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The genitalia of male ants are interpreted in the context of the broader Hymenoptera. For the first time muscle homologies are established for twenty six species of ants in nine subfamilies: Amblyoponinae, Cerapachyinae, Dolichoderinae, Ecitoninae, Ectatomminae, Formicinae, Leptanilloidinae, Myrmicinae and Ponerinae. Fifteen muscles in total are found in the external genitalia of male ants and all are homologous with the musculature of basal Hymenoptera (Schulmeister 2001). Secondary fusion, reduction or losses of muscles have occurred in different lineages. From generalized to derived, the functional morphology and qualitative mechanics of three taxa, Formica obscuripes Forel 1886, Messor andrei Mayr (1886) and Labidus coecus Latreille (1802) are described and compared. Special reference is made to the Ecitoninae, where the work of Birket-Smith (1981) is reinterpreted and volsellar homology is clarified. The true digitus of Labidus is revealed to be a small sclerite at the base of the distal volsellar element whichis re-identified as the cuspis.
Anatomy, army ants, kinesiology, morphology, reproductive biology
Insect male genitalia are spectacular mechanical adaptations with diverse morphology and function. Examples of this diversity include the twin hypodermic intromittent organs of the Drosophila bipectinata complex (
The morphology and mechanics of male ant genitalic musculature remain undescribed for all ~12, 000 species of ants (
Knowledge of genitalic functional morphology contributes to our understanding of the sexual selection pressures which act on these organs (
The goal of this paper is to discuss the homologies of male ant genitalic musculature in the context of the broader Hymenoptera and to provide descriptions of the functional mechanics of three distinct forms. The work of
All specimens examined in this study were from the author’s personal collection (BEBC) or the Longino collection (JTLC, University of Utah). Collections in 95% ethanol were made in Central America, and in the states of Arizona, California, Tennessee and Washington. Specimens were softened by boiling in water for 2–5 minutes. Dissections were carried out in petri dishes, with or without wax and minuten pins. A Leica MZ16 microscope with maximum magnification of 115× and mounted with a JVC Digital Camera KY-F708 3-CCD was used for dissections. Micrographs were generated for illustration using Auto-Montage Pro Syncroscopy (Synoptics Ltd., Frederick, MD USA) software. For especially small males, temporary mounts of minute sclerites and musculature were prepared for observation with a Leica DME compound microscope using glycerin as the mounting medium.
General hymenopteran terms preferred by the Hymenoptera Anatomy Ontology (
The mechanics of sclerite movement were assessed qualitatively by examining the angles of muscular origin and insertion, and the attachment or position of the sclerite relative to the local components of the genital capsule. Simple physical models were used to predict the mechanics of muscular contraction and element movement. Formica obscuripes Forel 1886, Messor andrei Mayr (1886) and Labidus coecus Latreille (1802) were chosen as models for functional morphology because their genitalia are well developed, represent cases of relatively generalized to specialized morphology, and among these three taxa all the observed genitalic muscles of male ants are represented. The more complicated movements described here should be treated as predictions to be tested either via direct observation of copulation or via neuroethological studies.
Anatomical terms of the external genitalia used herein, excluding specific membrane and apodeme names. Hymenoptera Anatomy Ontology (HAO) Uniform Resource Identifiers (URIs) link to concept definitions. Synonyms discussed or used are indicated.
External genitalia (Figs 1–16)
The external genitalia of ants are composed of five elements: sternum IX, the cupula, and the paired valves of the penisvalvae, volsellae, and parameres (Figs 1–4). Sternum IX (Fig. 4) is produced anteromedially into a process called the spiculum (
Lateral to medial, the three paired valves are the parameres, volsellae, and penisvalvae (Figs 2 and 3). Homology of the parameres of the Formicidae with those of other Hymenoptera is reasonable, as the following complex characters are shared: 1) at least four of the five penisvalvae muscles attach to the basimeres; 2) an intrinsic muscle has an origin in the basimere and inserts in the telomere; and 3) a membrane is present at the telomere articulation with the basimere. It should be noted that this telomeral membrane is variable in development for the Formicidae, from well-developed to absent across numerous clades (
Although the term aedeagus has a long history and is frequently used in the myrmecological literature to describe the middle-most valves (
Internal genitalia (Figs 16 and 17)
As the aedeagus is the interface between the internal and external genitalia, a general description of the internal genitalia is warranted (Fig. 16). Spermatozoa are produced in the follicles of the paired testes and pass through the vas deferens (
Musculature
The homology of genitalic musculature for each species examined is summarized in Table 2. Fifteen total muscles were observed among the examined ant taxa. Considerable variation exists in the morphology of tergites IX and X, but as these are not primary components of the genitalia these were excluded from this investigation. Three muscles, a, b and c, have their origins on sternum IX and insert on the cupula (Figs 4, 5–7). Muscle a is paired and originates on the spiculum from which it diverges to the ventrolateral margin of the cupula, except in the ecitonines where a is inserted in a median groove ventrad to the reduced foramen genitale. The origin of the unpaired muscle b is along the cranial apodeme; the muscle converges anteriorly to its ectal insertion on the medioventral margin of the cupula. Each element of the paired muscle c originates on an anterolateral lobe of the sternum and inserts on the ventrolateral margin of the cupula.
Muscle presence or absence per taxon. Only muscles recorded from the Aculeata are included. 0 absent; 1 present; 2 fused; ? presence/absence unconfirmed; ‡Amblyoponinae; §Cerapachyinae; |Dolichoderinae; ¶Ecitoninae; #Ectatomminae; ††Formicinae; ‡‡Leptanilloidinae; §§Myrmicinae; ||Ponerinae; ¶¶Sphecidae and ##Vespidae (final two from
| Species | Muscles | |||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| a | b | c | d | e | f | g | h | i | j | k | l | o | p | qr | t | |
| Prionopelta nr. modesta‡ | 1 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 1 |
| Cerapachys nr. augustae§ | 1 | 1 | 1 | ? | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | ? | 1 |
| Cylindromyrmex brevitarsus§ | 1 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 0 | 1 |
| Dolichoderus bispinosus| | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 1 |
| Eciton lucanoides¶ | 1 | ? | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 0 |
| Labidus coecus¶ | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 0 |
| Labidus praedator¶ | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 0 |
| Neivamyrmex longiscapus¶ | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | ? | 0 | 1 | 1 | 0 |
| Nomamyrmex eisenbeckii¶ | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 0 |
| Gnamptogenys mordax# | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 0 |
| Camponotus sansabeanus†† | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 0 | 1 | 1 | 1 |
| Camponotus atriceps†† | 1 | 1 | ? | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 0 | 1 | 1 | 1 |
| Formica obscuripes†† | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 1 |
| Prenolepis imparis†† | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 0 | 1 | 1 | 1 |
| Leptanilloides sp. ‡‡ | 0 | 1 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 1 | 0 | 1 |
| Aphaenogaster nr. rudis§§ | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 0 | 1 | 0 |
| Atta cephalotes§§ | 1 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 0 | 1 | 0 |
| Crematogaster nigropilosa§§ | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 0 | 1 | 1 | 1 |
| Messor andrei§§ | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 0 | 1 | 1 |
| Myrmica kotokui§§ | 1 | ? | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | ? | 0 | 1 | 1 | 1 |
| Pheidole californica§§ | 1 | 1 | 1 | 1 | 1 | 2 | 2 | 1 | 1 | 1 | 1 | 0 | 0 | 0 | 1 | 0 |
| Hypoponera opacior|| | 1 | 1 | 1 | ? | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 0 | 0 | 1 | 0 |
| Leptogenys donisthorpei|| | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 0 |
| Odontomachus chelifer|| | 1 | 1 | 1 | ? | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 0 |
| Platythyrea prizo|| | 1 | 1 | 0 | ? | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | 0 |
| Sceliphron caementarum¶¶ | 0 | 1 | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 0 |
| Dolichovespula adulterina## | 0 | 1 | 0 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 |
Four muscles, d, e, f, and g, have their origins on the cupula (Figs 1–3, 5–8). Muscles e, f, and g are paired, while d is unpaired.The origins of the cupula muscles are usually associated with an apodeme: muscle d with the ventral, e with the lateral, f with the dorsolateral and g with the dorsal apodeme (Figs 1, 2, 6). In general the gonostipital arms, or anteromedian projections of the basimeres, receive the insertions of d on the ventral margin and e ventrolaterally (Figs 1, 5, 7, 8). Both f and g insert ectally on the basimere, f on the lateral margin and g on the dorsal margin (Figs 1–3, 5–8). Loss of cupula muscles was observed for three species: Atta cephalotes L. (1758), Crematogaster nigropilosa Mayr 1870, and Leptanilloides sp. in which the cupula is reduced to a thin strip with only muscle e present.Muscles f and g are fused in Pheidole californica Mayr 1870, and muscle d could not be confirmed for Hypoponera opacior Forel (1893), Odontomachus chelifer Latreille (1802), and Cerapachys nr. augustae.
In total, five penisvalva muscles (Figs 13–15) were found; h, i, j, and k were consistently present except in the reduced genitalia of Leptanilloides sp. (Table 2), while l was lost in disparate taxa. These muscles are paired, with one element of each associated with a left or right penisvalva. Three of the penisvalva muscle origins are on the mesal face of the basimere: i on the ventral, j on the dorsolateral, and k on the dorsomedian face (j, k Figs 1–3; i Fig. 8). The origin of muscle h is variable, in some cases spanning the apices gonostipitum to the ventral apodeme of the cupula. Although the dorsomedial basimeral area where k originates has been termed the parapenis in basal Hymenoptera (
Across the taxa examined there was great variation in the form of the volsellae, with concomitant variation in the presence and form of the two muscles, p and qr (Figs 9–11) which both insert on the cuspal apodeme. Similar to the penisvalvar muscles, the volsellar muscles are paired with one element of each associated with a left or right volsella. Muscles m, n, o, s, and si (
A single intrinsic muscle of the paramere, t, was found in several of the taxa examined. Muscles u and v (
Genital capsule and sternum IX habitus and musculature. 1 Formica obscuripes lateral oblique view, 0.5 mm 2 Messor andrei dorsal view, 0.5 mm 3 Labidus praedator dorsal view, 1.0 mm 4 Formica obscuripes sternum IX anterior-to-posterior oblique mesal view, 0.5 mm. Abbreviations: Bm basimere; Cr cranial apodeme of sternum IX; Cs cuspis; Cu cupula; Di digitus; Dl dorsolateral apodeme of cupula; Do dorsal apodeme of cupula; Lc lateral apodeme of cupula; Lm valviceps lamina; Ma maculation; Pm penisvalva membrane; Pt phallotrema; Sp spiculum; Te telomere; Tm telomeral membrane; Vm volsellar membrane; Vu valvura. Muscles: a b c d e f g h i j k l p qr t;insertion of muscle ‘x’: xi; origin of muscle ‘x’: xo.
Muscles of thegenital capsule and sternum IX. 5 Formica obscuripes genital capsule anterior-to-posterior view, 0.5 mm 6 Neivamyrmex longiscapus lateral view, 1.0 mm 7 Messor andrei genital capsule anterior-to-posterior view, 0.5 mm 8 Formica obscuripes volsella and paramere mesal view, with all penisvalvar muscles removed except l, 0.5 mm. Abbreviations: Ad anterodorsal apodeme of basimere; Al anterolateral apodeme of basimere; Ap apex gonostipitis; Av anteroventral apodeme of basimere; Ba basivolsellar apodeme; Bm basimere; Bv basivolsella; Cs cuspis; Cu cupula; Dl dorsolateral apodeme of cupula; Do dorsal apodeme of cupula; Eb endophallic bladder; Fg foramen genitale; Ga gonostipital arm; Lc lateral apodeme of cupula; Pv penisvalva; Te telomere; Vc valviceps; Vo volsella. Muscles: a b c d e f g h i j k l p qr t;insertion of muscle ‘x’: xi; origin of muscle ‘x’: xo.
Volsella muscles and morphology; the parossiculus is composed of the basivolsella and distivolsella. 9 Formica obscuripes ventral view; inset mesal with digitus removed, dorsal left; both 0.5 mm 10 Messor andrei mesal view, dash indicates base of twisted lamina and part of setose ridge; inset dorsal; both 0.2 mm 11 Labidus coecus mesal view, 1.0 mm 12 Neivamyrmex longiscapus lateral view, 0.5 mm. Stippling on 9 and 12 indicate setal bases. Abbreviations: Bm basimere; Bv basivolsella; Ca cuspal apodeme; Cs cuspis; Di digitus; Vm volsellar membrane. Muscles: p qr t;insertion of muscle ‘x’: xi; origin of muscle ‘x’: xo.
Penisvalva muscles and morphology. 13 Formica obscuripes ectal view, 0.5 mm 14 Messor andrei ventral, 0.2 mm 15 Labidus coecus ectal view, ki is on mesal face of valvura, 1.0 mm. Abbreviations: Lp lateral apodeme of penisvalva; Vc valviceps; Vu valvura. Muscles: h h’ i j k l;insertion of muscle ‘x’: xi; origin of muscle ‘x’: xo.
Internal genitalia. 16 Formica obscuripes ventral oblique view, 1.0 mm 17 Labidus coecus ductus ejaculatorius with wedge sclerite ventral view, 0.5 mm. Abbreviations: Vs vasicula seminalis; Ag accessory gland; De ductus ejaculatorius; Eb endophallic bladder; Gc genital capsule; Gp gonopore; Sp spiculum; Te testis; We wedge sclerite. Muscle: a.
Formica obscuripes Forel, 1886
Among the taxa examined this species has relatively generalized genitalia, with all fifteen muscles present and without extreme modifications of the sclerites. The three muscles of sternum IX, a, b, and c, are present in this species (Figs 4 and 5). Simultaneous contraction of both elements of a contracts the genital capsule into the gaster to varying degrees. Antagonistic contraction of either the left or right element of a adducts the genital capsule towards the respective side, an action which is synergistic with the genital capsule protractor b. Muscle c inserts ventrolaterally on the ventral apodeme of the cupula, above the insertions of a and b (Fig. 5), and torques the genital capsule via elevation of the basal margin of the cupula toward the muscle’s origin.
Muscles of the cupula rotate the genitalic valves within the cupula similarly to a ball-in-socket mechanism. Each muscle is inserted on the anterior margin of the basimere except for d, which inserts on the anteroventral face of the endophallic bladder (Figs 5 and 8). Muscle e adducts the valves ventrally, while g acts antagonistically to elevate the apices while contracting the valves. Muscle e also adducts the telomere to the mediosagittal plane by drawing the medioventral apices of the basimeres dorsolaterally. Contraction of f may affect torsion of the valves.
All five muscles of the penisvalvae are present in this species (Fig. 13). The penisvalvae can move independently along the mediosagittal plane and are capable of rotation, pivoting on their lateral apodemes. Elevation of the valviceps is caused by contraction of h, while depression is caused by j. Muscle k is inserted basally on the valviceps and may dig the basal teeth into the female’s bursa copulatrix (
Two muscles, p and qr, are attached ectally and mesally to the cuspal apodeme of the volsellae, respectively, and act antagonistically to open or close the cuspides (Fig. 9). Adduction of the cuspis to the digitus is caused by contraction of qr, which bends the volsellar plate toward the penisvalvae while drawing the cuspal apex toward the digitus. The antagonistic abduction of the cuspis is induced by p, which extends laterally to the basimere from the ectal face of the cuspal apodeme (Figs 8 and 9).
The single intrinsic muscle of the paramere, t, is relatively well developed in this species. Contraction of t flexes the telomere ventrally via folding of the telomere corium. Upon relaxation of t, the elastic force of the sclerotic bridge between the basimere and telomere sclerite erects the telomere. Flexion of the telomere may also be affected by the penisvalvar muscle j which has a major portion of its origin along a mediodorsal inflection of the basimere. Intense contraction of j may roll the telomere ventrally via torsion of the inflection ectally, while also flexing the penisvalvae ventrally; this may function to strengthen the clamp-hold of the female.
Messor andrei Mayr, (1886)
With the loss of p, fourteen of the fifteen genitalic muscles are found in this species.Although the foramen genitale is somewhat constricted (Fg, Fig. 7), the muscular function of the cupula and sternum IX are not distinct enough from Formica obscuripes to warrant separate discussion. The clasping function of the paramere is maintained despite the replacement of most of the telomere corium by sclerite. In this case, there is very little telomere flexion. Rather, t adducts the apices of the telomere toward the mediosagittal plane while pulling the medioventral faces dorsally. The mesal face of the telomere has a cup-like lamina which, coupled with hypothetical secretions of the telomeral gland, may enhance the male’s grasp on the female.
The valviceps is produced dorsally with a laterally-produced lamina (Lm, Fig. 2) which is divided into anterior and posterior lobes by a constriction, making the valviceps similar in shape to an anvil. The dorsal laminae may function as a passive anchor in the female’s bursa copulatrix while the aedeagus is intromittent. Between the two valviceps, an ellipsoid chamber is formed with the penisvalvar membrane closing it dorsally. I hypothesize that this chamber acts as a mechanical sperm pump, forcefully pulsing ejaculate out of the anterior-directed phallotrema via contraction of l, which spreads the valviceps bases. Contraction of j may synergistically contribute to this hypothetical action via rotation around a vertical axis placed at the dorsal constriction, drawing the dorsal posterior lobes together while spreading the anterior lobes and valviceps apices ectally. Muscle j also adducts the valviceps ventrally. The basal wall of the valviceps, where k is inserted, is inflected dorsally and ventrally into a rough parabola with its vertex pointed anteriorly. This inflection divides k into dorsal levator and ventral contractor halves. Motion of l and k together may pull the valviceps anteroventrally, potentially digging the valviceps teeth into the female. Muscle h has most, if not all, of its origin along the endophallic bladder.
Clasping is not the function of this species’ modified volsella (Fig. 10), since the digitus is completely fused with the setose cuspal shaft, and is in the form of a crescent with a basal flange and apical process. The apical component of the cuspis is reduced to a small ellipsoid plate that articulates with the digitus laterally (Cs, Fig. 10, inset). Muscle p has been functionally replaced by qr which is divided into dorsal and ventral halves by the flange-like basivolsellar apodeme (Ba, Fig. 10). The dorsal margin of the cuspis is produced into a lamina which twists ~180° around the basal-distal axis. The reduced cuspis apex acts as a pivot for rotation of the digital crescent, while the twisted lamina provides strength and elastic flexibility for torsion of the volsella. Ventral contraction of qr inthe left volsella rotates the apex of the digitus medioventrally clockwise, while dorsal contraction rotates the digital flange dorsolaterally counterclockwise. Thus qr has a fine-tuned forward and reverse function via differential contraction of the dorsal or ventral sides coupled with the elastic torsion of the cork-screw cuspis. The copulatory function is hypothesized to be prying the terminal plates of the female open, akin to a lock-pick, via insertion of either the digitus or digital flange into the female followed by lifting or depressing torsion.
Labidus coecus Latreille, (1802)
The genitalia of Labidus have been greatly modified in form relative to the non-ecitonine taxa examined. Sternum IX is produced apically into a bifid lobe and muscle c is absent. An external longitudinal groove extends along the ventral face of the egg-shaped cupula (Cu, Fig. 8) and receives the insertions of muscles a and b. The origin of b has migrated up the anterolateral lobes of the sternum; the muscle extends mesally to its insertion in the ventral groove just posterad the insertion of a, which is ventrad the foramen and is longitudinally elongated. This elongation of a along the curvature of the cupulaprevents pronounced dorsal or ventral pitch during contraction of a. Muscle b may either torque the capsule contralaterally with contraction of the left or right half, or both halves together may adduct the apices of the genital valves ventrally.
Contrary to the arguments of
Contraction of the cupula muscles may cause splaying and movement in the mediosagittal plane. The cupula of all ecitonine species investigated is extremely strong and well-constructed to withstand the forces applied to it internally. A tall longitudinal apodeme is present along most of the length of the ring, from the foramen genitale to the apices gonostipitum. This apodeme may function as a strut, strengthening the cupula from the antero-posterior compression forces exerted by the cupular muscles. The anteroventral processes of the basimeres are extended anteriorly to a greater magnitude than the dorsal margins of the basimeres. Thus the dorsal margins of the basimeres act as a fulcrum for a third-order lever: synergistic contraction of d and e act as in-forces and cause the apices of the genitalic valves to swing ventrally. As muscle e converges mesally from its ventrolateral origin, it may also be able to fine-tune the yaw of the adduction motion. Muscle f contributes primarily to the spreading of the genitalic valves around a vertical axis of rotation between the apices gonostipitum and the bases of the valviceps.
With respect to the penisvalva muscles,
Four of the five penisvalva muscles seem to act synergistically by contributing forces to the ventral adduction of the penisvalva apex, while only h acts antagonistically. The muscles all act as in-forces for moving the penisvalva as a lever and may be conveniently grouped by the order of their lever mechanism: muscle h as first-order dorsal adductor;muscles k and j as first-order and i as a third-order ventral adductor. Besides contributing to ventral or dorsal adduction, h antagonistically closes or spreads the ventral apices of the penisvalvae.
The volsellae of Labidus are highly modified, which has led to confusion of their homology (
Both muscles of the volsellaeare present (Fig. 11), with their insertions on the cuspal apodeme.
The final point of contention with
The intersubfamilial homology of male ant genitalic musculature is described for the Amblyoponinae, Cerapachyinae, Dolichoderinae, Ectatomminae, Formicinae, Leptanilloidinae, Myrmicinae and Ponerinae for the first time, while the musculature and sclerite homology of the ecitonines Labidus and Neivamyrmex are clarified. Overall, the generalized male ant has fifteen muscles: three on sternum IX, four on the cupula, five penisvalvar, two volsellar and one paramere muscle. Muscles of sternum IX act to protract, contract or torque the genitalic capsule, which may aide in probing and intromittent behavior. The cupula muscles attach to the basimeres, and are most variable in presence or absence among the myrmicine taxa examined. Muscles of the cupula exert greater control over the genitalic valves relative to the muscles of sternum IX. The penisvalva muscles have functions which vary with the lineage, and muscle l is lost in some. In general, the penisvalva muscles may torque the penisvalvae apices, while adducting the individual penisvalvae in the mediosagittal plane. Volsellar mechanics varies greatly with the morphology of the volsellar sclerites. In species with opposable digiti and cuspides, the muscles may spread or close the apices. In other species, the digiti may torque clockwise or counter-clockwise, while movement in Labidus is restricted to the mediosagittal plane. Finally, the parameres have a single muscle which is lost in some lineages to be replaced by any of the proximal muscles.
The mechanics of the sclerites and their muscles are labile over evolutionary time: the axes of rotation, the pivots, and the muscular origins and insertions shift across the sclerites. Detecting the patterns of morphological and mechanical evolution of male ant genitalia may provide useful phylogenetic characters, especially for clades currently with few or without morphological synapomorphies (e.g. the Formicoid clade,
Thanks to my mentor Jack (John) T. Longino (University of Utah) for inspiration and review of this manuscript. For further reviews which improved the manuscript, I thank Marek L. Borowiec (University of California, Davis), Masashi Yoshimura (California Academy of Sciences), Erik V. Thuesen (the Evergreen State College, TESC), and two anonymous reviewers. Thanks to Eli M. Sarnat (University of Illinois), Erik B. Ordway (TESC) and Trisha Towanda (University of Rhode Island) for providing Adobe Illustrator assistance. This work was supported in part by the National Science Foundation grants DEB-0640015/DEB-1157383 and DEB-0072702 for the LLAMA and ALAS projects, respectively.