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On this page (which is divided into two parts to speed up downloading) I will provide a brief species description and then discuss in turn:
Crematogasteor opuntiae inhabits the hot deserts of Arizona, southern Nevada, and southeastern California. Wheeler and Wheeler (1986) describe the worker coloration as ranging from head and thorax yellow, gaster dark reddish brown to head dark reddish brown, thorax brownish yellow, gaster dark reddish brown with darker infuscation. In all cases the head and gaster are shiny while the thorax is only feebly so.
Beyond Pickett and Clark's (1977) study of the possible protection this ant provides Opuntia acanthocarpa and Wheeler's (1994) examination of the ultrastructure of its spermatheca and its associated gland, there is little detailed information on this ant.
It is worth noting, though, that other ants in this genus play an important role in various ecosystems. In both African and Asian wet tropical forests Crematogaster species are frequently competitive dominants in arboreal habitats (Davidson and McKey 1993). Many also partake in symbiotic relationships with plants ranging from the use of domatia to construction of ant gardens (Davidson 1988).
The ability of these ants to behaviorally dominate other ants species stems from several unusual evolutionary developments in the genus. Among Crematogaster species the sting is no longer functional but is instead flattened in order to bear droplets of viscous liquid poison (Forel 1928). The delivery of this poison is facilitated by the attachment of the postpetiole to the dorsal surface (instead of the anterior end) of the gaster and by the shape of the gaster (flattened on top and rounded at the bottom). These two morphological features allow workers to raise their gaster in a scorpion-like way with the tip pointed forward (Wheeler and Wheeler 1986). Still, such a defensive system might seem inferior to one where a functional sting is present. Davidson and McKey (1993) point out, however, that while stinging is a very effective strategy against vertebrate predators, it is often less effective than chemical warfare when the enemy is another ant species.
The scientific name of this ant implies a close association between it and the EFN-bearing cacti in the genus Opuntia. But, at my field site, C. opuntiae showed no special preference for Opuntias over other extrafloral nectar-producing cacti. What seemed to be most important to C. opuntiae workers -- and all extrafloral nectar gathering ants in my study for that matter -- was how much extrafloral nectar a particular plant was producing.
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On the whole, C. opuntiae was the most common EFN visiting ant at my site. Most healthy barrel cacti and cholla attracted dozens of C. opuntiae workers and, in the case of very large chollas, even thousands of them. If C. opuntiae workers were absent on a plant it was either because the plant didn't produce enough nectar to attract the ants or because some other ant species was excluding them: in no case could their absence be attributed to the lack of a nearby colony.
Two species of ants restricted C. opuntiae visitation. In the late summer and fall, Solenopsis aurea workers actively displaced C. opuntiae workers from many of their plants (see aggressive interactions between Crematogaster opuntiae and Solenopsis aurea). This, and the return of C. opuntiae workers to plants after S. aurea workers abandoned them, gave rise to an annual cyclical pattern of plant visitation as shown below.
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Changes in C. opuntiae visitation of chainfruit cholla and barrel cacti in response to S. aurea visitations. For a general description of this survey see Introduction web page: Study #1. |
What the above graph doesn't show is that C. opuntiae colonies were also occasionally displaced from their plants after dark by nocturnal Camponotus ocreatus workers. Unlike the situation with S. aurea, though, C. opuntiae colonies were often able to visit contested plants during the day and were even able to fend off C. ocreatus incursions with some frequency (see aggressive interactions between Crematogaster opuntiae and Camponotus ocreatus).
On my field site, C. opuntiae workers generally collected nectar from midmorning to well after midnight with the highest visitation to EFN-bearing plants typically occurring from shortly after dark until midnight. This was the broadest daily nectar collecting schedule of any ant species on my site and it varied only slightly with season. Along with F. foetidus, C. opuntiae workers were the only ants to regularly collect nectar during the heat of the day in summer. While there were probably benefits in collecting nectar at times when other ants wouldn't or couldn't, there were clearly costs to this behavior. On June 29, 1994 temperatures soared to 46 degrees Celsius (116 degrees F). By late afternoon on that day, workers like those shown below were on chollas all over my field site: workers who had climbed out onto spines and clamped down with their mandibles before dying.
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Dead C. opuntiae |
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While extrafloral nectar might make up the majority of C. opuntiae's diet, the ants do feed on other things. In addition to getting plant juices from EFNs, the ants also gather it at times from aphids. Although it might surprise some, spring time in the desert is occasionally cool and damp enough to support local outbreaks of these small plant-sucking insects. Like many ant species in more temperate climates, C. opuntiae workers collect their excrement, a substance known as honeydew.
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C. opuntiae workers |
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C. opuntiae's diet also includes meat. In addition to some scavenging, workers regularly capture termites. How they catch these insects is interesting. On my field site termites typically came above ground at night to collect plant debris. To protect themselves from various predators, they first covered anything they planned to collect with a plaster made from soil, saliva, and perhaps some of their feces. The photograph below shows an erect plant covered by plaster; most of the time, though, termites plaster over small areas of the desert and work underneath these coverings.
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C. opuntiae workers |
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C. opuntiae workers cannot break through the plaster once it dries and hardens. What they do instead is hang around the periphery of these plaster sheds and wait for new construction to take place. When it occurs the ants move to these areas of moist plaster and try to grab the termites who are doing the construction. Most of the time they are unsuccessful, but occasionally a worker will manage to stick its head up under the plaster roof and grab a termite. This worker then waves its gaster (rear-end) up and down, releasing a pheromone that attracts other ants to it. These other ants come and help dislodge the termite from beneath its protective cover.