Krombeinictus nordenae Leclercq 1996

Krombeinictus nordenae Leclercq

This little crabronine (Figure 22), 5–6 mm long, is one of the most colorful species in the Crabroninae. The black head and thorax provide a striking contrast to the ivory to pale yellow of the mandibles, apex of clypeus, antennae, extreme base and dorsum of the pronotum, scutellum, metanotum, and legs (except for the light red hindfemora and hindtibiae); the abdomen is light red except for small transverse blotches of light brown to black at the base and apex of the first four gastral terga.

Morphologically, Krombeinictus Leclercq shares with the Oriental Vechtia Leclercq the distinction of being the only crabronine wasps with a downcurved triangular lamella (FL, Figures 23, 24) overhanging the scapal basin in both sexes. The female is unusual among crabronine females in lacking a pygidium delimited by lateral carinae on the convex last abdominal tergite. The latter has scattered setae laterally, and medially there is a narrow strip of two to three rows of close-set setae forming a brush (B, Figure 25).

Most females of K. nordenae we collected had a dried secretion on the areas adjacent to and between the medial rows of setae on the last tergite, and some females had it on the last sternite as well (Figure 26). We believe that this secretion may be brushed by the medial rows of setae onto the thickened wall of the internode entrance as small droplets. The droplets may function as a deterrent to ants or other organisms while the adult wasp is foraging and the nest is vulnerable.

The Oriental Piyumoides Leclercq, considered by Leclercq (1996) to be the genus most closely related to Krombeinictus, also lacks a pygidium. Females, however, lack the median brush of setae on the last abdominal tergum, and there is no secretion from abdominal glands on this segment in females of three of the four known species in the NMNH collection.

Krombeinictus nordenae is unusual or unique among sphecid wasps in a number of behavioral characteristics. The female manifests extraordinary maternal care, rearing one larva at a time and feeding it progressively. The species also is remarkably different from all other Sphecidae in feeding the larva clumped masses of fine-grained pollen rather than paralyzed arthropods. We found such clumped masses of pollen of Humboldtia laurifolia on the hypostomal area of a female (Figures 27, 28) and recovered grains of the same pollen from the exuviae and meconium of postdefecated larvae within cocoons of K. nordenae (Figures 31, 32). The cocoon also is unlike that of any other known crabronine (Figures 34, 35); it is discussed at length below (see “Cocoon”).

Krombeinictus nordenae is a relatively rare wasp, and only seven females and two males were obtained at Gilimale. We found only 10 nests of K. nordenae at this locality compared with about 40 of Crossocerus mukalanae Leclercq. Two females and one male were in separate intemodes that did not contain evidence of nesting; they were appparently just sheltering there. Another female, mentioned in the section on ants within the discussion of diversity in a single stem (FRR 91-S-226), apparently was about to initiate nesting. The last abdominal tergite was coated copiously with the secretion that we believe may be used as an ant guard (Figure 18), and there was a substantial amount of pollen on the setae of the hypostomal area (Figures 27, 28). A third male was recovered from an internode in the Sinharaja Reserve.

We never found more than one adult female in a nest, and in half the nests there was no female. Our impression is that the females are timid and may take flight if the nest is disturbed.

NEST.—Only ten nests were found, four of them containing a female, presumably the foundress. A fifth nest had been layered dry, rather than immersed in alcohol; a male developed and emerged from the cocoon it contained. The fifth nest also contained five adult males of the crabronine C. mukalanae. They apparently were sheltering in the intemode as there was no evidence of nesting by C. mukalanae within this intemode. A sixth nest, also layered, contained two cocoons; one occupant was a mummified postdefecated larva, and the other occupant had emerged prior to the layering. The other four nests were identifiable as Krombeinictus by the characteristic cocoon containing either a postdefecated larva or a pupa. The dried pith had been entirely removed from the inner wall of the internode in each of the nests. We presume that the female does this before laying the first egg; however, it also is possible that K. nordenae may take over an internode that has been cleaned out by ants or other occupants and later abandoned.

One nest (Figure 33) provided an insight into some aspects of probable nesting behavior. The internode was 6.3 cm long and up to 4.0 mm wide. The interior cavity was 5 cm long, with a maximum width of 3.4 mm. When this internode was split downward from the opening, we found a small wasp larva, 2 mm long, on the cavity wall just below the entrance hole, a female wasp lower in the cavity, and a cocoon holding a well-colored wasp pupa at the narrowed lower end of the cavity. A second cocoon containing a postdefecated larva was 10 mm above the upper end of the lower cocoon.

It seems probable from these data that the wasp lays her egg on the inner wall of the cavity near the entrance. When the egg hatches, she feeds the larva progressively. The relative developmental stages of the two cocoons suggest that when the first larva is full grown, she carries it lower down in the cavity to spin its cocoon. She then probably deposits another egg near the entrance. When that larva hatches, it is fed until full grown, and is then transported lower in the cavity to spin its cocoon. Then the sequence is begun again.

A second nest was in an internode 5.7 cm long, with a maximum width of 5.1 mm. The cavity was 4.7 cm long, with a maximum width of 4.6 mm. The female was not in the nest. A cocoon at the narrowed bottom end of the cavity held a pale female pupa with black eyes. A second cocoon, 23 mm above the first, contained a postdefecated larva.

Another nest contained a well-colored pupa in a cocoon 14 mm from the inner end of the cavity. An empty cocoon, from which an adult had emerged, was 15 mm above the lower cocoon. At the bottom of the cavity was a female that had moved head inward to escape the alcohol. It is unlikely that this female is the foundress. Had the foundress still been active, there should have been a nearly full-grown larva near the entrance. It is more likely that this female emerged from the empty cocoon.

Another nest was layered in the field and was examined later in Washington, D.C. It appeared that at one time it contained four cocoons, but only two cocoons remained from which adults had already emerged. There was also a small, mummified, yellow larva of K. nordenae. The other nests contained only a single cocoon with either a postdefecated larva or a pupa; the yellowish color of fresh larvae was reminiscent of that of bee larvae. A final nest is discussed below (see “Associates”).

IMMATURE STAGES.—We obtained no information on duration of the immature stages nor on the size of the egg. Three post-defecated larvae from cocoons were 5.0, 5.5, and 6.0 mm long. Four pupae were 4.5 and 5.0 mm long, two of each length.

LARVAL FOOD.—The larva is fed fine-grained Humboldtia pollen that clumps together from an oily substance, pollenkitt, secreted on the pollen grains (Figures 29, 30). The female gathers the pollen in clumps on the hypostomal setae behind the mandibles (Figures 27, 28). We found Humboldtia pollen only on the hypostomal area of one female and voided with the exuviae and meconium of final instar larvae (Figures 31, 32) within cocoons of K. nordenae. The mechanics of transfer of food to the larva is unknown. Inasmuch as the larva is positioned on the inner internode wall just below the entrance, the female could just insert her head to feed the larva. However, because of the danger of predation by birds or lizards, the female perhaps is more likely to enter the internode to transfer food.

All females were part of the short type series. We did not sacrifice any to ascertain whether pollen also might be carried in the crop.

COCOON.—The cocoon is totally unlike that spun by any other crabronine. The normal crabronine cocoon is more or less ovoid, with the posterior end tapering rather narrowly, and the cocoon is circular in cross section. In contrast, the Krombeinictus nordenae cocoon (Figures 34–36) is broadly ovoid and tapers very slightly posteriorly. The exposed upper surface is gently convex so that, in cross section, the cocoon is more curved on the side appressed against the inner wall of the cavity and rather slightly curved on the surface not in contact with the wall. Cocoon dimensions (n=9) are 6–9 mm long, 2.6–3.4 mm wide, and about 2.0–2.1 mm high. The diaphanous cocoon wall is an irregular mesh of silken strands overlying a delicate semitransparent film (Figure 36). The meconium is plastered as one or two black splotches at the posterior end of the cocoon (Figure 37).

The cocoon is somewhat flattened, so that there is a space at least 2 mm high between its upper surface and the inner wall of the cavity (Figure 35). This permits the mother to crawl over a cocoon, if necessary, to carry a larva toward the lower end of the cavity, or to allow a newly emerged adult from a cocoon lower in the cavity to reach the entrance. Postdefecated larvae and pupae in cocoons were oriented with the head facing the entrance, except that one pupa in a cocoon 3 mm below the entrance (the only cocoon so close to the entrance) was oriented with the head away from the entrance. Had this misoriented occupant eclosed, it would have had no difficulty turning around in the cavity to make an exit. The unusual location of this cocoon so close to the entrance suggests that something happened to the nesting female at about the time the larva reached maturity. Otherwise, the larva probably would have been moved lower into the cavity.

ASSOCIATES.—One nest, not discussed above, had two unexpected occupants. The female Krombeinictus nordenae was not in the internode, but her slender, immature larva, about 5 mm long, was near the entrance, and there was a pale male pupa, 4.5 mm long, in a cocoon near the inner end of the cavity. In addition, there were in the internode a male parasitic anthophorid bee, Nomada wickwari Meade-Waldo, 6.4 mm long, and a female mutillid wasp, Physetopoda fumigata (Turner), 5.0 mm long. The former was certainly a chance visitor seeking shelter. The female mutillid, however, is possibly a parasitoid of K. nordenae.

We found no evidence that larvae of the keroplatid Platyceridion edax Chandler and Matile are predators in nests of K. nordenae, although they attack C. mukalanae, the other crabronine nesting in internodes. If secretions are placed around the entrance as an ant guard by female Krombeinictus nordenae, that also might inhibit oviposition within the nest entrance by a keroplatid female.