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Figure 1. Live shot of Dinomyrmex gigas minor worker under lab settings. Image © Gordon Yong Wenjie


Overview
Dinomyrmex gigas is one of the largest ants in the world and the largest in Southeast Asia [17][21]. It is also more commonly known as the giant forest ant. This large charismatic ant can be found in the forests of Singapore and Southeast Asia, however it is not often sighted by the public due to its predominant nocturnal activity [17][20][21]. Despite being considerably well studied compared to its smaller counterparts, most people know little about the giant forest ant beyond its large size. Most studies on this species focuses on the populations found in Malaysia, Sabah [19][20][21][22][23][27] and Brunei [17][18]. These studies have characterized many aspects of the natural history and biology of this species including its foraging behaviour [16][19][20]diet [16][20][22]territoriality [21]reproduction [18] and colony structure [18][20][21]. While much is known about the biology of this species in Sabah and Brunei, little is known about the populations in Singapore, where small and fragmented forest habitats may have an impact on the viability and behaviour of this majestic giant ant of the forest.

Table of Contents

Table of Contents


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Figure 2. Dinomyrmex gigas photographed in the Central Catchment Nature Reserve (natural habitat) at night. Image © Pierre Escoubas (Used with permission)


There is some confusion in the scientific name of this species, with most people knowing it by the latin name Camponotus gigas. Until the start of 2016, this ant was previously known as Camponotus gigas, under the subgenus Dinomyrmex, before new phylogenetic studies concluded that it was from a distinct lineage, resulting in the resurrection of the genus Dinomyrmex and the new combination Dinomyrmex gigas and Dinomyrmex gigas borneensis [25] as recognised latin names for the giant forest ant. 

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The giant forest ant is a common inhabitant of the rainforest in Southeast Asia from Thailand, Sumatra, Borneo, Malaysia to Singapore, with one recognized subspecies, Dinomyrmex gigas boreensis found in South Borneo [21]. It is found in a range of habitats from the peat swamps of a mangrove forest and up to montane forest 1500m above sea level [21].


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Figure 3. Map showing distribution of Dinomyrmex gigas retrieved from antmaps.org (pending permission). Note that the giant forest ant is not found in Philippines (Christian Peeters, pers comm.) The green dots on the map represent physical specimens that have been collected at that location. Click on image to see an interactive version of map with GPS coordinates.

How to identify the Giant Forest Ant in Singapore

The giant forest ant is one of the largest ants in the world comparable to Paraponera clavata (Bullet ant) and Dinoponera giganteawith head widths of more than 4mm [21] and the biggest ant you can find in Singapore and Southeast Asia! It is about the size of a one dollar coin (Figure 4) and can only normally be found in a good forest patch at night!


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Figure 4. (ia) front view and (ib) top view of Dinomyrmex gigas minor and major worker compared with a S$1 coin.


Morphology of ants
The image below gives a quick look at the morphology of an ant from the side view. The presence of a 'constricted waist' differentiates ants (Formicidae) from most other Hymenoptera. Note the following parts on the picture that will be important in the more accurate and scientific diagnosis to recognize Dinomyrmex gigas: The position of the metapleural gland for the species and the femur for the separation between species and subspecies.


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Figure 5. Image showing morphology of ant from side view. Image by Mariana Ruiz (User:LadyofHats) - Own work (data from Bert Hölldobler and Edward O. Wilson, The ants). Image renamed from Image: Ant worker morphology.svg (see below), Public Domain, https://commons.wikimedia.org/w/index.php?curid=2903050

Diagnosis

The following combination of features are used to identify the giant forest ant, Dinomyrmex gigas [25]
(i) Its large size. (Figure 4)
(ii) Presence of metapleural gland on the propodeum.
(iii) Presence of teeth on the masticatory and basal margin of the mandibles.


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Figure 6. (ii) image showing the metapleural gland on the propodeum. (iii) image showing the presence of teeth on both basal and masticatory margin of mandibles. Images © Gordon Yong Wenjie


The following feature can be used to distinguish between the species Dinomyrmex gigas and its subspecies Dinomyrmex gigas borneensis
(i) The presence of yellow femur on the all legs of Dinomyrmex gigas borneensis compared to the dark brown femurs on all legs of Dinomyrmex gigas


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Figure 7. (left) lateral view of Dinomrymex gigas borneensis (right) lateral view of Dinomyrmex gigas. Note the different color of the femur, light brown on Dinomrymex gigas borneensis and dark brown/black on Dinomyrmex gigas


Eusociality and Caste system
The term ‘eusocial’ was first coined by Suzanne Batra in 1966 to describe the nesting behaviour in Halictine bees. The terminology was subsequently extended to include other social insects such as ants, wasps and termites by E. O Wilson in 1971 [26]. He described the following three main features in ‘eusocial’ organisms:
1. Reproductive division of labour (with or without sterile caste)
2. Overlapping generations
3. Cooperative care of young
Most eusocial insects have a caste system of reproductive females and males and sterile workers that take care of the brood and soldiers that protect and defend the colony [13]. The castes in Dinomyrmex gigas includes the alates (winged reproductive males and females) [2], gynes (female reproductive or queen of the colony) [2] and bimorphic workers with major and minor workers in two separate subcaste. On average, major workers weighed 372 mg while minor workers weighed 135 mg [20][21]. Both minor and major workers exhibited size polymorphism while each caste differed in allometric growth of the head [21].


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Figure 8. Image of the different caste of Dinomyrmex gigas. Click on the red labels to look at head, top and side view of each subcaste. Images © Gordon Yong Wenjie


Female alate

The female winged reproductive of the species, the female performs mating flights and mates with male alates before losing its wings to become a gyne and found a new colony. Notice the swollen gaster of the female alate, it is swollen with eggs and food reserves, this is also known as a physogastric gaster. The female also has three ocelli on the top of its head and an enlarged thorarcic wing segment that house muscles that are essential for flight [24].


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Figure 9. (a) Front view (b) lateral view (c) top view of Dinomyrmex gigas alate with scale bars attached. Images © Gordon Yong Wenjie

Major Worker

The major worker is a sterile female whose primary role is to defend the nest and protect foragers. Notice the large heart shaped head of the major and the large mandibles with sharp mandibular teeth! The major workers also possess a single reduced ocellus on the top of its head, its function however is still a mystery but suggests a distinct development pattern from minor worker (Christian Peeters, pers comm.). These ants are also rarely involved in foraging [17][21] but perform colony border patrol and ritualistic fights [22] and have a strong bite that would result in a small cut on human hands (Gordon Yong, pers obs.).



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Figure 10. (a) Front view (b) lateral view (c) top view of Dinomyrmex gigas major worker with scale bars attached. Images © Gordon Yong Wenjie


Minor Worker
Large Minor Worker
Minor workers are highly polymorphic with a big size range of average head width 3.56mm (SD=0.53mm). They lack ocelli unlike the major workers or alates. Minor workers are also most involved in foraging and transporting of food from nest to nest [20][21]. Note the more rectangular-ovalish head shape compared to the heart shape of the major worker, these two caste are often confused for each other.


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Figure 11. (a) Front view (b) lateral view (c) top view of Dinomyrmex gigas large minor worker with scale bars attached. Images © Gordon Yong Wenjie

Small Minor Worker
Note the smaller size and the more ovalish head shape for the smaller minor worker compared to the large minor worker. Small minor workers do not grow up to become large minor workers but remain the same size throughout their lifespan.


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Figure 12. (a) Front view (b) lateral view (c) top view of Dinomyrmex gigas small minor worker with scale bars attached. Images © Gordon Yong Wenjie


Male alate
The male alate is the winged reproductive male of the colony and engages in mating flights to with female alates. Male alates are not always produced by the colony and are only produced for the sole purpose of mating with winged females, they have a short lifespan of about 1 to 8 days [19]. Note the small head compared to the body and the enlarged thoracic wing segment on the thorax (alitrunk) that house the muscles for mating flights. The males also have 3 well developed ocelli on the top of the their heads that are essential for flight.


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Figure 13. (a) Front view (b) lateral view (c) top view of Dinomyrmex gigas male alate with scale bars attached. Images © Gordon Yong Wenjie

Natural history and biology

Colony structure and foraging range

The giant forest ant has a moderately sized colony compared to other ant species, however its large size allows it to forage over a much greater area. The best studied colony in Sabah had a population of about 7000 workers with a foraging range of 0.8 hectares. Numerous studies concluded the giant forest ants to have polydomous colonies, with workers of each colony distributed unevenly among numerous nests [20][21]. These nest were most often found in the soil between the buttress roots of emergent trees and also found in the hollows of fallen or decaying tree trunks and at the base of smaller trees [21]. Excavation of a queenright nest in Brunei also yielded 784 minor workers, 45 major workers, 2 male alates, 157 larvae and 34 eggs [18]. The giant forest ant was found to be monogynous, with only one queen per colony. Excavation of nests in Sabah also revealed some ant guests staying in the nest of the gigas, namely a myrmecophilous cricket, Camponophilus irmi Ingrisch 1995 and a cockroach, Eroblatta borneensis Shelford [21].


Figure 14. Image showing the distribution of nests of a single colony studied in Sabah. Each black dot represents a single nest, with Q being the nest with the queen of the colony. Black solid lines represent artificial trails made of bamboo used to link the nests and broken lines represent natural trails in the canopy that were abandoned by the giant forest ants after bamboo trails were introduced. HT represents the foraging trees with large groups of associated trophobionts. Figure from Pfeiffer & Linsenmair, 1998 [20] (Fair use).


Foraging 
The giant forest ant is active and forages predominatly at night [17][20][21]. Some reasons hypothesized to explain its nocturnal activities include the high temperatures and low humidity in the canopy where they forage in the day, the need to reduce predation on Phorid flies that need light to lay their eggs in the wounds of majors and to avoid competition for foraging trails such as trunks and branches, which are used by other diurnal ant species for foraging during the day [20]. The giant forest ants also use a system of dispersed central foraging as a result of their polydomous colonies. Such a system involves the foragers bringing food back from a food source such as a trophobiont associated tree, into satellite or 'source' nest within their territory such as nest E and Z in Figure 11 on one night. Trophollaxis from these foragers to specialized transporter ants waiting in these nest then occur before these specialized transporter ants carry the food from the 'source' nests to the 'sink' nest where the queen and most of the brood are found [21].

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Before the start of each foraging night, large number of ants have been observed to accumulate at the entrance of each nest (Christian Peeters, pers comm.) at dusk, before the sudden leaving of these foragers in large numbers from the nest within a short span of 50 min [20][21]. This sudden leaving of foragers have been termed as an exodus by researchers describing this phenomenon [21]. The exodus of foragers often climb into the canopy, where they use a network of natural trunk trails such as tree branches, fallen trees, bamboo and vines as highways to navigate to foraging trees with large groups of associated trophobionts in Sabah [21][23]. In Brunei, foragers have been observed to travel quickly on such highways from a fixed distance, before ending in a slow meandering search pattern at the end of such highways [17].



Figure 16. Image showing the accumulation of minor workers at the entrance of the nest guarded by major workers before the exodus. Image © Gordon Yong Wenjie



Diet 
In Sabah, the giant forest ants collect honeydew as their main food source but also take some bird dropping and other insects such as winged termites to supplement nitrogen intake. It was suggested that rainfall may affect the items they collected for nitrogen supplements, with termites being collected after heavy rains that wash away the bird droppings and bird droppings collected when there was no or little rain [21]. These ants collect honeydew in large amounts from different trophobiotic associations with wax cicadas, Membracidae, Fugloridae and and Coreidae [21][23]. The giant forest ant also frequently collects extrafloral nectar, plant sap and fed on rubber directly from open wounds on the plant [21]. In Brunei however, fungi was found to be the main bulk of the diet of the giant forest ant, supplemented by arthropods, small fruits and seeds [17]. In Singapore, the giant forest ant has been observed to be foraging in large numbers up a tree at night and coming down with full gasters, suggesting the collection of honeydew (Gordon Yong, pers obs.). The ants were also observed to collect insect parts, termite alate without wings and bird droppings on separate ocassions (Gordon Yong, pers obs.).



Figure 17. Gif showing Dinomyrmex gigas with full gasters after collecting honeydew using a branch as a highway. Giant forest ants also observed to be carrying termite alates while walking on the branch. Gif converted from video © Gordon Yong Wenjie



For its main food source of honeydew, the giant forest ants depend on the excretions of Homoptera, hence forming a trophobiotic interaction with these insects. In particular, Dinomyrmex gigas was observed to herd but not protect Bythopsyrna circulata (wax cicadas) and provide protection to Mictis cf. longicornis (coreid bugs) to obtain honeydew from these insects. Three distinct behavioural patterns were observed in the trophobiotic association of the giant forest ant with wax cicadas. The first was collecting which involved the flicking of honeydew droplets by the wax cicada and the catching of the flicked droplet using the head or leg of the waiting ant. The wax cicada has no specific morphological adaptations to control the output of honeydew droplets, resulting in the flicking of the droplet. The second pattern is antennating from ahead which involves an ant sitting laterally or above the cicada while antennating its front part. The last pattern was secondary gathering which involved a worker ant shuttling between different collector ants and grooming them when needed. The grooming involved gathering the droplets that had hit them and that they were unable to collect by themselves. These worker ants also frequently prompted the collectors to perform trophallaxis to transfer collected honeydew from the collector to the worker before heading back to the nest once they are full [23].



Figure 18. Image showing trophobiotic interactions between wax cicada (Bythopsyrna circulata) and Dinomyrmex gigas in Sabah, Borneo. (1a) Shows tending of wax cicada adults by minor workers of D. gigas. (1b) A collector ant below the cicada with another ant above it antennating, notice also the droplet on the mid leg and head of the collector (circled in red). (1c) Liquid food exchange (Trophollaxis) between two minor workers. (1d) Overview of the trophobiotic interaction between B. circulata and D. gigas. Edited from Figure in Pfeiffer & Linsenmair, 2007 [23] (Fair use)

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Figure 23. Phylogeny of the subfamily Formicinae based on phylogenomic analysis. Legend for maximum likelihood bootstrap support values shown on top left hand corner of the picture. Camponotus gigas as it was previously known is found in the tribe Camponotini and can be seen to have a different lineage (100 bootstrap node support) compared to Camponotus sensu stricto, leading to the resurrection of the subgenus Dinomyrmex to genus and the new combination Dinomyrmex gigas as the recognized name for the giant forest ant. Figure from Ward et al., 2016 (Fair use)

References

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