Reduviidae - Phasmatocoris labyrinthicus
Family Reduviidae Group Guild Status
Phasmatocoris labyrinthicus Troglobiont Predator Rare
Size (exclusive of front legs): male 11.8 mm; female 12.0 mm; adults about 17 mm with partially extended front legs.
This species of thread-legged bug (subfamily Emesinae) was first recorded in the cave during the initial study in 1990, when it was observed in several widely separated areas of the cave. Early in the recent study we found several dead adults of this species, but we were initially unable to locate a live animal. A concerted effort to find this species in portions of the cave where most of the dead animals were being found proved successful 9 months into the study on August 14, 2010. On that date we located three live adult animals, one (the first, a male; photo 1 at left) in the upper Tarantula Room, and two females adjacent to the Jackrabbit Shaft door (cave interior side of the door). We coaxed an adult female spider (Eidmanella pallida) from lower on the wall up to the vicinity of one of these bugs, and it captured and fed on the spider (photo 2 at left). The same day, prior to finding the first live animal, we had also found 6 shed skins (photo 3 at left) and one dead adult in the Tarantula Room near where the first live adult was found. That same day, in the Millipede Tomb Room, we found another dead animal that had died in a life-like pose, and was covered with pure white fungus (photo 4 at left). We have observed three live juveniles of this species in the Jackrabbit Room, a pure white fifth (last) instar nymph on January 3, 2011 and a probable second instar nymph on (September 24, 2011). A fifth (final) instar nymph was observed on June 11, 2011. We do not have enough data at this point to make any conclusive comments on the life cycle of this species, but based on the seasons we have observed adults and various instar juveniles, we presume that the species has an annual life cycle in the cave.
Based on our observations we suspect that this species is an opportunistic predator and is preying on any animals that it can capture, whether the prey is a cave resident occasional visitor, or an incidental. We have only the single instance of observed feeding, which was prompted by our efforts, but that occurred with no hesitation on the part of the animal. We do, however, have additional circumstantial evidence for feeding by this species in the cave. On the day the first adult animal was found in the upper Tarantula Room we found two dead insects on the floor directly below where the thread-legged bug was stationed on the cave wall. One was a small (11 mm long) spider wasp (Agenioideus biedermani), and the other was a wingless female wasp (family Mutillidae [or possibly Bradynobaenidae]). Both of these animals were intact, and may have been fed on by the bug. The method of feeding of thread-legged bugs (and all predatory hemiptera) is to suck the vital fluids from their prey. Discarded carcasses, particularly hard-bodied species, show no obvious evidence of harm since their exoskeleton retains its structure even when they have been “drained”. We have also found dead ants (Pheidole rhea) just inside the Jackrabbit Shaft door, directly below where (and when) Phasmatocoris have been observed at that location.
The construction of the Tarantula Tunnel tour access into the cave penetrated the cave in what we now recognize as the heart of the habitat for this species (as is also true for the nicoletiid (Speleonycta anachoretes). Since the animals are still present and apparently successfully breeding in the cave, disruption of their habitat from the commercial development of the cave apparently has not had a consequential long-term effect on the species.
Since we have confirmed that this species has a long-term presence in the cave, still occurring twenty years after it was originally observed, and with the additional evidence of shed skins (indicating presence of the animals as they develop), juveniles active in the cave, and feeding of the animals on at least one resident invertebrate of the cave (E. pallida), we have demonstrated that this species in not an incidental, but an active element of the ecology of the cave (Pape, 2013). We have conducted two UV lighting efforts just outside the main Entrance Sink of the cave to see if the species leaves the cave, or is present independently on the surface. Emesines (thread-legged bugs) are typically readily attracted to UV lights at night. Environmental conditions for these efforts were perfect, but none of the animals were found at the lights. The tests were conducted during the season that winged adults are active in the cave. We believe the species does not occur on the surface, is a true troglobiont at Kartchner Caverns, and is likely a relictual population.
This species of Phasmatocoris is distinctly different morphologically form any known species in the genus. It is also the northernmost and first Nearctic species in this primarily Neotropical genus. See Pape (2013) for additional information on this species.
© 2020 R.B. Pape. All rights reserved.