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Ants (Hymenoptera: Formicidae) have great potential to exert influence over the morphological evolution of their obligate mutualist partners. Obligately myrmecophilic mealybugs are noted for their unusual morphology, and while this is often attributed to their relationship with ants, a quantitative assessment of this link is lacking. We address this need by evaluating morphological change among mealybugs as a function of ant association. This study considers the associates of 2 independent ant clades—AcropygaRoger, 1862 ants associated with root mealybugs from the families Xenococcidae and Rhizoecidae and herdsmen ants from the Dolichoderus cuspidatus(Smith, F., 1857) species-group associated with mealybugs from the tribe Allomyrmococcini (Pseudococcidae)—and compares them to free-living or potentially myrmecophilic species sampled from among the mealybugs and root mealybugs. We use a combination of geometric morphometric and linear datasets to evaluate characteristics of body shape, body size, leg metrics, and ostiole development. Obligate myrmecophily significantly influences both body shape and size. Myrmecophilous mealybugs are smaller than their free-living counterparts and are either pyriform or rotund in shape rather than oval. Ant-associates from Rhizoecidae also have significantly reduced anterior pairs of ostioles compared to free-living species. Ostioles are involved in defense against natural enemies and mutualist ants typically protect their partners, presumably supplanting the need for structures like ostioles among myrmecophilous species. We discuss the influence ants have on the evolution of their associates in the context of domestication and offer avenues for future exploration.

The fruitful study of associations between ants and scale insects yields insight into the mechanisms that shape these symbioses. Field collections provide the basic information linking partnered species, and as such it is critical that collection techniques from the field reflect true species-to-species partnerships in the published literature. It is equally critical that such practices limit the potential for mistaking free-living “neighbors” for symbiotic partners and publishing erroneous associations. This article describes a protocol for collecting subterranean scale insects and associated AcropygaRoger ants, which relies upon the activity of worker ants to sort and distinguish symbionts from free-living scale insects that happen to live near the colony. By collecting samples of ants and scales into nest boxes and allowing a resting period of several hours, worker ants will gather symbiotic partners into dense, protected clusters in which symbionts are actively tended. Free-living scale insects neighboring the colony can be collected from soil along with colony samples, but these free-living individuals are excluded from protective clusters and ignored by workers. Following confirmation of ant attendance, true symbiotic partners can be confidently collected, preserved, and recorded for future study. We illustrate the value of employing this collection protocol using a case study from Peru.

Let's travel far back in time to a clear, sunny morning between 15 and 20 million years ago in a tropical Caribbean forest. A gentle rain the night before has […]