The morphology of Colpocephalum pectinatum (Phthiraptera: Amblycera: Menoponidae) under scanning electron microscopy

Here, we describe under scanning electron microscopy (SEM) the morphology of Colpocephalum pectinatum (Phthiraptera, Menoponidae), an ectoparasite found in burrowing owls, Athene cunicularia. We devote particular attention to the morphology of the main structures of the head (antennae and mouth-parts) and legs (tarsi and femoral ctenidia). Moreover, we describe the main peripheral sensory organs, located in the labial palpi and the distal end of antennae. We also detected that the structure of antennae and antennal sensilla arrangement are very similar to that described for other Colpocephalum and Menoponid species, and we discuss the function of each type of sensilla. We suggest that SEM studies combined with other microscopy and physiological techniques could be useful for elucidate the function of each structure, lice behaviour, as well as their taxonomy.

Sensory features of the human louse antenna: new contributions and comparisons between ecotypes

Body and head lice are known to be exclusive ectoparasites of human hosts. Current genomic and transcriptomic data suggest that both louse types represent ecotypes of the same species. They exhibit morphological and physiological differences that probably arose from living in different biotopes. Sensory traits represent suitable candidates to be affected by environmental heterogeneity. Therefore, through scanning electron microscope studies, this study analysed and compared the sensory structures of both ecotypes' antennae. Seven types of sensory structures were identified in both ecotypes: bristles, tuft organs, pore organs, single pore and three morphotypes of sensilla basiconica. Notably, the single pore and the morphotypes of sensilla basiconica were described for the first time in the body louse antenna. This study's comparative analysis mainly revealed size differences across the sensory structures of the ecotypes. Bristles of the flagellomere 2 of the body louse antenna were longer than the head louse bristles. In addition, the pore organs of the head louse antenna presented a higher diameter than those of the body louse. The possible relevance of size differences regarding the biotopes exploited by the body louse and the head louse is discussed. Yet, physiological studies may help to fully understand the phenotypical differences of both ecotypes.

Mouthpart sensory structures of the human head louse Pediculus humanus capitis

Head lice are exclusive human parasitic blood-sucking insects. Distributed worldwide among school-age children, this parasitosis generates scalp irritation and sometimes social prejudice. Understanding how head lice detect and perceive their human hosts is crucial to control transmission. Here, we describe the sensory structures present on the mouthparts of Pediculus humanus capitis and their possible contribution to the feeding decision-making process. On the anterior zone of the clypeus around the haustellum two morphological types of sensilla, invariable in location and number, were identified: fourteen short clypeus bristles (SCB) and six long clypeus bristles (LCB). During feeding these structures contact the host skin but not its blood. Located antero-dorsally on the everted haustellum and between the epipharyngeal teeth, a third sensillar type was identified: about four short peg epipharyngeal (SPE) sensilla. These structures are bathed with the incoming blood, when head lice feed, so may have a gustatory role. In behavioural experiments antennectomy of lice did not interfere with feeding behaviour, suggesting that the sensory structures on the mouthparts could be involved in host assessment.

The Sensory Machinery of the Head Louse Pediculus humanus capitis: From the Antennae to the Brain

Insect antennae are sophisticated sensory organs, usually covered with sensory structures responsible for the detection of relevant signals of different modalities coming from the environment. Despite the relevance of the head louse Pediculus humanus capitis as a human parasite, the role of its antennal sensory system in the highly dependent relation established with their hosts has been barely studied. In this work, we present a functional description of the antennae of these hematophagous insects by applying different approaches, including scanning electron microscopy (SEM), anterograde antennal fluorescent backfills, and behavioral experiments with intact or differentially antennectomized lice. Results constitute a first approach to identify and describe the head louse antennal sensilla and to determine the role of the antenna in host recognition. SEM images allowed us to identify a total of 35-40 sensilla belonging to seven different morphological types that according to their external architecture are candidates to bear mechano-, thermo-, hygro-, or chemo-receptor functions. The anterograde backfills revealed a direct neural pathway to the ipsilateral antennal lobe, which includes 8-10 glomerular-like diffuse structures. In the two-choice behavioral experiments, intact lice chose scalp chemicals and warm surfaces (i.e., 32°C) and avoided wet substrates. Behavioral preferences disappeared after ablation of the different flagellomeres of their antenna, allowing us to discuss about the location and function of the different identified sensilla. This is the first study that integrates morphological and behavioral aspects of the sensory machinery of head lice involved in host perception.

The tuft organs of the human body louse, Pediculus humanus corporis--cryofixation study of a thermo-/hygrosensitive sensillum

The tuft organs of the human body louse, Pediculus humanus corporis, display a complex cuticular apparatus without wall pores, consisting of the cuticular cone, cuticular pocket, and the sensillar peg with 5-6 tuft processes. The cuticular wall of the sensillar peg exhibits several layers of differing electron density tightly surrounding the dendrites. The sensillar peg is innervated by the unbranched dendrites of two receptor cells which do not reach into the tuft processes. A third receptor cell forms a lamellar outer dendritic segment which wraps around the two other dendrites and terminates before reaching the base of the sensory peg. This type of receptor cell is named stratoterminal while for those which end within the cuticular apparatus the term conoterminal is proposed. Five auxiliary cells are associated with each tuft organ. The pore organs display a porous cuticular plate innervated by the branching dendrites of four receptor cells. A fifth receptor cell terminates more proximal and resembles the stratoterminal cell of the tuft organ. From the fine structure it is evident that the tuft organs represent thermo/hygrosensitive sensilla whereas the pore organs may have an olfactory function.