Page 1 of 2 | © 2005-2010 text by Stephanie Shorter, Ph.D.
This is the first study to demonstrate selective predation by a spider for sources of vertebrate blood. Previous field observations revealed that Evarcha culicivora, a small-bodied jumping spider [Salticidae] found only in east Africa, routinely prey on blood-engorged female mosquitoes. But the way in which these spiders are able to identify prey that are carrying vertebrate blood from a recent meal has remained a mystery. The goal of this study was to discover the sensory cues that enable the selection of blood-filled prey while also ruling out possible confounding contributions of mosquito behavior.
Laboratory-reared salticids were maintained on a diet of blood-fed female mosquitoes and lake flies, sympatric midges that make up a large part of the diet of E. culicivora in the wild. Mosquitoes (Anopheles gambiae) were both cultured and collected from the wild. More than a dozen other species of arthropods were also collected to be used as prey during the research; none of these organisms consumed blood. Most of the experiments were conducted with dead arthropods prepared as cork-mounted lures. Preliminary data collection showed that testing with live prey yielded equivalent results. Spiders of both sexes and different ages/sizes were tested. Body size of the prey item was usually closely matched to the size of the spider except as noted below when preferred prey size was the measure of interest. Spiders were fasted a week prior to testing to raise motivation to stalk the prey presented during the testing period.
Each spider was tested only once in a choice task inVolving the simultaneous presentation of the sight or smell of two prey items. The spider's choice was defined by approaching one prey item in a testing arena where the different prey options were placed in different limbs. For example, one testing arena was a Y-shaped apparatus that disambiguated choice—moving towards one prey item took the spider farther away from the prey item not selected. Spiders made these predation choices based on visual cues alone (i.e., the lures) or by olfactory cues alone. In the olfactory experiment, a gentle controlled airflow wafted the smell of two samples of different prey arthopods down either limb of the Y-shaped arena. These prey items only served as an odor source; they were out of view of the spider.
The main result was that E. culicivora spiders chose to prey upon a blood-fed female mosquito in the majority of trials (80% or more trials across several experiments). Arthropods that were not carrying blood, including male mosquitoes, were infrequently chosen. When blood-fed and sugar-fed female mosquitoes were simultaneously available, the spiders tended to go for the blood-fed mosquito. Vision-based and odor-based choices were strikingly similar.
This preference for consuming blood was so strong that it trumped the preferred prey size. Spiders of different sizes were tested with prey of different sizes. Not surprisingly, chosen prey size scaled with spider size: small spiders tended to seek small prey and larger spiders went for larger prey. However, a reversal of preference was found in a subsequent test in which prey lures differed in both size and blood content. Smaller spiders chose larger mosquitoes that were fed blood over more appropriately sized mosquitoes that were fed on sugar. Larger spiders chose small mosquitoes that carried blood, opting for a smaller meal with blood over a larger meal without it.
In another study, E. culicivora were reared in conditions where they were never exposed to blood-fed prey. Nevertheless, these spiders selectively chose prey that carried blood when their preferences were measured later. This finding suggests that prior experience cannot account for the prey-choice behavior. Indirectly seeking vertebrate blood through selectively targeting female mosquito prey seems to be an innate predisposition of these salticids.
Previously, it had been proposed that E. culicivora predation of female mosquitoes may simply be opportunistic, that the mosquitoes likely move slower when carrying blood and so are easier to capture. Although discriminating sources of blood in potential prey may be facilitated by this sluggishness or other differences in mosquito behavior, this collection of experiments convincingly show that prey behavior is not critical and E. culicivora can identify what meal will contain blood relying only on excellent visual acuity or solely based on odors produced by the prey.
For more information: www.pnas.org