A New Thrips-Hunting Microstigmus from Costa Rica (Hymenoptera: Sphecidae, Perphredoninae).
Psyche 77(1):120-126, 1970.
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A NEW THRIPS-HUNTING MICROSTIGMUS
FROM COSTA RICA
(HYMENOPTERA: SPHECIDAE, PEMPHREDONINAE) BY ROBERT W. MATTHEWS
Department of Entomology
University of Georgia, Athens, Ga. 30601 In the two years since the discovery (Matthews, 19683.) b) of the first social member of the Sphecidae, Microstigmus comes Krombein, which preys on Collembola I have received a number of additional preserved nests of this genus from the same Costa Rican locality. Two of these, collected by Miss Robin M. Andrews and Dr. Daniel H. Janzen, are quite distinctive and have proven to be those of a new species whose prey is Thysanoptera. 0. W. Richards has very graciously consented to describe this new species; I have added notes on the unique prey and distinctive nest structure. Microstigmus thripoctenus 0. W. Richards, new species Female. Pale testaceous; disc of metanotum, propodeurn just above petiolar attachment, distal end of petiole, two oval spots on gastral
tergite 2 and sometimes a central spot on tergite I, blackish suffused. Antenna1 segment 12 sometimes darker. Tips of mandibles piceous. Most of prothorax, tegulae, coxae, trochanters and narrow bases of femora, white. Wings hyaline-iridescent, venation very pale, ptero- stigma with a large dark distal spot. Eyes evidently green in life. Length ca. 2.5 mm.
Head between eyes nearly as broad as mesoscutum, with eyes clearly broader than distance between outer edges of tegulae. Anten- nal scape not extending quite halfway to median ocellus; segment 2 clearly longer than 3, about two and a half times as long as broad, 3 almost one and a half times as long as broad, 3-1 I very gradually decreasing in length, penultimate segments hardly more than quad- rate, 12 a little longer than 2 but considerably thicker. Head smooth and shining, area above antennae with a fine reticulation; clypeus transversely rather convex, ventral margin feebly emarginate and like the outer edge of mandibles with very long pale hairs. Orelli in a triangle somewhat narrower than equilateral. Oculo-malar space considerably wider than long. Ventral tooth of mandibles long, four times as long as the subtruncate dorsal one, but both *Manuscrzpt received by the editor December 8, 1969,
19701 Matthews - Microstigmus 121
teeth shorter than in some of the other species. Transverse keel of pronotum very strong but surface otherwise smooth. Mesoscutum shining with some feeble subcoriaceous sculpture in front half, sur- face with rather close, short stout pale bristles, recurved backwards. Scutellum strongly raised from in front and behind to a sharp transverse keel. Metanotum with a well-marked transverse keel just behind the front margin, a central longitudinal keel with a weaker keel on each side. Propodeurn with a number of strong keels, mostly transverse 01- longitudinal, defining about twelve large rectangular or polygonal areas. Mesopleuron generally smooth but with a system of irregular keels, especially one curving well in front of mid coxa, two irregularly sinuous ones leading upwards from near it, and a keel defining a prepectal area, distinctly below but only feebly above. Male. Rather smaller and paler than the female. Gaster without dark marks. Face from a little way below median ocellus, most of scape, much of side of pronotum, front part of mesopleuron, much of femora, white. Head wider, oculo-malar space longer, but not quite quadrate. Antennae very similar though with one more seg- ment, segment 3 if anything shorter. Distal part of gaster somewhat distorted but tergite 7 apparently not emarginate, with rather long bristles on each side; sternites with much longer apical bristles than female; sternite 7 triangular but with a short, straight apical truncation, a little upcurved, with numerous long bristles beneath. Holoty~e 9, 29 I
paratypes, Costa Rica, Prov. Puntarenas, Osa peninsula, 2/2 miles S.W. Rincon, do& N, 83'29' W, 16 Feb., 1969, ex nest (R. Andrews). One paratype from nest in same locality 26 Feb. 1968 (D. H. Janzen). Holotype and two para- types deposited in U. S. National Museum, Type No. 70759; other paratype in British Museum.
Discussion. There are more species of Microstigmus than is usually supposed; as a result several references to the type of the genus, M. theridii Ducke, 1907 cannot be accepted without reserve. The British Museum, however, has a female from Ducke's original series from Tefe.
gives a key to the genera of Pemphredonini in- cluding the Neotropical Microsti.g?nus. A key to the described species was given by Richards (1932) ; since then, only M. comes Krombein ( 1 967 ) has been described. The species of Aficrostigmus fall into two groups; those like the present one in which the third f ments
antenna1 segment is less than twice as long as broad and s.g
122 Psyche [March
4-1 I are hardly longer than quadrate and those more like M. theridii in which the third segment is fully twice as long as broad and the remaining segments are also clearly longer than broad. In the present group three species have been described and I know of no others. These are M. wagneri du Buysson, 1907, de- scribed from near Tijuca, State of Guanabara, Brazil; M. guianae Rohwer, 1923, (= hingstoni Richards, 1932) from Guyana (not British Columbia as stated by Rohwer) and M. myersi Turner, 1927, from Trinidad. M. wagneri has the apical portion of the clypeus considerably produced, a little upturned, and separated from the proximal part by a shallow depression.
The mesoscutum is finely
rugose. In color it is brownish-black, with head, pronotum, meso- scutum and metanotum reddish, legs testaceous. The nest is said to resemble that of M. theridii. M. guianae is testaceous with a few brown markings. The mesoscutum is clathrately sculptured. The nest is made of fibre and rotten wood with a pedicel 6 cm long. M. myersi is a brown species and the metanotum is hardly convex. The mesoscutum is smooth. The nest is said to resemble that of Ad. theridii, but to include some small mud pellets. BIOLOGICAL NOTES
Nest Construction. In the same Costa Rican rain forests1 where Microstigmus comes occurs so abundantly, the two nests of 44. thripoctenus were obtained at heights of 4 and 7 feet above ground. One was found hanging beneath a palm frond tentatively identified as being of the genus Geonoma, and the other was suspended from the 12 cm long leaf of an unidentified vine. Both nests hung firmly from the approximate center of the leaf midrib (Fig. I). Their pedicels, measuring 22 rnm (Janzen nest) and 45 mm (Andrews nest) in length, increased in thickness toward the nest proper, from 0.2 mm in diameter at the point of attachment to 0.8-1.0 mm just above the nest entrance.
At first glance, both nest bags appeared somewhat like greyish shriveled pears, but close examination revealed 5 distinct pouch-like lobes about the periphery of the lower half of each. One nest bag proper measured I I rnm long by 10 mm at greatest width (Janzen) ; the other was 12 mm long by 7 mm at its widest point (Andrews). The nest entrance, situated to one side of the pedicel attachment, was shielded in both nests by a distinctly protuberant hood-like over- hang. Such a hood is never present in M. comes nests. 'See Matthews, 1968b, for a more nearly complete habitat description.
19701 Matthews - Microstigmus 123
Nest of Microstigmus thripoctenus 0. W. Richards. (Photo by
Daniel H. Janzen.)
Relatively coarse and irregular chips of punky wood form the basic construction material for the nests. The particles, varying from grey to brown 01- black and resembling sawdust when dry, are securely bound up in a silken matrix. The entire nest interior, in- cluding the cells, is also coated with a silk-like material. Little or no unincorporated nest material remains in the bottom of the nest. In general construction, the nest of M. thripoctenus bears a re- semblance to that of M. guianae which is also made of rotted wood pellets and has a straight, non-spiraled pedicel (see photo in Howes, 1925, p. 276). The nest of another undescribed species from Ecuador is also similar in appearance.
Because the two nests were suspended from different plants, it appears that M. thripoctenus does not exhibit the host plant specificity so characteristic of M. comes. Neither, as one might suspect from this, does M. thripoctenus appear to derive its nest material from the host leaf in the manner of Ad. comes; the leaf undersurface bears no evidence of a scraped area in the vicinity of the nest attachment. And although the nests of both species are coated with a silk-like material, M. thripoctenus females have only sparsely scattered hairs on the apical tergite where M. comes exhibits its well developed setal brush.
Cell Contents. Upon dissection, each nest was found to contain six pocket-like cells, each corresponding to one of the pouch-like lobes (with the extra cell in the center). As with M. comes, the
cells were mass provisioned, and no two cells were at the same stage of development. Three cells of the Andrews nest contained pupae: one fully pigmented female, one male with cinnamon eyes, and one completely white female. All were oriented with head in the bottom, anus at the cell opening. The remaining three cells of the nest contained a prepupa, a small larva with 171 thrips, and an incompletely provisioned cell containing 58 thrips with no evi- dence of egg or larva. The Janzen nest, which was less complete, contained one newly emerged adult with freshly expanded wings and, in another cell, a pupal female. A third cell contained a shriveled egg or young larva with 70 thrips. Of the remaining cells, one contained a dozen thrips and two were empty. The Andrews nest was collected at night when all adults were presumed to be present. It yielded three females and one male. The other nest, collected during the day, contained no adults except for the newly emerged female still in her cell. In the absence of further data, the social status of M. thripoctenus must remain un-
19701 Matthews - Microstigmus 125
certain, although the presence of three females in one nest seems indicative of some degree of social cooperation, which may perhaps be similar to that of M. comes.
Prey. Use of Thysanoptera as prey has not previously been re- ported for the genus Microstigmus; those species for which prey records are known (M. comes, M. theridii) use Collembola for provisioning their cells. However, it seems possible that M. wagneri and M. guianae may also be found to utilize thrips, for they share similar morphological characteristics with M. thripoctenus. Other
Sphecidae known to prey on thrips include members of the genera Spilomena and Ammoplanus, pemphredonines which, so far as known, nest in various pre-formed cavities.
All of the over 300 thysanopterans in the Andrews nest were immature individuals, belonging to apparently 4 species (differences may be in stadium rather than taxon). The largest individual mea- sured nearly I mm long, but most were about 0.5 mm in length. With but two exceptions, the 82 thrips from the Janzen nest were also immatures, belonging to 3 or more apparent species. Kellie O'Neill of the U.S. National Museum determined the Janzen nest prey as species of Thripidae and identified the two adult females as Leucothrips sp. and Bradinothrips n. sp. In her letter, she states that the prey "belong to groups of minute, pale, solitary leaf feeders that jump readily and are difficult for humans to capture." Indeed, she added, Bradinothrips was previously known only from a unique individual collected in British Guiana.
Within the M. thripoctenus nests, the prey appeared to be piled loosely in the cell bottoms rather than packed in compact masses like the Collembola of M. comes.
Since several specimens were dis-
lodged from their cells during shipment, exact prey counts per cell were not possible. However, the I 71 individuals that remained in one cell of the Andrews nest seems an extraordinary number; by contrast, the most Collembola present in any of 22 M. comes cells examined was 58.
The new sphecid wasp, Microstigmus thripoctenus 0. W. Richards, is described and its relationships to other species of Microstigmus discussed. Its 11-12 mm long bag-like nest is sus- pended on a straight pedicel from the underside of leaves in the Cost?, Rican rain forest. Constructed of punky wood chips bound together in a silken matrix, the nest has a hood-like overhang above
the entrance and a bottom half with lobes roughly corresponding to the cells within.
The six cells are mass provisioned with Thysan- optera (Leucothrips and Bradinothrips)) a new prey record for the genus, with as many as 171 individuals found in one cell. One male
and 3 female wasps were present in one nest collected at night. LITERATURE CITED
1907. Nouveau genre de sphkgides (Hym.). Ann. Entomol. Soc. France, 76: 28-29.
DU BUYSSON, R.
1907. Description d'un sphbgide nouveau (Hym.). Ann. Entomol. Soc. France, 76: 29-30.
HOWES, P. G.
1925. Master architects of the jungle. Nature Mag., 6: 274-276. KROMBEIN, K. V.
1967. A new Collembola-hunting Microstigmus with notes on M. guia- nensis Rohwer (Hymenoptera, Sphecidae) . Entomol. News, 78 : 253-256.
MATTHEWS, R. W.
l968a. Microstigmus comes: sociality in a sphecid wasp. Science, 160: 787-788.
1968b. Nesting biology of the social wasp Microstigmus comes (Hymen- optera : Sphecidae, Pemphredoninae) . Psyche, 75 : 23-45. PATE, V. S. L.
1937. Studies in the pemphredonine wasps (Hymenoptera, Sphecidae) I. New genera and species of the ammoplanoid complex. Trans. Amer. Entomol. SOC., 63: 89-125.
RICHARDS, 0. W.
1932. A note on the genus Microstigmus Ducke, with the description of a new species. Ann. Mag. Nat. Hist., (10) 9: 372-377. ROHWER, S. A.
1923. Three new pemphredonine wasps (Hymenoptera). J. Wash. Acad. Sci., 13 : 369-371.
TURNER, R. E.
1927. A new species of Microstigmus (Hym. Sphegid.). Bull. Entomol. Res., 20: 407-408.
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