Aneuretus simoni

Aneuretus simoni is the only species of this genus and is the only extant member of the subfamily Aneuretinae. Long believed to be one of the basal lineages of Formicidae, molecular phylogenetics have shown the subfamily is closely related to the Dolichoderinae (see the subfamily phylogeny).

Identification
In the field workers resemble small yellow Pheidole minors in their morphology. Behaviorally they have a tendency to keep their long petiole folded up against their propodeum with their gaster slightly elevated.

Distribution
A Sri Lanka endemic. Jayasuriya and Traniello (1986) reported: colonies were found only in the humid forests of the Gilimale area. The distribution of A. simoni appears to be correlated with rainfall (200 - 500 cm/year) and geographic elevation (above 450 meters).

This taxon was described from Sri Lanka.

Habitat
Aneuretus was found in areas of primary rainforest and dense secondary growth. However, most observations were made in secondary growth because of its greater accessibility. Both forest types are extremely humid, but only the secondary growth regions were thickly carpeted with leaf litter during the period of study. The predominant tree genera in the A. simoni habitat are Bombax, Dipterocarpus, CaUophyUum, Myristica, Artocarpus, Syzygium, Doona, and Mesua (Jayasuriya and Traniello 1986). Wilson et. al (1956) noted colonies were most abundant at the edge of clearings.

Biology
The following information is based on and includes excerpts from Jayasuriya and Traniello (1986) and Wilson et. al (1956).

Foraging/Diet
In the field (as in the laboratory), Aneuretus appears to be a scavenger, and workers feed on nectar and prey upon small insects. The sting is used to paralyze prey. Thus, the feeding habits of Aneuretus are unspecialized. Carbohydrate intake is in the form of nectar from fallen fruits and flowers. Moreover, several Aneuretus nests contained coccids and it is possible that Aneuretus feeds on the sap extruded by these insects. Leaf litter samples contained abundant small insects (entomobryoid collembolans, and cerambycid and carabid larvze). Many of these litter organisms are likely to be potential prey of Aneuretus, and constitute the protein component of its diet.

Larvae feed directly upon prey delivered to them by workers. Adult trophallaxis was observed in the lab.

Workers primarily forage in and on the leaf litter, favor shaded areas, and have been observed in foliage up to 1 1/3 m above ground. Odor trails are used to recruit nestmates to food. Workers appear to forage preferentially at low temperatures, and colonies are active in the morning and late afternoon, ceasing foraging at noon.

In foraging, workers move at about an average gait for ants of comparable size; very roughly speaking, they are slower than most dolichoderines, such as Technomyrmex albipes, but faster than most ponerines, such as Ponera coarctata. Like many of the higher dolichoderines, they are much given to halting abruptly at intervals, standing motionless for a moment, and then abruptly starting off again, often in another direction. When they are disturbed away from the nest, this behavior is intensified, with the workers dashing for a short distance, then halting and remaining' perfectly still for a while, then dashing off again; under these conditions they show themselves capable of considerable speed and agility.

Colony Attributes
Aneuretus colonies can contain multiple queens and have approximately 65 minor workers. Major workers comprise only two to three percent of the colony population. Sexuals are found in nests from mid-July to August.

Colonies appear to be polydomous. In the field, a mature colony is usually divided into two or three neighboring groups. The chambers, which comprise the subunits of a nest, are not necessarily connected by galleries, but worker traffic on the surface of the ground is often seen between subnests, and can involve the transfer of brood and queens. In the laboratory, approximately half of one colony moved into a second nest tube, even though adequate space for the entire colony was available in any single tube.

Nesting Biology
Aneuretus nests in pieces of rotting wood on the ground, in fallen branches from twig size up to 75mm in diameter. Many nests were located in the hollow core (about 0.5 cm in diameter) of small twigs, a few were found under the bark of rotting logs, and one was under a layer a leaf litter.

Reproduction
Alates were found in nests collected after mid-July. July and August are the dryest months in the Gilimale region, and they are followed by the monsoon in October. Thus, assuming that alates are released before October, we speculate they are produced during the dry season under conditions favorable for dispersal, but at a time shortly preceding the monsoon, to insure sufficient moisture during colony foundation.

Behavior
A. simoni workers typically avoid contact with workers of sympatric ant genera such as Pheidole, Paratrechina, Tetramorium, Odontomachus, and Monomorium. When alien conspecifics are encountered, aggression results, indicating that nestmate recognition and perhaps intraspecific territoriality is well developed.

Nestmate recognition is very evident in A. simoni, and introduction of conspecific into a nest resulted in marked aggression. The otherwise timid workers engaged in combat for hours, biting and stinging each other. Thus, Aneuretus workers appear to show territorial behavior (at least in nest defense) only in intraspecific interactions, but usually abscond in most other situations.

When a nest is first broken open, the minor workers swarm out aggressively and run over any alien object offered them, but they do not attempt to bite or sting. Major workers seem to be more timid, and do not venture out in this fashion. The aggressiveness of the minor workers is usually short-lived, and they soon retreat back into the undisturbed part of the nest, leaving unprotected any brood which may happen to be in the exposed chambers. Later, they venture out singly to retrieve the brood.

An ethology study (Traniello and Jayasuriya 1985) found: "Observed repertory sizes were 5 acts (the queen), 14 acts (major workers), 28 acts (callow minor workers) and 31 acts (mature minor workers). These repertory sizes are comparable to those observed in other ant species. Major workers, which average less than two in number per colony, do not show brood care. Majors were also never observed to participate in colony defense. The behavioral repertory of callow minor workers includes queen-related acts, brood care, and foraging. A comparison of social organization in A. simoni and other so-called primitive and advanced species indicates that social behavior is very similar to that of dolichoderine species."

Chemistry
Trail and alarm communication in Aneuretus simoni are mediated by the secretions of the sternal and pygidial glands, respectively. The sternal gland is composed of a glandular epithelium and an associated reservoir located in the 7th sternum. This gland produces a relatively long-lived mass recruitment pheromone. The pygidial gland opens between the 6th and 7th tergites and produces a secretion that releases aggressive alarm. The evolution of glandular physiology in the Aneuretinae and Dolichoderinae are discussed in view of these findings.

Conservation Status
This species is restricted to a small area in Sri Lanka. Available information suggests this ant is under threat due to habitat loss.

Castes
The single extant species of the genus has dimorphic workers (see illustration in next section).

Wilson et al (1956): "the major worker differs from the minor primarily in its proportionately larger and broader head, and in its relatively shorter, stouter propodeal spines. In the queen, it will be noted both of these allometric trends are carried a bit further. The head is wider with respect to its length than in the worker castes, and the propodeal spines are reduced to mere angles."

Description
From Wilson et al. (1956):

Worker
The following description is based on the minor worker. Eyes small, containing only about 30 ommatidia. Antennae 12-segmented, with the segments of the flagellum increasing distally both in length and width. Clypus broad and flat, lacking a median carina, the anterior border strongly emarginate in fullface view. Mandible as shown in Plate 2B. Mandibular dentition reminiscent of Dolichoderus (s. str.), relatively constant except in the following two charaeters: (1) the shapes of all the individual teeth vary somewhat, and (2) the number of the reduced median teeth (located between the three apical and three major basal teeth) varies from four to right. Maxillary palp 3-segmented, the terminal segment showing a variabile median constriction or local cuticular thickening, which may well indicate a phylogenetically recent fusion site. Labial palp 4 - segmented.

(Plate 1. A, Wings of queen. B, Wings of male. C, G, Queen. D, Male. E, H. Worker major. F, I, Worker minor.)

Alitrunk as figured. Propodeal spines spike-like, tapering abruptly near the tips to form sharp points; seen from above, they diverge from one another strongly. Metasternal gland bulla prominent, its openings supplied with a small number of coarse guard hairs originating from both the dorsal and ventral lips. Anterior peduncle of petiole long and slender, somewhat narrowed toward its anterior attachment. The petiolar node well differentiated from the anterior peduncle by dorsal and lateral swellings. Seen from above, the node proper is nearly twice as broad as long. Posteriorly, it narrows abruptly to the point of attachment to the gaster.

The gaster lacks any form of intersegmental constriction whatsoever, The sting is well developed, sclerotized, and exsertile.

Integument relatively thin and collapsible, resembling in this respect that of the higher dolichoderines and formicines. Color of living and fresh alcoholic material varies within most nest series from light yellow (in callows) to medium yellowish orange. Body lightly shagreened overall, subopaque to feebly shining; the dorsal and most of the lateral surfaces of the propodeum are in addition transversely rugulose.

Pilosity generally sparse. Appendages covered with abundant short, oblique pubesence but lacking longer hairs except on the coxae and tarsal claws. Body supplied with scattered, relatively long, pointed erect hairs distributed as shown in Plate 1; body pubeseence appressed and sparse.

Queen
Queen. (Plate 1.) The queen is much larger in size than either of the worker subcastes and differs further in its proportionately broader head and greatly reduced propodeal spines. In addition, it is much darker, being overall dark brown. It is essentially similar to the workers in petiolar structure, mandibular dentition, pilosity, sculpturing, etc.

The wing venation (of both the queen and the male) is relatively generalized and similar to that of the most generalized true dolichoderine, Dolichoderus s. str., as typified by [Dolichoderus attelaboides] (Fabr.). The latter species, as pointed out by Brown and Nutting (1950) in their important review of formicid wing venation, possesses all of the venational elements of the primitive ponerine pattern except for the first radial cross-vein. Aneuretus simoni shows two significant further advances. First, Mf2 is completely contracted, so that the ends of Rs2 and the cross-vein m-cu are contiguous. Second, Rs4 has contracted to bring about the alignment of the cross-veins 2r and r-m.

Male
Plates 1 and 2 show almost all of the important features of external morphology of this castre Especially noteworthy are the conservation of the" myrmecioid" petiolar structure shown also by the worker and queen, the relatively generalized structure of the mandible, and the generalized structure of the genitalia, especially the volsella. One perhaps significant advanced feature is the absence of serration on the ventral edge of the penis valve.

Larva
Immature larva. (Text-fig. 3.) Body length (through spiracles) 1.1-2.1 mm. Straight length 0.8-1.6 mm. Somewhat sigmoidal in sideview; thorax forming a stout neck which is bent ventrally at 90 degrees; abdomen rather stout, diameter greatest at abdominal somite IV; abdominal somites IX and X small and directed posterodorsally, the tenth forming a terminal naked knob. Anus posterior. Leg and wing rudiments present. Spiracles small, decreasing in diameter posteriorly. No spinules seen on the integument. Body hairs short, moderately numerous, uniformly distributed, of three types: (l) 0.027-0.045 mm. long, slightly curved or flexous, with a few minute denticles, without alveolus and articular membrane, the most numerous type; (2) O.045-0.0n mm. long, longest and most numerous anteriorly, slightly curved or flexuous, with a few minute denticles, with alveolus and articular membrane; (3) minute (about 0.001 mm. long), few, widely scattered. Head moderately large, feebly cordate; cranium transversely subelliptical, with the occipital border impressed and the corners rounded. Mouth parts large. Antennae small, each with three sensilla, each of which bears a moderately long spinule. Head hairs moderately numerous, uniformly distributed, slightly curved or flexuous, long (0.027-0.072 mm.), with numerous minute denticles on the distal 2/3. Labrum short and broad (breadth 2.8 times the length), bilobed (due to a rather deep impression of the ventral border; anterior surface of each lobe with one minute hair and three sensilla; ventral border with two sensilla near the middle and with a few minute spinules; posterior surface of each lobe with one large and three minute sensilla; posterior surface spinulose, the spinules minute and in numerous short arcuate rows, the rows radiating from each dorsolateral corner. Mandibles moderately large, heavily sclerotized and subtriangular in anterior view, longer than broad; slightly curved posteriorly; apex forming a rather long tooth which is slightly curved medially, two rather large subapical teeth on the medial border; anterior surface with numerous short longitudinal ridges (rows of exceedingly minute spinules); posterior surface with several acute denticles on the distal half and numerous ridges on the basal half, some of the ridges bearing a row of minute spinules. Maxillae lobose, with the apex a small conoid directed medially, with minute spinu1es in short rows which are arranged in longer rows encircling the galea; palp a frustum bearing four apical (two encapsulatrd and two with a spinule each) and one lateral (with a spinule) sensilla; galea a tall peg with two apical sensilla. Labium hemispheroidal and protruding; with all surfaces spinulose; the spinules more numerous and in more numerous rows on the anterior surface, and larger, fewer, and in short rows or isolated on the posterior surface; palp a slight elevation with five sensilla, three encapsulated and two bearing a spinule each. Opening of sericteries wide and salient. Hypopharynx spinulose, the spinnles so long that those in adjacent rows overlap; the rows numerous and radiating from each dorsolateral angle.

The larva of Aneuretus shows only one dolichoderine character --the short, broad labrum -- and even this is not distinctive. The one and only character that affiliates it with the Dolichoderinae is, strangely enough, one which is not characteristic of the subfamily at all, namely the terminal knob projecting posteriorly or posterodorsally. Now, a terminal knob occurs in the larvae of several genera in other subfamilies, but there it projects posteroventrally, ventrally, or anteroventrally. Only in the most advanced dolichoderine genera Engramma, Tapinoma and Technomyrmex have we previously found it projecting posteriorly or posterodorsally. (See G. C. and .J. Wheeler, 1951.)

In marked contrast are the many non-dolichoderine characters of Aneuretus larvae. These are given in the accompanying table. It will be seen that all the non-dolichoderine characters of Ane1tJ'etus except 3 and 4 are more generalized than their dolichoderine counterparts. 'fhus we find Aneuretus standing out conspicuously as a generalized ant larva when considered in reference to the Dolichoderinae.