Evidence for idiothetic and allothetic control of thermo-orientation in feather-feeding lice.

Thermal cues are widely used by ectoparasites to find and exploit hosts. Recently, the wing louse Columbicola columbae (Phthiraptera: Ischnocera) was shown to use thermo-orientation when migrating between host microhabitats. Here, we study the control systems governing thermo-orientation by motion tracking wing lice on spatial and temporal heat gradients. As previously demonstrated, lice placed on spatial heat gradients successfully located nearby heat targets. Unilateral antennectomies were then used to remove spatial aspects of the thermal cue. These lice were still capable of locating heat targets, suggesting their response relied on tracking changes in the cue over time (idiothetic control). Course control was accomplished via angular corrections after louse body-angles deviated from the heat target. Louse behavior on temporal heat gradients provided additional evidence for idiothetic control- lice altered turn size and velocity after temperatures uniformly shifted without any spatial reference. We also show that lice are likely capable of responding to spatial aspects of the cue, consistent with allothetic control. On the spatial heat gradient, lice with two antennae were more efficient at locating heat targets as compared to those with unilateral antennectomies. Additionally, when traveling along temperature isoclines (where lice can detect spatial but not temporal aspects of the cue), lice with two antennae consistently turned towards the heat target, while those with unilateral antennectomies showed no preference. In all, we find evidence that lice can use both idiothetic and allothetic control during thermo-orientation, and likely integrate information from these two systems to guide movements on hosts.

Thermo-orientation and the movement of feather-feeding lice on hosts.

Temperature variation on the host is known to influence ectoparasite distributions. Ectoparasites may also use temperature gradients between host regions when moving on the host; however, tests are rare. Feather-feeding wing lice (Phthiraptera: Ischnocera) spend the majority of their time on the flight feathers of their avian hosts where they insert their bodies between feather barbs to escape host preening. However, because wing lice feed on downy abdominal feathers, they must repeatedly migrate between the flight feathers and body regions of their hosts. We performed a series of experiments that tested thermo-orientation in wing lice and evaluated its potential use during louse migrations between host regions. We found that wing lice can rapidly and accurately locate nearby heat targets that approximate host temperatures (37 C), demonstrating a capacity for directed thermo-orientation. We next tested the preference of wing lice for temperatures found along migration routes between bird flight feathers and their body regions. Wing lice could distinguish between temperatures found within distinct bird regions, and lice that had recently fed preferred the cooler temperatures (32 C), similar to those within bird flight feathers where they typically reside. However, when starved for 18-20 hr, wing lice shifted their preferences toward temperatures typical of bird body regions where they feed (36 C), demonstrating an ability to use thermal cues when moving between bird regions. We discuss the use of thermal cues during louse migration and microhabitat selection, as well as other potential impacts of thermo-orientation on host-parasite interactions.