mesosoma in one pair (propodeal spine), with other possible locations include pronotal spine and petiole spine.

Subcastes. Roles of ants in the colony, a further subdivision based on minor differences in morphology and role under their respective castes.

Supercolony. The unusual case where multiple colonies unite and are not aggressive with one another, often occur in invasive species in introduced regions. Individuals range from millions up to billions.

T

Tramp species. Species that are associated with habitats frequently-disturbed by human activities. They may further be dispersed by human to places beyond its native range.

Trophic eggs. Unfertilized eggs that function as food. In ants, they are usually produced by workers and offered to queens and larvae.

W

Waist segment(s). The segment(s) connecting mesosoma and gaster of an ant. The first segment is called ‘petiole’ and the second segment (when present) is called ‘postpetiole’.

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