Rober W. Matthews.
Nesting Boilogy of the Social Wasp Microstingmus comes Hymenoptera, Sphecidae, Pemphredoninae).
Psyche 75:23-45, 1968.

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NESTING BIOLOGY OF
THE SOCIAL WASP MICROSTIGMUS COMES
(HYMENOPTERA: SPHECIDAE, PEMPHREDONINAE) Museum of Comparative Zoology
Sociality in the Hymenoptera has been achieved independently at least ten times (Wilson, 1966). Of these, the majority have oc- curred in the Apoidea, which in addition exhibit most of the inter- mediate stages in the evolution of sodality. While no truly social Sphecidae have been previously recorded, presocial sphecids parallel- ing both types of social evolution in bees have been reported (Evans, 1~64 1966), and undoubtedly more examples will be found as fur- ther studies are made. This paper describes the nesting biology of a unique pemphredonine wasp, Microstigmus comes Krombein (Fig. I), from Costa Rica and presents evidence indicating that it is to be regarded as the first case of well-developed social behavior in the family Sphecidae.
Except for brief accounts by Myers (1934) and Howes (1925, 1933); little is known of the biology of Microstigmus. Myers found eleven M. theridii nests suspended from the undersides of Coccoloba pubescens leaves in the forests of Trinidad; each was constructed of reddish fibers taken from the underside of the leaves, loosely bound by strands of a silken material, and suspended from the leaf by a slender coiled pedicel. Nests contained one to eight cells, and the prey was Collembola; some nests were parasitized by pteromalid wasps. Although Myers noted the presence of more than one female in some nests, this fact has apparently been overlooked. Howes gives a very general account of the nests and biology of M. guianensis from British Guiana including photographs of nests, which he thought to be made of lichen and moss fragments. Significantly, Howes re- cords only one adult wasp per nest, though he does not state how many nests he observed. However judging from his nest photographs his observations may have involved more than one species. During the first two weeks of March, 1967, I studied Micro- stigmus comes on the Osa Peninsula of Costa Rica. This wasp constructs white bag-like nests suspended from the under surface of fan-shaped fronds of the palm Crysophila guagara Allen, a com- Manuscript received by the editor April 4, 1968 23
Pswhe 75:2J-45 (1%). hup Ytpsychc tnlclub orgi75175-021 html



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24 Psyche [March
mon plant in primary forests of that region. Over 40 nests were collected during the course of this study; 27 of these were collected at night, when all adults were presumed to be inside. Up to 18
cells and 10 females were found per nest, with many nests containing more than one female.
THE STUDY AREA
The study plot, approximately 44 meters square, was in a flat tract of undisturbed Tropical Rain Forest (terminology of Leopold, 1959) south of Agua Buena Creek behind the Tropical Science Cen- ter Building, 3.5 miles southwest of Rinch, Osa Peninsula, Punta- renas Province, Costa Rica. The canopy, ranging from 35 to 60 meters in height, was relatively closed. Understory plants were not particularly dense; it was possible to walk through the forest with relative ease.
During the period of study (March 5-1 I, 1967), relative humid- ity in the forest was above 80% both night and day and rain fell briefly nearly every day. The air temperature one meter above ground on a representative day ranged from 2zå¡ at 0600 to 27.5OC at I 200.
Fig. 1. Female of Microstigmus comes Krombein.



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Eighty-eight Crysophila guaguru Allen plants were found in the study area. Figure 2 is a map of their distribution and the distribu- tion of wasp nests in the study
area. These plants ranged from
seedlings to mature trees approximately 40 feet tall, but nearly half of the plants in the plot were 5 feet or less in height. The number of fronds per plant ranged from 4 on the smallest seedlings to a maximum of 22 on some of the mature trees. Seventy-four wasp nests from 38 plants were counted in the plot; this was probably less than the total number of nests present, as it was difficult to see nests in the tallest trees. Nests were most easily seen after dark, when they stood out plainly in a flashlight beam against the white undersides of the leaves. Figure 3 shows the distribution of nests as related to plant size class. Trees of 20 feet or less in height were the preferred nest sites, with the greatest number of nests occurring on trees between 6 and 10 feet tall. Sixteen plants had one nest only; 14 had 2 nests; 7
had 3 nests; one had 4 nests and one had 5 nests. This appears to be a high density for a primary forest predaceous insect, but may be a result of the ease of finding nests. By contrast, 800 sweeps with a net in similar vegetation in an adjacent area yielded only one adult of this wasp. While there appeared to be plenty of suitable nest sites available, Figs. 2 and 3 indicate certain sites were pre- ferred. This fact may be significant in the evolution of social behavior in this species since a clumped nest distribution is likely to increase the probability of nest-mates being relatives (see Hamilton, 1964) 4
The nest (Fig. 4) is suspended from near the midrib of the leaf, usually about half the distance to the tip, and approximately at the point where the leaf curvature is greatest. Occasionally there were two active nests on a single leaflet, and once three were found. The broad leaf protects the nest from the rain that would almost certainly destroy an exposed nest. Nests were always on the younger fronds. Older fronds having a large amount of epiphilous algal growth had no active nests although they often showed evidence of having once had them. According to D. H. Janzen (pers. comm.), leaves begin to acquire epiphilous growth at about 5 months of age, suggesting that the life of a wasp nest is less than six months. Indeed, T. C. Emmel
(in litt., 22 Aug. 1967) states that he could find no nests in the same locality during August 1967, despite extensive searching. NEST STRUCTURE AND CONTENTS
Nests are constructed entirely from the waxy bloom coating the



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26 Psyche [March
under surfaces of the Crysophila leaves, and were exclusively confined to these plants.
This material is scraped from a roughly oval area approximately 10 cm in diameter, the nest being attached at the approximate center of the scraped area (Fig. 5). While no ex- traneous foreign materials were incorporated in any M. comes nest, nests of M. myersi are reported to have earth pellets incorporated in the walls (Myers, 1934) and nests of M. hingstoni (= guianen- sis) are said to have pellets of rotted wood included in the nest sac (Richards, 1932). By contrast, Howes (1925, 1933) states that nests of the latter species in British Guiana are made of moss and lichen fragments.
The nest pedicel is loosely spiralled, usually with two coils, and when outstretched measures an average of I 5 mm (range 12-18 mm). The single entrance, located at the nest apex beside the attachment of the pedicel, opens to one side (Fig. 4) ; in one case there was a double entrance opening to both sides. The nest sac is ovate or obconical; it averaged I 7.4 mm long (range I 2-26 mm) and 12. I mm (range 8-15 mm) at its widest diameter. The outer surface varied in texture from relatively smooth to granulose. Only loosely bound together, the flocculent bottom half of the nest probably serves as a reservoir of material used for cell construction. In basic structure, M. theridii nests (Myers, 1934) are similar, measuring 12 mm long with a coiled ~edicel of the same length. Photographs of sup- posed M. guianensis nests given by Howes (1933, repeated in Clark, 1937) show a very short, uncoiled pedicel. However, unless ~edicel length is highly variable in the species, these nests are probably in- correctly ascribed. Rather, an earlier photograph of a nest with a very long, apparently uncoiled pedicel (Howes, 1925, p. 276) prob- ably represents the true Af. guianensis nest, for this agrees closely with the nest of M. hingstoni, recently placed in synonymy under JI. guianensis (Krombein, 1967), having a pedicel measuring 60 mm (Richards, I 932).
The upper half of the nest is hollow and covered internally with a smooth translucent coating, giving rigidity to the sac. The same
material is found on the ~edicel and lip of the entrance, and on the internal surfaces of the cells. The coating of the cells with a transparent substance seems to be unique in the S~hecidae but parallel cases are known in some bees (e.g. Colletidae). It is prob- able that the pedicel may be entirely secreted, for Myers (1934) noted that the reddish Af. theridii nest sacs were held together by whitish silk-like strands which became thicker toward the nest apex



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19681 Matthews - Alicrostigmus
NUMBER OF NESTS
PER PLANT
0 NONE
ONE
TWO
b THREE
FOUR /
Fig. 2.
Distribution .of Crysophila palms and M. comes nests in the study plot near Rincon, Costa Rica. Each symbol represents a single Crysophila plant.
and "wholly predominated in the pedicel which looked whitish in consequence". However, lack of differential coloration in the secre- tion and plant fibers obscured such a distinction in Ad. comes. The source of this translucent material is unknown; however, wasps frequently worked up and down the pedicel and about the entrance, stroking and tamping these areas with thrusts of the abdomen (see Nest Maintenance and Defense, below). Examina- tion of the abdomen tip reveals a dense brush-like cluster of hollow,



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0 ALL PLANTS
PLANTS WITH ONE
OR MORE NESTS
Height Size Class (in feet)
Fig. 3. Histogram of height size distribution of Crysophila guagara anc distribution of M. comes nests in the study plot. blunt-tipped setae at the end of the apical tergite (Fig. 6). Associ- ated with this setal brush is a rather large gland, the cells of whici" each have a well developed cuticular vesicular organelle and tubule (Fig. 7) similar to that described for the defensive gland of Eleode: longicollis by Eisner ct al.
(1964). Several tubules fuse and emptj
into the hollow bases of each of the brush setae. This distinctive



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glandular apparatus is very prominent in tergites treated with 10% KOH and viewed under high magnification. While the function of the gland is not yet known, it may prove to be the source of secreted material applied by the setal brush.
Within the nest are the pocket-like cells, situated in the lower half of the sac 8 to 11 mm below the entrance. Initially they are arranged about the periphery of the nest, with the central area gradually filled as the number of cells increases. Each cell, approxi- mately cylindrical in shape, is about 4 mm long and 1.8-2.1 mm wide at the mouth. Diagrams representing cell distribution and con- tents of a large and small nest are given in Figs. 8 and 9. A total of 139 cells (mean, 3.6 cells per nest) were found in 39 dissected nests; the largest number of cells in a single nest was 18. All but 7 nests contained 4 or fewer cells; six nests contained no cells, but four of these also lacked adults, indicating that they may have been abandoned. Ten nests contained but one cell. The cell contents from night-collected nests are summarized in Table. I. Typically a four-celled nest would contain one pupa, one mature larva, one fully provisioned cell with egg or newly hatched larva, and one partially provisioned cell. Indeed, no two cells of the same nest were ever found to be in the same state of develop- ment, a fact also noted for M. theridii (Myers, 1934), suggesting that only one cell is constructed and provisioned at a time. Even in a 13-celled nest that contained 10 adult females, only 2 cells had eggs (each at a distinctly different stage of development) and a third cell was partially provisioned.
Cells of Af. comes nests were mass provisioned with Collembola of the families Entomobryidae and Sminthus-idae (Table 2). Use of Collembola as prey is, so far as known, unique to Aficrostigmus with- in the Sphecidae; however, they are an abundant and probably little exploited food source in the tropical rain forest.* The Collembola, averaging about I mm in length, were virtually all sub-adult; in completed cells with eggs, the number of Collembola per cell aver- aged 46 (range 31-58; N = 22). The prey are packed together into a compact, more 01- less spherical mass, but can be easily sep- arated in alcohol. Myers ( I 934) recorded predominantly Entomo- bryidae as prey of M. theridii on Trinidad, one cell containing about *Since this paper was written, I have received an unusual nest and adult of an undescribed species of Microstigmus from the same locality, in which the prey is thrips.
The single provisioned cell contained 70 thrips, mostly immature instars.




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19681 Matthews - Microstigmus 31
30 individuals of varying ages; he noted that the prey were frequent- ly dismembered, presumably by the wasps, but no such dismember- ment was noted for M. comes prey.
Completed food masses (Fig. 10) averaged 1.2 mm in diameter and were firmly stuck to one side of the cell near its midpoint. In every case these food masses were of mixed species composition: 6 species were found in I cell, E) species in 10 cells, 4 species in 8 cells, 3 species in 3 cells, and 2 species in 2 cells. Two of the six species, Lepidocyrtm sp. X and Paronella sp., comprised 70% of the prey sample (Table 2).
The hunting behavior of M. comes was not observed. However, R. J. Snider (pers. comm.) states that, insofar as known, the species used as prey are all epigean and sun-loving; furthermore, all are brightly and distinctly colored and (except Deuterosminthurus) possess scales which reflect in sunlight. These facts suggest that M. comes hunts primarily among low vegetation, probably in sun flecks, and relies upon visual cues in its search for prey. The sausage-shaped egg measures 1.4 mm long and 0.4 mm wide at the middle; it is draped over the food mass and attached by one end, the other end projecting free (Fig. 10). The egg is not laid until provisioning is complete;
normally but one egg was found
per nest ( 10 of 15 nests with eggs), but four nests had two eggs and one exceptional nest had 3 cells with eggs. Freshly provisioned cells are weakly closed with a rather haphazard crisscrossing fabric consisting of a few secreted strands. However, cells containing mature larvae, prepupae and pupae are open. Pre- pupae and pupae are always found oriented with the head in the bottom of the cell, the anus attached at the cell opening. As in most other known members of the Pemphredoninae, no cocoon is spun. The distinctive larva of M. comes is described elsewhere (Evans and Matthews, I 968).
Nothing is known regarding the relative lengths of the develop- mental stages of M. comes. Attempts to rear eggs and larvae in gelatin capsules failed, probably due to dessication. However, sev- eral adults subsequently emerged from pupae in nests kept in covered petri dishes. Howes (1933) reports that eggs of Af. guianensis hatch in two days and the larvae feed for one week, then pupate and emerge as adults two weeks later.
EXPLANATION OF PLATE 2
Fig. 4.
Microstigmus comes nest attached to underside of a Crysophila frond. Note nest entrance and doubly coiled pedicel. (Photo by C. W. Rettenmeyer.)




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Psyche
[March
ADULT BEHAVIOR
Provisioni/z~. -The earliest any wasps flew from the nest in the morning was just before 0800 hours, the times of first departures on three consecutive days being 0806, 0751 and 0824. Altogether
15 trips for prey were timed; these required from 8 to 140 minutes, the mean being nearly 37 minutes. At this rate, it would require a single female about three days to collect the 46 Collembola needed to provision the average cell. In one nest observed continuously, the two females made a total of 13 trips for prey over a period of 7 hours 28 minutes.
^Parasite egg on mature M. comes larva in one case. Table 1. Summary of contents of 22 active Microstigmus comes nests, collected at night and preserved immediately. When the females returned to the nest they invariably alighted on the side of the nest, then quickly crawled up and inside. This behavior made it possible to determine whether prey was being car- ried, and to ascertain that it was held in the mandibles. Generally the female spent less than a minute at the nest between prey hunting trips. Usually she was in and out again quickly, having spent less than 5 seconds inside. This was followed by walking slowly over the outside of the nest for up to 60 seconds before departure, usually visibly grooming the mouthparts during much of this time. The ob- served wasps never departed directly from the nest entrance. Often the females made brief "orientation" flights before disappearing; these consisted of two or three circular flights about 5 cm beneath the nest.
Nest Afaintenance and Defense. -All nests observed were found to have at least one adult present at all times, and wasps were active on the outside of the nest throughout the day. This extra-nest activity was classified as "inspection" trips or "maintenance" trips. Usually, "inspection" trips were brief, with the wasp crawling once around the nest quickly from top to bottom or in a spiral pattern
and then reentering the nest. These trips were made at irregular intervals throughout the day and seldom did five minutes elapse without a wasp crawling on the nest surface at least once. In one representative hour, 29 "inspection" trips were made, re- Number
of Adults:
Female Male
56 19
Immatures :
Pupae Prepupae Larvae Eggs
28 5 14 17
Incom-
plete
Food
Masses
11
Total
8 1
Para-
sites
5l
Empty
----
2




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quiring from 4 to 36 seconds each (average, 8.5 seconds). During another hour, 22 trips were made, ranging from 4 to 50 seconds (average, I I seconds), Apparently both sexes in this be- havior, and on occasion two or three individuals came outside almost Fig', 5.
Underside of a Cry~ofliVa leaflet and attached nest; note oval area from which nest material has been scraped. (About natural size.)
(Photo by C. W. Rettenmeyer.)




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34 Psyche [March
simultaneously. The significance of this "inspection" behavior may be protection against predators and parasites by enchancing the likeli- hood that an intruder will be encountered before endangering the nest.
IÌö- one instance a small beetle larva was deliberately placed on the nest. Its movements quickly attracted the two wasps inside the n,est. At the first encounter the wasps attempted to grasp the in- truder in the mandibles but were repelled. There followed a period of extensive rapid crawling over the nest during which the wasps encountered the intruder several times but failed to successfully re- move it. After about 7 minutes, the larva dug into the loose material in the bottom of the nest, and about 2 minutes later the wasps re- turned to the inside of the nest and no longer seemed excited. The following day the beetle larva emerged from the nest and again created considerable excitement until it again burrowed inside the nest about 4 minutes later. When the nest was collected that after- noon, the beetle larva was found inside. On another occasion, a
female braconid wasp parasite (see following section) was observed to crawl down the pedicel and then run quickly onto the nest sur- face. In less than two seconds, a Microstigmus emerged from the nest, presumably stimulated by the braconid's movements. In circling the nest it encountered the braconid almost immediately, causing the parasite to fly.
'Maintenance" trips took three forms. One type involved carry- ing small chunks of fibrous material from the interior of the nest to the outside, where they were dropped. Typically this behavior oc- curred in spurts, several such pieces being carried out over a three or four minute period. The material to be discarded was carried in the mandibles and usually down to the bottom half of the nest. Then, while standing on the hind two pairs of legs, the wasp re- moved the material from its mouthparts with the fore tarsi and the material floated away.
Fig. 6. Dorsal view of M. comes ( ? ) apical tergite, treated with 10% KOH. Note cluster of blunt-tipped setae (sb) at apex; cuticular portions of associated gland (gl) are also visible. Central "line" (arrow) is edge of tergite, bent back upon itself.
Fig. 7. Cuticular vesicular organelles (VsO) and associated tubules (Tu) isolated from glands of apical tergite by KOH treatment. Note the
indistinct peripheral fringe swellings (PF) covering surface of each or- ganelle.




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MATTHEWS - MICROSTIGMUS COMES




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36 Psyche [March
Two other types of "maintenance" involved work on the exterior of the nest or pedicel. In one the wasp worked on the nest proper, removing numerous small pieces of nest material from the surface with the mandibles and discarding them in the manner described above. In one such case, a wasp pulled off 37 pieces of nest in less than eight minutes. The third variation of "maintenance" behavior consisted of work primarily on the pedicel or about the nest entrance. When on the pedicel the wasp repeatedly crawled up and down it in a spiraling manner, with the body axis roughly perpendicular to the nest pedicel. Here the tip of the abdomen was used extensively, often vigorously stroking or tamping the substrate, suggesting that the setal brush and associated gland may function in this activity. This latter behavior was noted primarily in the early morning, and excursions often lasted 10 or more minutes. PARASITISM
Nine cells in five nests were found to be parasitized by the braconid wasp HeterospiZus microstipmi. This wasp was first described by Richards ( 1935) from M. theridii nests on Trinidad, and my speci- mens have been compared with the type in the British Museum by G. E. J. Nixon. While no other parasites of M. comes were found in Costa Rica, Myers (1934) records a pteromalid wasp as a fre- quent parasite in M. the& nests from Trinidad but does not men- tion any braconid parasites.
The parasitized nests contained six pupae, two larvae and an egg. Adults subsequently emerged from some pupae. While the larvae and egg could not be positively identified, there seems little doubt that they were braconids. The egg was found attached to the ventral surface of a mature Microstigmus larva between the head and thorax. It measured 0.4 mm long and 0.1 mm wide at the middle, and was