Megalomyrmex

Longino (2010) - Although widespread in the Neotropics, from southern Mexico to northern Argentina, Megalomyrmex species are never abundant. They occur in low to middle elevation wet to dry forest habitats. Some species are free-living with large diffuse nests in the soil (e.g. Megalomyrmex modestus) or small nests in dead wood (e.g. Megalomyrmex drifti) (Brandão 1990, 2003). Others are specialized social parasites or predators of Attini (e.g. Megalomyrmex mondabora, Megalomyrmex symmetochus, Megalomyrmex wettereri; Wheeler 1925, Weber 1941, Kempf & Brown 1968, Adams et al. 2000, Adams & Longino 2007). Some species have unusual alkaloids (Jones et al. 1991a; Jones et al.1991b; Jones et al. 1999).

Identification
Brandão (1990) - Megalomyrmex is clearly a natural group and can be separated from all other myrmicines by the monomorphic workers with mandibles with, at least, 5 teeth, palpal formula 4:3, 3:2 or, rarely, 3:1.

Nothidris (sensu Snelling, 1975) shows strong affinities with Megalomyrmex, but can be readily distinguished by the presence of soldiers (without intennediates) and the lack of a carina connecting the posterior plates of the propodeum in ordinary workers. Nothidris queens present petiole broader than the postpetiole and forewings with open discoidal cell (m-cu not complete). Nothidris has a transandean distribution.

Boudinot et al. (2013) - Males of Megalomyrmex are morphologically heterogeneous; some species resemble “generalized myrmicines” whereas others resemble the strange attenuated males of Neotropical Aphaenogaster. The generalized males may be confused most easily with those of New World native Monomorium, but the reduced petiolar and postpetiolar nodes of Megalomyrmex males, relative to workers, is sufficient to separate the two genera. However, separation of Megalomyrmex males from those of Aphaenogaster deserves special mention, due to the remarkable convergence in body form (even including a kinked and distally swollen metabasitarsus in one case). Males of Megalomyrmex may be separated from those of Aphaenogaster by the following characters: (1) absence of an anterior mesonotal bulge (present in A. honduriana); (2) region of head capsule posterior to ocelli never produced into an elongated and flared collar (present in the A. phalangium complex); (3) absence of an anterobasal metacoxal bulge; (4) presence of the foraminal carina (if developed); (5) presence of the basipetiolar carina.

Megalomyrmex species groups

Biology
Boudinot et al. (2013) - The Neotropical ant genus Megalomyrmex is diverse in form and behavior. Several species tend sternorrhynchans or are predacious (Brandão 1990); others are associates of fungus-growing Attini, having lestobiotic (thief ant), agro-predatory (garden usurper), or xenobiotic (cohabitating guest ant) relationships with their hosts (Adams 2008; Adams, Shah et al. 2012).

Additional information about the biology of this genus can be found in details provided on many of the individual species pages.

Nomenclature

 *  MEGALOMYRMEX [Myrmicinae: Solenopsidini]
 * Megalomyrmex Forel, 1885a: 371. Type-species: Megalomyrmex leoninus, by monotypy.
 * [Type-species not Formica bituberculata, unjustified subsequent designation by Wheeler, W.M. 1911f: 167.]
 * Megalomyrmex senior synonym of Cepobroticus, Wheelerimyrmex: Ettershank, 1966: 101; Brandão, 1990: 415.
 * CEPOBROTICUS [junior synonym of Megalomyrmex]
 * Cepobroticus Wheeler, W.M. 1925d: 168 [as subgenus of Megalomyrmex]. Type-species: Megalomyrmex (Cepobroticus) symmetochus, by monotypy.
 * Cepobroticus junior synonym of Megalomyrmex: Ettershank, 1966: 101; Brandão, 1990: 415.
 * WHEELERIMYRMEX [junior synonym of Megalomyrmex]
 * Wheelerimyrmex Mann, 1922: 29 [as subgenus of Megalomyrmex]. Type-species: Megalomyrmex silvestrii, by original designation.
 * Wheelerimyrmex junior synonym of Megalomyrmex: Ettershank, 1966: 101; Brandão, 1990: 415.

Worker
Brandão (1990) - Total length 2-12 mm. Basal area of mandibles with open trulleum and linear mandalus, canthellus not reaching the basal margin. Masticatory blade with 1 to 3 apical teeth. Dental formula 1+4, 2+n (n=5 to 12) or 3+3. Distal margin of labrum bilobed, longitudinal cleft more deeply excavated than in other genera of the group. Palpal formula 4:3, 3:2 or occasionaly 3:1, without apparent fusion of segments. Prementum with 2 large setae at the center of the plate. Clypeus, in side view, divided in two regions by a square angle. Anterior portion with anterior border either straight or round, with or without a median denticle. Posterior region with median area prolonged between the frontal carinae. The anterior border of the posterior region may have up to four denticles, the inner ones leading to marked carinae, that marginate the central depression. Frontal carinae always present, may be parallel or divergent behind the antennal sockets. Frontal area sometimes indistinct. Frontal suture impressed in most species of the Leoninus and some species of the Modestus group.

Vertex angles round giving to the head capsule a round shape. Occipital margin raised or not; when raised may be visible in head frontal view.

Compound eyes with 5-25 ocular facets at largest diameter. The cavities that accommodate the eyes may be encircled by 1 or 2 concentric striations. Ocelli may occur in gamergates of the Leoninus and Pusillus groups, the anterior ocellus being always larger than the lateral ones, which may be vestigial.

Antennae always with 12 articles, clubbed. Scapes, laid back over the head, attaining at least the occipital border, but in general exceeding it. Scapes largest diameter at 7/10 of its length. First funicular joint equal in length to the second in the Leoninus group but at least twice the second in other groups. Antennal club with generally 3 segments, in cases with 4 or 5. the involved segments being always longer, but not necessarily broader than preceeding ones (except for the Pusillus group where clubbed segments are much broader and longer than ordinary ones).

Humeral pronotal angles always round. In dorsal view the pronotum is always the largest part of the trunk; it may exhibit a shallow median depression, separating two low inflated lateral areas (in the Leoninus group only). Promesonotal suture generally impressed laterally and dorsally, except in the Silvestrii group as a whole and in some species of other groups. When the suture is not impressed the dorsal profile of the pronotum is continuous. Contrariwise when impressed, the mesonotum may be higher than the pronotum. Mesoternum and metasternum may have acrotergites in form of paired sharpened teeth. Anepisternum and katepisternum, even when not clearly separated by sutures, distinguishable as low inflated areas posterior and lateral to the mesonotum.

Metanotal groove may be rather deep and sometimes marked by gross parallel rugosities from the dorsum to the pleural region, completely separating the katepisternum from the propodeum.

Propodeum generally with dorsal face clearly distinguished from declivity. Anterior margin of dorsal face may be elevated over the metanotal groove. In dorsal view this margin may also be notched, giving to it an ear-lobe aspect in side view. The entire propodeum dorsum may be strongly depressed, the depression suited to accommodate the pedicel when hold over the body. In side view this depression in not visible because of the lateral crests. At the declivity these lateral portions may be marked by carinae. Propodeum angles never dentate nor acuminate.

Declivity, seldom incospicuous, may have concentric rugosities over the foramen encircling the epipetiolar carina. Posterior propodeal plates always round and translucid. Metapleural gland always well developed, with irregular rugosities over the bulla. Propodeum spiracles in general large, at times fitting in a low elevation, at an angle of 45° to the main axis of the body.

Petiole pedunculate or not. When pedunculated the node distinctly separated from the peduncle. Ventrally the petiole may bear a denticle anteriorly that may originate a longitudinal flange reaching the spiracle. Node always high and as large at the base as the peduncle. Dorsal border, in frontal view, round with distinct apex or subquadrate with gentle dorso-lateral angles. In some species of the Leoninus group this border may present a shallow median notch, giving the node a cordiform aspect in frontal view. Posterior collar of petiole always striated.

Postpetiole never pedunculate. Node globose, generally lower and broader than the petiole. In some species of the Pusillus and Silvestrii groups the spiracles protrude laterally. The ventral face of the postpetiole may bear an acute tooth anteriorly and/or a round process posteriorly.

Anterior border of gaster straight or subtruncate in dorsal aspect. Venom apparatus with subquadrate spiracular plate, with spiracle near the ventral margin. Anterior apodema less developed than in Solenopsis. Posterior tubercle obsolete. Quadrate plate similar with that of Huberia (Kugler, 1978, fig. 27) and anterior margin reduced. Anterior apodema more developed in the Pusillus group. Anal plate with 5-12 long setae at distal margin. Oblong plate of the anterior arm with 3-4 intervolvifer sensillae. Subterminal tubercule low and obsolete. Anterior apodema narrow in side view.

Legs slender, unremarkable, with well developed spurs on middle and hind tibiae in the Leoninus and Modestus groups, but obsolete in the other groups. Apex of femora round in most species, but genual plates at times acuminate in species of the Leoninus group. Tarsal claws simple without arolia.

Sculpture

In general the body surface is smooth and shiny but may have rugose areas on the sides of clypeus, apex of katepisternum, metanotal groove, propodeum declivity and ventral face of petiole. Some species may also have striations around the antennal sockets, dorsally on the mandibles and petiole and postpetiole collars. Megalomyrmex wallacei (Modestus group) is the only species in the genus with most of the integument covered by gross punctures and non-oriented rugosities.

Pilosity

Clypeal set complete (in some individuals the median seta may be lacking, but its insertion is visible as a scar). In most species there are two kinds of hairs. Long hairs (comparable in size to the diameter of compound eyes) occurring on frontal carinae, pronotum dorsum, dorsal border of petiole and postpetiole nodes and apex of gastric segments. Small hairs (size from .1 to .2 mm) ocurring uniformly over the integument.

Queen
Brandão (1990) - Always larger than respective workers. Anterior ocellus in general more developed than the lateral ones, which may be vestigial. Parapsidal sutures impressed on mesoscutum. Notaulus, when present, impressed only anteriorly. Queens may have only one type of hair and larger areas of striation than conspecific workers. Wing venation with discoidal cell always closed, M leaving Rm at r, m - cu always complete, cu - a reaches A in a square angle.

Peeters & Adams (2016) - Ergatoid queens in both Megalomyrmex foreli (Leoninus group) and Megalomyrmex wallacei (Modestus group) but they differ strikingly in fecundity and morphological dimorphism relative to their workers.

Male
Boudinot et al. (2013) - The diagnosis provided below is derived from examination of the males of the following taxa: M. adamsae, M. brandaoi, M. foreli, M. incisus, M. megadrifti, M. miri, M. milenae, M. modestus, M. mondabora, M. mondaboroides, M. silvestrii, M. symmetochus, M. wallacei, M. wettereri, and M. male 01. Among the Central American Myrmicinae, characters which are unique for Megalomyrmex are indicated in italics.

1. Antenna with 11, or more often 13 antennomeres.

2. Funiculus of antenna as long as, or more often longer than mesosoma.

3. Antennomeres often kinked (distinctly bent or curved).

4. Pedicel cylindrical or subcylindrical, never swollen.

5. Palpal formula 4,3; 3,3; or 3,2.

6. Mandibles worker-like, with distinct basal and masticatory margins and with three to numerous teeth.

7. Head proportional for body size (i.e. head length about one half mesosoma length, HL/ML = 0.47–0.62).

8. Occipital carina and vertex never markedly elongated and flared as a collar.

9. Notauli absent or weakly indicated by short diagonal sulcus with transverse carinae.

10. Metacoxa without an anterobasal bulge.

11. Metasternum with or without a spine-like process.

12. Tibial spur formula 1s–b,1s–b.

13. Ventral margin of tarsal claws smooth, without teeth.

14. Foraminal carina usually present and well-developed, may be absent.

15. Propodeal foramen with a strong foraminal carina, several arcing carinae of equal strength, or no distinct carinae.

16. Pterostigma present, well-developed.

17. Forewing: submarginal cell 1 closed.

18. Forewing: marginal cell 1+2 open (i.e. apical abscissae of R and Rs not fusing at apex of wing).

19. Hindwing: claval region narrow.

20. Petiole and postpetiole nodiform; nodes reduced in height relative to workers and queens.

21. Basalmost area of petiolar dorsum delimited posteriorly by a transversely arcing carina or two lateromedian, sinuate carinae (i.e. basipetiolar carina present).

22. Postpetiole with narrow posterior attachment (i.e. helcium of first gastric tergum narrow).

23. Gastral shoulder present (i.e. first gastric sternum with a transverse angle laterad the helcium).

24. Pygostyles present.

25. Telomere tall in the sagittal plane; not dorsoventrally flatted. Body predominantly shining: pronotum, mesoscutum, mesopleuron and lateral face of propodeum never areolate, foveolate, foveate, or scabrous; these surfaces without lamellae, carinae, denticles, or spines.

Notes/Comments 1. Specimens may (very infrequently) differ in antennomere count from left to right antenna due to variable suturation.

3. One of various antennomeres of the funiculus is always kinked in the silvestrii-group, excluding M. wettereri. Both M. wallacei and M. foreli have the third antennomere apically flattened and kinked. The fourth antennomere of M. mondabora, M. mondaboroides, and M. male 01 is kinked. The function, if any, of the kinked antennomeres of male Megalomyrmex and other genera (e.g.Basiceros and Aphaenogaster) is unknown.

11. This is character six in Bolton’s (2003) diagnosis of the solenopsidine tribe group, which he classifies as not having a metasternal process, with the exceptions of M. latreillei (=M. foreli) and Vollenhovia pertinax. We observed presence of a metasternal process, in addition to M. foreli, in the male and both castes of female of M. leoninus and M. modestus.

12. The tibial spur formula follows Bolton (2003), in this case indicating that the meso- and metatibia have one simple to barbulate spur each.

14. The foraminal carina has various states of development and is effectively absent in M. drifti and M. foreli. We predict that the male of M. nocarina will not have a foraminal carina.

21. The basipetiolar carina is present in all sexes and castes of Megalomyrmex; it is weakly developed in the male of M. modestus. A similar carina is present on the males of several ponerine lineages, but this must be considered homoplasious.

23. This is the twelfth character in Bolton’s (2003) diagnosis of the Solenopsidini.

26. Prior to the discovery of M. longinoi, no Megalomyrmex species were known to have costate sculpture. If the male of M. longinoi has similar sculpture to the worker, then this would be an exception to this character.

Larva
Brandão (1990) - Although I have at hand larvae of only 4 species of Megalomyrmex, some characters seem constant and may be used as a brief characterization. I include the description of M. symmetochus larvae (Wheeler & Wheeler, 1955), that I have not studied. I have studied larvae of the following species: Megalomyrmex ayri; Megalomyrmex balzani; Megalomyrmex iheringi and Megalomyrmex silvestrii.

Profile pogonomyrmecoid (M. balzani) to pheidoloid (all other species and M. symmetochus). Subhexagonal head with smooth hairs (kind lAl in Wheeler & Wheeler, 1976) unbranched, slightly curved or straight; antennae of the usual type not elevated; bilobed labrum in all species but in M. balzani (round); mandibles pheidoloid or ectatommoid (in M. symmetochus); galea and laxinia loboses (paraboloid in Wheeler & Wheeler, 1955); maxillary palp longer than galea, ventral face of abdomen without gonopods or leg vestiges. Spiracles with simple atrial wall.

Anchor tipped hairs (kind 1A4 of Wheelers) at the posterior end of the dorsum. Unbranched tip denticulate (kind 1 B4) at the posterolateral portion of the ventral face.

In M. balzani there is a protuberance shaped as a subcone, better classified as a frustrum with an apical hair not capitate in Wheeler's nomenclature.

Biological References

 * Adams, R.M.M. & Jones, T.H. (2010) The evolution of venom alkaloids in Megalomyrmex ants, from predators to social parasites. International Society of Chemical Ecology 26th Annual Meeting (p. Pster P110, pg. 301). Tours, France.
 * Adams, R.M.M. & Longino, J.T. (2007) Nesting biology of the arboreal fungus-growing ant Cyphomyrmex cornutus and behavioral interactions with the social-parasitic ant Megalomyrmex mondabora. Insectes Sociaux, 54(2), 136-143.
 * Adams, R.M.M., Jones, T.H. & Jeter, A.W. (2010) Male specific tyramides from three additional myrmicine genera. Biochemical Systematics and Ecology, 38(3), 454-456.
 * Adams, R.M.M., Mueller, U.G., Schultz, T.R. & Norden, B. (2000) Agro-predation: usurpation of attine fungus gardens by Megalomyrmex ants. Naturwissenschaften, 87(12), 549–554.
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