Stigmatomma silvestrii

Identification
Total length of workers 3.5 - 4.5 mm. Body color yellowish brown to reddish brown.

Distribution based on Regional Taxon Lists
Oriental Region: Taiwan. Palaearctic Region: China, Japan, Republic of Korea.

Biology
S. silvestrii feeds mainly on centipedes. Masuko (1993) discovered and reported the habit whereby queens wound larvae non-lethally and imbibe their haemolymph as food.

Masuko (1986) provides the following account:

Description of larval hemolymph feeding (LHF)

As established by my laboratory observations, LHF of S. silvestrii is a behavior in which adult ants pierce the dorsal integument of the upper abdomen of older larvae by pinching with their sharpened mandibular tips, and then feed upon hemolymph leaking from the puncture (Fig. 1). The larvae are most often pinched at the two intersegmental grooves between the 2nd and the 4th abdominal segments (Fig. 2). In general, queens (inseminated females) perform LHF; except when the colony is starved, it is seldom seen in workers and dealate, uninseminated females. The latter castes instead feed directly on prey, the cuticle of which has been opened either by larvae or adults. Uninseminated females, which are often encountered in field-collected mature nests, are ordinarily observed to labor in a workerlike manner. Queens start each bout of LHF by active antennal stroking and handling of a larva, and then pinch it from either side. Invariably, after several pinchings, the mandibular tip applied to the larval dorsum punctures its integument, and a droplet of hemolymph flows from the wound. The queen at once attaches her lower mouthparts to the puncture to lick the droplet. She then "freezes" motionless for several minutes, during which time the lapping movement of her lower mouthparts, still attached to the puncture, usually ceases, indicating that she is imbibing hemolymph from the larval body cavity using her pharyngeal pump. Each bout of LHF usually involves a single larva. Following LHF the queen ordinarily discharges one or two pure white infrabuccal pellets onto the nest floor, probably the filtered dregs of hemolymph. Queens at this time are often groomed at the mouthparts by workers, which remove the pellet and usually discard it outside the brood chamber.

LHF by queens of developed colonies

LHF is not only performed solely by queens, but it is a queen's only way of obtaining nutrients. Table 1 A shows the feeding time budget of three queens observed in laboratory colonies that were monogynous and mature, i.e., including more than 20 workers (the field data suggest that S. silvestrii colonies become mature, that is, start production of sexuals, when their worker population exceeds about 10). All three queens indulged in LHF to the exclusion of other modes of feeding, even though captured prey items (centipedes or mealworms) were available in the brood chamber throughout the observation period. To confirm this again and to ascertain the time intervals between bouts of LHF, two further queens of monogynous mature colonies were observed with a VTR. One, who was observed continuously for 5 days at 27±1°C under constant light (LL), repeated LHF at intervals of 5.3±0.7 h (mean±SE). The other queen was observed under both LL and DD (constant dark), each for 3 days, at 22±1°C; these periods were preceded by 3-day light and dark adaptation periods, respectively (an infrared light was used for observation in darkness). The difference between mean intervals of LHF by this queen under LL and DD was not statistically significant (ANOVA, F(1,11)=0.508), so the data were combined to yield an interval of 9.1±1.8 h (mean±SE). Also in this VTR study, it was noted that the queens obtained their nutrients exclusively by LHF despite continuous accessibility to prey. It is thus confirmed that nearby prey remains are ignored by S. silvestrii queens, while they repeat LHF at intervals of several hours.

Scarred larvae

Larvae subjected to LHF were always mature or near-mature individuals of the 5th instar, which is the final instar for both female and male larvae in this ant (the larval instars of S. silvestrii are easily determined based on the body size and the chaetotaxy). Usually several members of the 5th instar population are pinched repeatedly, which causes their scars to become increasingly evident (Fig. 2). These observations are confirmed by field collected material (Fig. 3): 149 5th-instar larvae belonging to eight winter nests were collected at Manazuru during December 1982 and April 1983. Their body size, which was measured when alive as the width of the 4th abdominal segment with an ocular micrometer accurate to 0.025mm, ranged from 0.475-0.800 mm, with a mean of 0.647 mm. Those with scars thought to have been made during the preceding active season, and characteristic of LHF, were all in the size class above 0.625 mm. Figure 3 shows that among 106 larvae of this class, only 44 (41.5%) had scars: seven (15.9%) had only "faint" scars, 19 (43.2%) had "faint" to "intermediate" scars, and 18 (40.9%) had at least one "bold" scar (for classification of scar degrees see Fig. 2).

The punctures close shortly after LHF by hemolymph coagulation and scarred larvae do not die from their wounds nor are they cannibalized, as would generally be the case with damaged larvae of higher ants. In well-fed laboratory colonies most scarred larvae were observed to continue feeding and to succeed in pupation. However, if these colonies are starved, larvae most exploited for LHF and hence the most debilitated were devoured and consumed before the same-sized but unscarred or weakly scarred larvae.

Among 169 larvae contained in cocoons collected in the field during 1981 and 1984, 125 (74.0%) had scars characteristic of LHF. These facts suggest that, even under natural conditions, the potentially destructive effect of LHF on the larval population is not profound. It is still probable, however, that LHF has some unfavorable effects on its recipients, e.g., causes delay in their development or lowers their rates of survival, especially in colonies with few larvae.

LHF by foundresses

The possibility of LHF by foundresses (colony founding queens) is of interest. Since the appearance of the first workers is crucial to subsequent colony survival, it is expected that foundresses will show a low rate of LHF compared to queens in developed colonies, in order to rear the first workers as quickly as possible. Unlike the foundresses of higher ants, which maintain their first brood on nutrients regurgitated while claustral, Amblyopone foundresses forage alone outside the nest (Wheeler 1933; Haskins and Haskins 1951; Haskins 1970).

Table 1 B shows the feeding time budget of three haplometrotic (singly colony-founding) queens, each of which was collected with her immatures and later observed in the laboratory. Foundress 82-299 showed both LHF and prey feeding, but she spent much more time at the latter. Only prey feeding was observed in the case of foundress 84-40. Prey availability was experimentally lowered during observation of foundress 81-96. In her case only prey feeding and cannibalism of the pupae were recorded during the studied 10-h period. Outside this period, however, she was observed to perform LHF on some particular larvae, which were later cannibalized. Thus, as predicted, their feeding characteristics differed greatly from those of queens in mature colonies, when prey was available.

Nomenclature

 *  silvestrii. Stigmatomma silvestrii Wheeler, W.M. 1928d: 97 (w.) JAPAN. Ogata, 1987: 103 (m.); Terayama, 1989b: 345 (q.); Masuko, 1990b: 221 (l.). Combination in Amblyopone: Brown, 1960a: 169 (see also p. 197); in Stigmatomma: Yoshimura & Fisher, 2012: 19. See also: Masuko, 1993: 35.