Dolichoderinae, Formicidae, Hymenoptera, Insecta, Arthropoda, Animalia
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Additional images:
Queen, face view (alternate specimen) (small, large), petiole, lateral view (small, large).
Worker, petiole, lateral view (small, large), tibia (small, large).
Range
Kempf (1972) lists A. instabilis from scattered localities in Brazil, the Guianas, Peru, Colombia, Panama, Costa Rica, Honduras, Guatemala, Belize, and Mexico. I have examined material from Mexico, Honduras, Nicaragua, Costa Rica, Panama, and Bolivia.
Identification
Queen
Measurements (n=5): HLA 2.49 (2.42-2.52), HW 2.47 (2.39-2.55), SL 1.50 (1.42-1.50), CI 100 (98-101), SI 60 (58-62).
Palpal formula 6,4; ocelli large (OCW > 0.20mm); middle and hind tibia with prominent pectinate apical spur; dorsal surface of mandible smooth, very faintly microareolate, with moderately abundant small piligerous puncta, setae in puncta short, suberect, larger puncta with long setae near masticatory margin; medial and lateral clypeal lobes at about same level (medial lobe not projecting anteriorly); head quadrate, with sides somewhat convex, cordate posteriorly; petiolar node tall, strongly compressed into thin scale at apex; posteroventral petiolar lobe evenly convex from front to back, broad and flat laterally, ending before posterior margin of sternite, leaving small rim formed by posteriormost portion of sternite; scape with abundant erect setae, about as long as one half maximum width of scape; middle and hind tibia with abundant erect setae, longest of these about as long as one half maximum width of tibia (MTSC 30-35); sides of head without erect setae; posterior margin of head with abundant erect setae; pronotum with erect setae on anterior and posterior margins, absent from medial area; mesoscutum, scutellum, and propodeum with abundant erect setae; petiolar node with variable number of long setae on apex, abundant long setae on posteroventral lobe; all gastral terga with abundant erect setae; color red brown.
Worker
Measurements (n=4): HLA 1.88 (1.76-2.10), HW 1.85 (1.80-2.13), SL 1.39 (1.34-1.41), CI 101 (95-102), SI 74 (67-76).
Palpal formula 6,4; middle and hind tibia with prominent pectinate apical spur; dorsal surface of mandible strongly microareolate, dull, with moderately abundant small piligerous puncta, setae in puncta short, erect, larger puncta with long setae near masticatory margin; medial and lateral clypeal lobes at about same level; head with convex sides, strongly cordate posterior margin; mesosoma in lateral profile with sloping pronotum, mesonotum forming separate convexity, posterior mesonotum and dorsal face of propodeum together forming single shallow concavity, with no notch marking metanotal groove; scape with abundant erect setae, length of setae about one half maximum width of scape; mid and hind tibia with abundant erect setae, longest setae about one half maximum width of tibia; sides of head without erect setae; posterior margin of head with abundant erect setae; pronotum, mesonotum, and dorsal face of propodeum with abundant long erect setae; color dark brown to light orange brown, if somewhat bicolored gaster is darker then mesosoma.
Similar Species
Azteca instabilis queens are readily identified by the large size and large ocelli. Azteca gnava and A. sericeasur are similarly large but have much smaller ocelli (OCW < 0.15mm). Larger workers of A. instabilis are identified by the dull dorsal surface of the mandibles, large size, and densely setose tibiae.
Natural History
Azteca instabilis is the largest species in the genus. It occurs in wet to dry forest habitats, usually below 500m elevation.
Azteca instabilis makes its nest in the hollow trunks of trees which have a large crevice or fissure at the base. When I encounter them, workers are issuing from the fissure in large numbers, usually ascending the tree and often blanketing the surrounding forest floor. In one case I was able to peer up into such a fissure, and I observed a very large carton nest filling the trunk cavity. Colonies can be long-lived. During extended field work at Sirena in Corcovado National Park, I frequently passed a large tree with a very active A. instabilis colony in the trunk, with workers flowing in and out of a large fissure at the base. I returned to the site 16 years later to find the same fissure with A. instabilis workers still active.
When examining Azteca queens in museum collections, A. instabilis is usually the most abundant species because the large queens frequently come to lights at night. This is in sharp contrast to most other Azteca, whose alate queens are usually encountered as diurnal strays, occasional specimens in Malaise traps, or parts of nest collections. This correlates with the fact that A. instabilis queen ocelli are absolutely and relatively far larger than ocelli on any other Azteca queen I have examined. The enlarged ocelli are not the result of an allometric relationship with head size, since A. sericea and A. sericeasur, with queens nearly as large, have tiny ocelli typical of smaller queens. Ocelli concentrate light and detect light of low intensity (Chapman 1982), suggesting a functional relationship between large ocelli and nocturnal habits.
Workers frequently forage on the surface, both day and night. They visit extrafloral nectaries, and may tend coccoid Hemiptera under small carton shelters.
Page author:
John T. Longino, The Evergreen State College, Olympia WA 98505 USA. longinoj@evergreen.edu
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