COMPARATIVE ANATOMY OF THE VENTRAL REGION OF ANT LARVAE , AND ITS RELATION TO FEEDING BEHAVIOR

The morphology and systematics of the larvae of ants have been studied in great detail by George C. and Jeanette Wheeler in many articles and is summarized in Wheeler and Wheeler (1976). From t’he studies of many authors, especially the former and William M. Wheeler (1918, 1920), it is apparent that the mature larvae of many species of ants are fed solid food by the adult worker ants, which place the food on the ventral region of the larvae. Although the Wheelers describe the general morphology of ant larvae, they have not studied the ventral feeding region as a distinct unit, except in larvae where this region is the most specialized (e.g. Pseudomyrmecinae and Camponotini). Consequently, we intend in this study to examine the fine detail of the ventral region of larvae in the hope of clarifying homologies in morphological adaptations for feeding on solid food.

Moser.Five specimens examined with SEM.
Formicinae Colobopsis pylartes Wheeler.Collected from Davy Crockett National Forest, TX (July 19, 1973), and identified by A. C. F. Hung.Three specimens examined with SEM.Camponotus rasilis Wheeler.Reared from colony collected from College Station (Brazos Co.), TX (early spring, 1976), by M. R. Barlin.Identified by J. Mirenda and R. Petralia.Four specimens examined with SEM.All species were identified from adults.Collected material from which specimens were obtained is available in the insect collection of the Department of Entomology, Texas A&M University, College Station, Texas, 77843.Both adults and larvae and/or prepupae are available for all species except P. barbatus and C. laeviuscula (adults only).
P. barbatus, C. laeviuscula, M. pharaonis, S. molesta, and /. pruinosum were collected and reared according to the techniques described for Solenopsis invicta Buren (Petralia and Vinson 1978).C. rasilis was reared in a large bucket, brood being maintained in Wilson cells and petri dishes covered with a red plastic film.Atta texana was reared by Dr. John Moser (USDA Forest Service, Southern Forest Expt.Station, 2500 Shreveport Hwy., Pineville, Louisiana 71360).Larvae of N. nigrescens were fixed in 70% ethanol for 2 years.The remaining species were obtained from the insect collection of the Department of Entomology, Texas A&M University, and had been fixed in 80% ethanol for up to 6 years.Specimens from.the insect collection and N. nigrescens were subse- quently refixed in 3% glutaraldehyde-3% formalin for 3 days or more, post-fixed in osmium tetroxide, dehydrated, critical point dried, Psyche [December mounted on metal stubs with Tube-CoatTM, metal coated, and exam- ined in a JEOL JSM-35 SEM at 20-25 kV.Specimens of all other species were mounted on metal stubs with Tube-Coat TM and examined alive in a JEOL JSM-35 SEM at 15 kV.Sectioning of the trophothylax of P. g. mexicana and P. pallidus was carried out after refixation, but prior to post-fixation.This was accomplished by splitting the larva longitudinally with a hand-held razor blade.

PONERINAE, PONERINI
Leptogenys elongata--(Figs.3-6).Ventral body region bears only a few minute hairs, which are most numerous on the thorax (which attenuates to form the "neck" (Fig. 3)).Large cone-shaped tubercles laterally flank this region, although they become more medial after the 3rd abdominal segment (Figs. 3,4).Each tubercle bears 6-9 long, simple hairs approximately 1/3 of the distance from the base; this basal 3 is distinctly broader than the tapering apex.The apex bears a few blunt spinules or papillae.There are numerous rows of poste- riorly projecting spinules on the ventromedial surface of the thorax (Fig. 6) and a few rows on the 1st abdominal segment.A unique euticular structure is found on the 3rd abdominal segment (Fig. 4).It is roughly ovoid, tapering posteriorly, and consists of 9-10 transverse ridges (Fig. 5) surrounded by small papillose protuberances.
Pachycondyla villosam(Figs.7-9).The distinct neck is formed by the thorax and st abdominal segment.The ventral body region bears only a few, scattered, minute simple hairs.This region is flanked laterally by elongate, cone-shaped protuberances bearing spinules and a few hairs.These protuberances gradually approach the ventral midline as they near the anus, thus enclosing an elongate, sub-ovoidal region from the neck to the anus.Spinules in short rows are formed in a unique arrangement on the ventral region.Those on the neck form      Petralia & Vinson Anatomy of Ant Larvae 381 an almost semicircular arrangement on each segment, with anterior spinules pointing posteriorly and lateral spinules pointing medially (Fig. 7).A similar arrangement of spinules is found on the remaining abdominal segments anterior to the anus.However, on these seg- ments the concentric circle of short rows of spinules is complete, including posterior spinules which point anteriorly (Figs. 8, 9).

PONERINAE, ODONTOMACHINI
Odontomachus clarus .Neck attenuated as in the Ponerini (Fig. 10).Ventral body region bears only a few small, simple hairs.This region is flanked laterally by elongate cone-shaped protuberances (which continue onto the dorsal surface) bearing 4-6 long simple hairs near the apex (Fig. 10).Numerous rows of posteriorly pointing spinules occur medioventrally on at least the first 8 body segments (Figs.11, 12).The ventral surface of the prothorax also bears a pair of fleshy tubercles anteriorly.

PSEUDOMYRMECINAE
Pseudomyrmex pallidus (Figs. 13,14).The structure and vesti- ture of the trophothylax are similar to those in P. g. mexicanus although in the specimen examined, the trophothylax was tear-drop shaped in longitudinal section and the segments were indistinct.
Spinules on the thoracic and 1st abdominal segments point towards the posterior boundary of each segment (Fig. 15); those on the 2nd abdominal segment point towards the anterior boundary of this segment.The latter spinules cover the posterior lip of the tropho- thylax.The posterior portion of the 2nd abdominal segment, as well as the succeeding segments, bears numerous, straight, simple hairs (Fig. 16).In specimens where part of the trophothylax is extruded, the spinules on the extruded region point posteriorly (Fig. 17).The lateral lips of the trophothylax bear spinules pointing medially (Fig. 7).

1979]
Petralia & Vinson Anatomy of Ant Larvae 383 rows of posteriorly pointing spinules are on the anteroventral region of abdominal segment 2, while a very few (less than 10) anteriorly pointing spinules are on the posteroventral region of this segment (Fig. 19).Numerous short rows of posteriorly pointing anteroventral spinules and anteriorly pointing posteroventral spinules occur on each of abdominal segments 3-5 (Figs. 20, 21).Abdominal segment 6 bears only a few spinules pointing in each direction, while segment 7 bears very few spinules.Adult queen ants were observed to place solid food on the posteroventral region of larvae, which readily fed upon it.

MYRMICINAE, SOLENOPSIDINI
Monomorium pharaonis (Figs.24-26).Head close to body, thorax curved but not forming a distinct "neck" (Fig. 26).Hairs of anteroventral body region all bifid, except for those on prothorax, which bears simple, branched, or bifid hairs.Apices of hairs of ventral region of prothorax and mesothorax (and most of meta- thorax) straight, whereas most other body hairs with hooked apices (Fig. 25).Spinules of thoracic region mostly point posteriorly (Fig. 24), while those of 2nd and 3rd abdominal segments point anteriorly.
Each segment bears a single transverse row of about 8 simple hairs.The medial bare region between the most medial pair of hairs is especially wide on the thoracic segments.Remaining body hairs are bifid with hooked tips.Hairs papillose (Fig. 29).The rows of spinules on the medial region of each thoracic segment point posteriorly, while those of the 1st 3 abdominal segments point anteriorly.
Spinules very blunt and point posteriorly, those on lateral flanks of praesaepium (lst 2 abdominal segments) point postero-medially; rows of spinules on meso-and metathoracic segments very well developed and prominent (Fig. 40).

DISCUSSION
The morphology of the ventral body region can be related to feeding behavior.Thus, in larvae which are reported in the literature to be fed only by regurgitation of liquid food from the adult workers, the morphology of this region is relatively unspecialized.In this category can be placed Crematogaster laeviuscula and Iridomyrmex pruinosum, in which the head is so closely appressed to the body, that it cannot reach the ventral body region with its mouthparts.Larvae of these species bear few or no spinules in this region and the hairs and hair pattern are unspecialized.The papillose regions on C. laevius- cula may be vestiges of the rows of spinules found in other ants.Wheeler and Wheeler (1976) state: "... the immobility of a body with a dolichoderoid or crematogastroid shape precludes self-feeding.
Hence these larvae must be fed by regurgitation".
The remaining larvae described in this study are reported in the literature to be fed solid food by adult workers.Figs.36-38.Fig. 36.Lateral view of Iridomyrmex pruinosum.Head (h).Fig.  37. Anteroventral body region of L pruinosum.Note short, simple hairs (arrow).Praesaepium (arrow).

Psyche
[Decernber However, larvae of Neivamyrmex nigrescens (Dorylinae) do not show specializations for holding food.In studies based on the labora- tory culture methods of Topoff and Mirenda (1978a, b), workers placed prey near larvae, which then attached themselves to this food and appeared to suck out the body fluids (Mirenda, pers.comm.).Thus, larvae of N. nigrescens do not require a specialized ventral feeding region as food is never placed on them.Wheeler and Bailey (1920:270) do not describe how ant larvae in the Dorylinae are fed "pellets made of the flesh of insects".
The simplest arrangement of morphology for holding solid food is found in some Myrmicinae.The 2 species from Solenopsidini des- cribed above (Monomorium pharaonis, Solenopsis molesta) bear similar general patterns of vestiture on the anteroventral region, i.e.
with anterior spinules pointing posteriorly and posterior spinules pointing anteriorly, and with hairs mostly simple.Furthermore, the position of the head and curvature of the thorax give them the ability to reach the anteroventral region.Similar morphology was described for S. invicta (Petralia and Vinson 1978, 1979).The anteroventral region is least specialized in M. pharaonis and it is not known whether this region is used for holding food.Larvae of S. invicta and S. molesta feed on solid food placed on them (Petralia and Vinson 1978, Wheeler and Wheeler 1955), and in S. invicta the hairs and spinules of the anteroventral region are efficient for holding solid food while the larva feeds upon it (Petralia and Vinson 1978).The hairs surrounding the hairless medial area enclose the food particle to form a "food basket", while the medial spinules are arranged to assist attachment of the food.S. molesta appears to have a somewhat similar "food basket".
A comparable means of holding food is found on larvae in the Attini.Trachymyrmex septentrionalis bears well developed hairs only on the ventral region, with the majority being in the anteroven- tral region.These hairs appear to be aranged in a similar pattern as in Solenopsis.Thus, this may also be a "food basket" since Wheeler and   Wheeler (1974, 1976) state" "The ventral hairs of the attine larvae keep the fungal mass [food] firmly in place while the larva is feeding".Atta texana is almost bare of hairs but possesses a ventral prothoracic boss with outward pointing spinules and minute hairs.G. C. Wheeler  (1948) describes how adult workers place solid food on the mouth- parts of these larvae.We speculate that larvae may use this structure     1979]   Petralia & Vinson Anatomy of Ant Larvae 391 to brace or anchor the food while feeding, because of the close proximity of this boss to the face of the mouthparts, although this is not described in the literature.
The most efficient method for holding solid food, within the Myrmicinae, may be in larvae of Pogonomyrmex barbatus.The segments in the posteroventral feeding region (especially abdominal segments 3-5) bear spinules and strongly denticulate hairs, both pointing to the center of each segment.The attenuated thorax allows larvae enough dexterity to feed easily from this region.
This attenuation is even more pronounced in the Ponerinae.The larvae of Ponerinae described here have a long neck which allows them to reach solid food placed on the wide, flattened posteroventral body region which Wheeler and Wheeler (1976) refer to as a food "platter."W. M. Wheeler (1918) describes the feeding behavior of Odontomachus: "These larvae are placed by the ants on their broad backs, and their heads and necks are folded over onto the concave ventral surface, which serves as a table or trough on which food is placed by the workers."This "food platter" is visible in O. clarus and Pachycondyla villosa where a row of tubercles demarcates this area laterally.It is better developed in P. villosa in which a circle of spinules on each segment probably holds food more efficiently than the posteriorly pointing spinules in O. clarus.The arrangement of spinules in P. villosa may be comparable to that on the posteroventral feeding region of P. barbatus (Myrmicinae).
The feeding region on larvae of the ponerine Leptogenys elongata may be even more specialized by the development of a unique cuticu- lar process on the ventral surface of the 3rd abdominal segment.The cuticular,ridges in this structure seem to be formed from a fusion of the small, blunt papillae or spinules, as is visible in the peripheral parts of the structure.Thus, these ridges are probably homologous to the rows of spinules common on ant larvae.We speculate that this structure may form a "food tray," efficient in holding solid food while the larva feeds upon it. 3Recently, John Mirenda and the authors have made preliminary observations on larval feeding in live colonies of L. elongata.Adult workers place larvae head first into the par- 3Dr.G. C. Wheeler and J. Wheeler suggest that this structure might function as a stridulatory or food-holding structure or both (pers.comm.).

Psyche
[December tially eaten bodies of isopods, where larvae actively feed.Since Wheeler (1918) states that the larvae of North American Ponerinae feed on food placed on their ventral region, or on insect parts placed near them (observed in L. elongata; 1910), it is possible that several methods of feeding are present in the same species.However, an alternative function of the ridged structure is a holdfast.When not feeding, larvae typically curve their "necks" so as to rest the ventral surface of the prothorax or the posteroventral surface of the head against the ridged structure.Perhaps posteriorly pointing spinules hook on the ridges, allowing the larva to maintain the resting position with minimum effort.
The most advanced specializations for holding food are found in the Pseudomyrmecinae and the Camponotini.Many larvae of the Camponotini possess a well developed pocket for holding solid food called a praesaepium (Wheeler and Wheeler 1953).It is best deve- loped in Colobopsis as is evident in C. pylartes.In both Campono- tus rasilis and C. pylartes the posteriorly pointing spinules on the anteroventral region may function to force the food against the posterior wall of the praesaepium (the posterior wall in Colobopsis is the large ventral welt).This is supported by illustrations of a food pellet held in the praesaepium of C. gasseri (Forel) (Wheeler and  Wheeler 1970).
The "food pocket" is most highly developed in the Pseudomyrmecinae and is called the trophothylax (Wheeler and Bailey 1920; Wheeler and Wheeler 1956).The spinules in the trophothylax are positioned to help hold food in it, although this assistance may be unnecessary.
Specialized feeding regions are also described in larvae of other social Hymenoptera, including the transitory praesaepium of allo- dapoid bees and some vespids, and the shelf of Mischocyttarus (Vespidae: Polybiinae)(Wheeler and Wheeler 1979).We thus assume that morphological adaptations for holding food on the ventral region have evolved independently in numerous taxa within the social Hymenoptera.
In conclusion, the various morphological specializations in the feeding regions of ant larvae are analogous, although not always homologous.Thus, 3 basic types of feeding regions are evident.The first is the "food basket" of larvae of some Myrmicinae, in which the specializations are in the arrangement of homologous groups of     1979]   Petralia & Vinson Anatomy of Ant Larvae 393 hairs and spinules.The second is the posteroventral "food platter" of larvae of most Ponerinae, in which specializations are in the arrangement of homologous groups of spinules and tubercles.The 3rd is the anteroventral "food pocket" of larvae of Pseudomyrmeci- nae and Camponotini, in which specializations are in the arrangement of homologous groups of spinules and in the homologous development of the pockets.