Difference between revisions of "Melissotarsus"

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Save for brief periods of sexual activity and colony founding by queens, ''Melissotarsus'' ants live in splendid isolation from the outside world. Accordingly they are poorly known, even though they appear widespread throughout Africa and Madagascar, with host trees belonging to over 20 families.
 
Save for brief periods of sexual activity and colony founding by queens, ''Melissotarsus'' ants live in splendid isolation from the outside world. Accordingly they are poorly known, even though they appear widespread throughout Africa and Madagascar, with host trees belonging to over 20 families.
  
Silk-producing glands are present in the head of adult workers (a trait unique to the genus), and silk is spun and combined with wood particles to seal cracks in tunnel walls (Fisher & Robertson 1999).
+
Silk-producing glands are present in the head of adult workers (a trait unique to this genus as well as ''[[Rhopalomastix]]''), and silk is spun and combined with wood particles to seal cracks in tunnel walls (Fisher & Robertson 1999).
 
Workers cannot forage outside the tunnels because the second pair of legs project upward (Delage-Darchen 1972), a novel adaptation for bracing the body while chewing (Khalife et al. 2018).  
 
Workers cannot forage outside the tunnels because the second pair of legs project upward (Delage-Darchen 1972), a novel adaptation for bracing the body while chewing (Khalife et al. 2018).  
  

Revision as of 03:53, 15 September 2019

Melissotarsus
Temporal range: 23–0 Ma
Early Miocene – Recent
Melissotarsus beccarii
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Hymenoptera
Family: Formicidae
Subfamily: Myrmicinae
Tribe: Crematogastrini
Genus: Melissotarsus
Emery, 1877
Type species
Melissotarsus beccarii
Diversity
4 species
1 fossil species
(Species Checklist)

Melissotarsus beccarii casent0102042 profile 1.jpg

Melissotarsus beccarii

Melissotarsus beccarii casent0102042 dorsal 1.jpg

Specimen Label

Evolutionary Relationships

Rostromyrmex






Cardiocondyla



Ocymyrmex






Nesomyrmex



Xenomyrmex





Terataner




Atopomyrmex



Cataulacus










Carebara



Diplomorium






Melissotarsus



Rhopalomastix






Calyptomyrmex



Tetramorium





Cyphoidris



Dicroaspis










Aretidris



Vollenhovia





Dacetinops



Indomyrma








Crematogaster



Meranoplus






Lophomyrmex




Adlerzia



Recurvidris






Stereomyrmex




Trichomyrmex





Eutetramorium




Royidris




Malagidris



Vitsika








Huberia



Podomyrma





Liomyrmex



Metapone













Kartidris



Mayriella





Tetheamyrma




Dacatria



Proatta









Dilobocondyla



Secostruma






Acanthomyrmex



Myrmecina





Perissomyrmex



Pristomyrmex









some Lordomyrma



Propodilobus





Lasiomyrma




[some Lordomyrma




Ancyridris



some Lordomyrma









Paratopula




Poecilomyrma




Romblonella



Rotastruma








Gauromyrmex



Vombisidris





Temnothorax




Harpagoxenus



Formicoxenus



Leptothorax













Based on Ward et al. (2014) and Blaimer et al. (2018).

Existing literature on all known species of Melissotarsus reveals similar habits of relying on armoured scale insects (Diaspididae) for food within the confines of tunnels chewed in healthy wood. The ants' anomalous morphology (head and legs) represents extreme adaptations (tunnelling through live wood and silk spinning) for their obligate mutualism with diaspidids. Unlike other symbiotic associations between ants and scale insects, honeydew is not the currency of exchange for Melissotarsus. Indeed, diaspidids do not produce honeydew. In addition to gaining access to underlying meristem tissue, diaspidids benefit from highly effective protection (enemies and weather) by the ants. Accordingly, they stop building their trademark shields even though they continue secreting wax and proteins as construction material. These secretions, together with exuviae and anal excretions, can provide an adequate array of nutrients for the ants (Peeters et al. 2017). In addition, workers can also feed on dead or dying diaspidids. Claims that the ants use diaspidids as 'domestic cattle' (Schneider et al. 2013) are revisited by Peeters et al. (2017).

Save for brief periods of sexual activity and colony founding by queens, Melissotarsus ants live in splendid isolation from the outside world. Accordingly they are poorly known, even though they appear widespread throughout Africa and Madagascar, with host trees belonging to over 20 families.

Silk-producing glands are present in the head of adult workers (a trait unique to this genus as well as Rhopalomastix), and silk is spun and combined with wood particles to seal cracks in tunnel walls (Fisher & Robertson 1999). Workers cannot forage outside the tunnels because the second pair of legs project upward (Delage-Darchen 1972), a novel adaptation for bracing the body while chewing (Khalife et al. 2018).

Photo Gallery

  • Melissotarsus worker showing dorso-ventral enlargement of the head, hypertrophied coxae, large basitarsi, and the peculiar orientation of the midlegs (all highlighted in red). Modified from Sketchfab https://skfb.ly/6vFQC
  • Melissotarsus worker chewing a tunnel in living wood, using specialized mid and hind legs to brace against the walls. Note how the basitarsi, and not the terminal tarsi, directly rest against the walls. Drawing by Roberto Keller
  • Chambers dug beneath the bark by ants, containing symbiotic coccoids and scattered ant larvae. From northern Mozambique. Photos by Christian Peeters.
  • Melisso coccoids&ants Peeters.jpg
  • Dense aggregation of ant-associated Morganella conspicua diaspidids. Photo by Christian Peeters.
  • Naked diaspidids together with two rare shields (on right). The latter is evidence that the secretory glands of Morganella conspicua continue to be functional. Photo by Christian Peeters.
  • Ventral view of the greatly enlarged coxae of mid and hind legs in worker, housing muscles that move the trochanter-femur complex away from the body to brace during tunnelling. Scanning electron microscopy by Roberto Keller.
  • Distal end of the enlarged basitarsus in the hindleg of a Melissotarsus worker, showing stout pegs that give traction against the walls. Note the groove that allows to bend the thin terminal tarsi out of the way during tunnelling. Scanning electron microscopy by Roberto Keller.

Hita Garcia, Wiesel and Fischer (2013) - The four species of this Afrotropical and Malagasy genus are rarely encountered. The Afrotropical species were revised by Bolton (1982) who also provided a good species identification key. These ants are special in several ways. They build their nests in healthy wood by tunnelling through the living tissue beneath the surface (Bolton, 1982; Fisher & Robertson, 1999; Belshaw & Bolton, 1994) and are rarely found outside of their nests, which could be the main reason for their relative scarcity in museum collections (Bolton, 1982). They live in close association with symbiotic coccids that are kept inside the nest. Adults are able to produce silk, which is used to close exit holes or to seal cracks (Fisher & Robertson, 1999).


Identification

Bolton (1982) - Myrmicine ants with moderate to conspicuous size variation in most nest samples, living under bark and in wood of live trees. Mandibles short, when unworn armed with a long finger-like apical tooth followed by two much smaller teeth and sometimes also by a minute basal denticle. With wear these gradually become an undifferentiated blunt margin. Palp formula 0,1 (weissi). Median portion of clypeus bluntly triangular in shape and somewhat raised, not projecting back between the frontal lobes. Lateral portions of clypeus simple and unmodified. Frontal lobes narrow, confluent centrally and separated only by a narrow impressed line; the anteriormost parts of the frontal lobes abut the posterior clypeal margin. Antennal scrobes absent. Frontal carinae absent. Antennae with six segments, the scapes very short (SI 39-47), the two apical segments forming a strong club. Eyes present, distinctly longer than broad and set in front of the midlength of the sides. Alitrunk short, fusiform and box-like, without dorsal sutures or impressions except in the very largest individuals where rarely a metanotal impression is shallowly present. Propodeum unarmed and rounded. Metapleural lobes absent. Lateral portions of pronotum reduced to a narrow V -shaped wedge below the level of the conspicuous mesothoracic spiracle. Anterior coxae small, much smaller than the massively developed middle and hind coxae. Propodeal spiracle round, situated low on the side of the propodeum and just behind its mid length. Metapleural gland system easily visible through the cuticle. Basitarsal segment of each leg greatly swollen, as wide as the preceding tibia, terminating apically in a circlet of small teeth on the anterior (leading) edge on the middle and hind basi tarsi. Petiole with an anterior peduncle and a small low posteriorly situated node which is broadly attached to the postpetiole; the latter broadly attached to the gaster. Dorsal alitrunk finely longitudinally costulate throughout. Elongate fine hairs present dorsally on head and body, and also present on the upper surfaces of the scapes and outer surfaces of the tibiae.

The genus most closely related to Melissotarsus is Rhopalomastix Forel, represented by three or four poorly defined species distributed throughout the Oriental and Indo-Australian zoogeographical regions and utilizing the same lifeway as Melissotarsus. Since Emery (1922a) and Wheeler (1922) produced their classifications these two small genera have always been placed together in a tribe of their own (Melissotarsini) and it is fairly certain that they represent two stages on a single adaptive line. Rhopalomastix is the more generalized of the two, Melissotarsus decidedly the more specialized, but the modifications seen in the latter are foreshadowed in the former genus. It is the accentuation of these adaptive specializations which separates the genera, as follows.

Rhopalomastix

Antennae 10-segmented. Lateral portion of pronotum extensive, distinctly larger than the mesopleuron. First coxa as large as or larger than the second and third coxae. Petiole sub-sessile, with a strong ventral process. Free posterior face of petiole node long, its articulation with the postpetiole narrow. Basitarsal segment of each leg not swollen, without apical circlets of teeth. Sting long and strong.

Melissotarsus

Antennae 6-segmented. Lateral portion of pronotum very reduced, forming a V-shaped narrow wedge which is smaller than the mesopleuron. First coxa much smaller than the swollen second and third coxae. Petiole short-pedunculate, with feeble or no ventral process. Free posterior face of petiole node very short, its articulation with the postpetiole very broad. Basitarsal segment of each leg strongly swollen, with apical circlets of teeth. Sting very reduced and probably non-functional.  

Keys to Species in this Genus

Distribution

Distribution and Richness based on AntMaps

Biology

Bolton (1982) - This small genus, of which only four uncommon species are presently recognized, is restricted to the Malagasy region (1 species) where it is rare, and the Afrotropical region (3 species) where it is, however, very widespread. The species nest in the healthy wood of living trees, apparently tunnelling their own galleries below the surface. For this reason most collections of Melissotarsus are made more by luck than by intent as their presence in the wood is usually not detectable on the surface. Delage-Darchen (1972) has shown that the method of walking in these ants is very strange; they progress on their front and hind legs with the middle pair projecting upwards, and presumably in contact with the gallery roof. She also noted the presence of coccids inside the galleries, also discussed by Ben-Dov (1978). It seems probable that coccid secretions form a major, if not the main, item in the diet of Melissotarsus species.

Khalife et al. (2018) - Both the head and legs of Melissotarsus workers show novel skeletomuscular adaptations to increase their ability to tunnel through living wood. The head is greatly enlarged dorsoventrally, with large mandibular closer muscles occupying most of the dorsal half of the head cavity, while ventrally-located opener muscles are also exceptionally large. This differs from the strong closing: opening asymmetry typical of most mandibulated animals, where closing the mandibles requires more force than opening. Furthermore, the mandibles are short and cone-shaped with a wide articulatory base that concentrates the force generated by the muscles towards the tips. The increased distance between the axis of mandibular rotation and the points of muscle insertion provides a mechanical advantage that amplifies the force from the closer and opener muscles. We suggest that the uncommonly strong opening action is required to move away crushed plant tissues during tunnelling and allow a steady forward motion. X-ray spectrometry showed that the tip of the mandibles is reinforced with zinc. Workers in this genus have aberrant legs, including mid- and hindlegs with hypertrophied coxae and stout basitarsi equipped with peg-like setae, and midleg femura pointed upward and close to the body. This unusual design famously prevents them from standing and walking on a normal two-dimensional surface. We reinterpret these unique traits as modifications to brace the body during tunnelling rather than locomotion per se.

Mandibles of young Melissotarsus worker, (A) dorsal (B) ventral. From Khalife et al. 2018
Histological section of the mandibles of a Melissotarsus worker, showing zinc reinforcement on the tips. Photo by Johan Billen

Castes

Winged queens and workers differ little in body size. However reproductive queens become physogastric as a result of highly active ovaries.

Worker, dealate queen and male of Melissotarsus. From Gorongosa, Mozambique. Photo by Adam Khalife.
Physogastric queen of Melissotarsus, i.e. abdomen is greatly enlarged by stretching the intersegmental membranes. From Toamasina Province, Madagascar. Photo by Christian Peeters

Morphology

Worker Morphology

 • Antennal segment count 6 • Antennal club 2 • Palp formula 0,1 • Total dental count 1-4 • Spur formula 0, 0 • Sting present

Male Morphology

 • Antennal segment count 11; 12 • Antennal club 0 • Palp formula 0,1 • Total dental count 0-2 • Spur formula 0, 0

Nomenclature

The following information is derived from Barry Bolton's New General Catalogue, a catalogue of the world's ants.

  • MELISSOTARSUS [Myrmicinae: Melissotarsini]
    • Melissotarsus Emery, 1877b: 378. Type-species: Melissotarsus beccarii, by monotypy.

References