Shelter selection in the kissing bugs Triatoma infestans and Rhodnius prolixus: Intra- and inter-specific interactions and quality evaluation.

Triatomines display most of their activities during the night. Before sunrise, they search, select and occupy adequate shelters to stay during the photophase, avoiding exposure to diurnal predators. In this work, we first explored the interactions between individuals of the same or different species in the shelter selection process of Triatoma infestans (Klug 1834) and Rhodnius prolixus (Stål 1859). When two groups of insects (either of different nutritional status, nymphal instar or species) were released together over an experimental arena containing two identical shelters, all nymphs were distributed randomly, suggesting the absence of intra- and inters-pecific interactions. Secondly, we analysed their preferences for particular features of shelters by releasing one group of insects (either T. infestans or R. prolixus) over an arena containing two different refuges. Nymphs exhibited preferences for darker shelters with a vertical orientation of its substrate and elevated from the ground, highlighting the importance of such features in a shelter selection context. We conclude that these species disregard the presence of other individuals but evaluate certain characteristics of the shelters to choose them. This information may contribute to understanding the colonization/recolonization dynamic processes of these Chagas disease vectors.

Experience-dependent modulation of the attraction to faeces in the kissing bug Triatoma infestans.

Triatoma infestans is the main vector of the Chagas disease in Latin America. These nocturnal bugs spend most of the daylight hours aggregated with conspecifics inside crevices in roofs and walls. Around the entrances of the shelters T. infestans deposits faeces that contain chemical cues that attract conspecifics. In this work we investigated whether attraction to faeces can be modulated by experience in this insect species. First, we analyzed if the attraction of nymphs to faeces is innate or acquired through previous sensory experiences. Results show that after hatching, 1st instar nymphs are attracted to faeces even if they had never been in contact with them before, thus indicating that this attraction is innate. Second, we studied if attraction to faeces can be influenced by the presence of con-specifics. No differences were found in the attraction to faeces of nymphs released alone or in groups, suggesting that attraction to faeces is independent of the presence of other individuals. Third, we examined if the innate response to faeces of nymphs can be modulated by experience. After pre-exposing nymphs to faeces during 24h, insects were no longer attracted to faeces. Finally, by pairing the presence of faeces with an aversive mechanical disturbance, nymphs switched from attraction to avoidance of faeces. These results show that although faeces attraction has a strong innate component, it can be modulated by experience. The learning and memory capacities of triatomines have been studied only recently, and our work is the first report on the effects of experience in the aggregation context.

Antennal pathways in the central nervous system of a blood-sucking bug, Rhodnius prolixus.

The haematophagous bug Rhodnius prolixus has been a model system in insect physiology for a long time. Recently, several studies have been devoted to its sensory systems, including olfaction. However, few data are available on the basic organisation of the nervous system in this species. By means of neuronal backfills, histology, confocal microscopy and three-dimensional reconstruction methods, we have characterized the projection patterns of antennal sensory neurons within the central nervous system of this disease-vector insect. We established the first partial three-dimensional map of the antennal lobe (AL) of a hemipteran insect. The ALs of this species are relatively diffuse structures, which nevertheless show a glomerular organisation. Based on computer reconstruction of the AL, 22 glomeruli with a radius of 8-25 microm could be identified. No obvious sexual dimorphism of the glomerular architecture was observed. Antennal afferents project not only into the deutocerebrum, but also some fibres descend through the ventral nerve cord to ganglia belonging to the abdominal segments.

Flight dispersal of the Chagas disease vectors Triatoma brasiliensis and Triatoma pseudomaculata in northeastern Brazil.

The present paper reports for the first time the capture of wild Triatoma brasiliensis and Triatoma pseudomaculata by means of light traps in Brazil. We tested commercially available lighting devices powered by batteries to attract the bugs to a white piece of cloth in the field. Two main findings showed to be significant: first, the results presented here show that light traps can be used for sampling these species in wild environments; second, they reveal that house colonization by triatomines may also happen as a consequence of the arrival of flying sylvatic bugs guided by artificial light sources. In addition, we discuss the effect of some environmental and biological factors on triatomine flight activity modulation.

Take-off activity and orientation of triatomines (Heteroptera: Reduviidae) in relation to the presence of artificial lights.

We analysed the flying activity of Triatoma infestans and Rhodnius prolixus when confronted to artificial lights of different spectral quality. We found that the presence of light sources (white or ultraviolet) did not affect their spontaneous take-off rate. The comparison between species showed that R. prolixus was more prone to fly than T. infestans. Females of T. infestans initiated flight more frequently than males of the same species. Although the same tendency was observed in R. prolixus, no significant differences were assessed between sexes. Concerning the orienting behaviour of triatomines at take-off in relation to the position of the light source, T. infestans showed a significant tendency to fly towards white light, but a non-oriented response when confronted to UV light or in the absence of a light source. R. prolixus also preferred to fly towards a source of white light and exhibited a non-oriented response with no light. However, when the UV light was presented, these bugs exhibited a bimodal attraction/repellence-behaviour. Our results support true attraction by white light rather than menotaxis or arrival by chance. These findings are discussed in relation to the colonization of human dwellings by Chagas disease vectors.

Circadian rhythm of behavioural responsiveness to carbon dioxide in the blood-sucking bug Triatoma infestans (Heteroptera: Reduviidae).

The temporal modulation of the behavioural response to carbon dioxide and its chronobiological basis were investigated in larvae of Triatoma infestans. We analysed the orientation towards CO(2) of insects kept under three different illumination regimes: (1) 12 h light/12 h darkness cycles (L/D), (2) constant darkness (D/D) and (3) constant light (L/L). When maintained under L/D conditions, insects exhibited an oriented response towards airstreams added with 1500 ppm of CO(2) during the first hours of the scotophase only. Bugs maintained under D/D also showed a positive orientation response towards CO(2) during the first hours of the subjective night, while bugs kept under L/L did not show a rhythmic oriented behaviour. Thus, T. infestans displayed a daily rhythm of orientation towards CO(2) (i.e. a potential food source) only at the beginning of the scotophase. The persistence of the rhythm under constant darkness reveals the existence of an endogenous circadian control of this behaviour.

Chronobiological basis of thermopreference in the haematophagous bug Triatoma infestans.

The chronobiological basis of the daily dynamics of thermopreference was tested in adults of Triatoma infestans, continuously registering the preferred temperature of insects released over a temperature gradient for 13 days. We found that the thermopreference in T. infestans is a dynamic process that depends on the time of the day and the post-feeding time. When submitted to a 12:12 h light/darkness cycle (L/D), the preferred temperature reached the highest and the lowest values at the end of the light and dark phases, respectively. This daily rhythm persisted under constant conditions of illumination (D/D and L/L), suggesting the existence of an internal oscillator controlling this behaviour. Statistical analysis revealed that the thermopreference of insects kept under L/D exhibited a ca. 24 h periodicity, while insects kept in D/D and L/L showed free-running periods of tau((D/D))=23.35 and tau((L/L))=27.35 h, respectively. The persistence of a cyclic pattern of thermopreference under constant conditions, and free-running periods, close to, but different from 24 h, demonstrate the existence of an endogenous control of the thermopreference in this species. The biological relevance of these results is discussed in the light of the hypothesis that both the length of time elapsed since feeding and the time of the day modify thermopreference in these bugs. The gradual decline in preferred temperature following feeding may be associated with energy conservation during starvation. The rhythmic modulation of thermopreference may be associated with the daily rhythm of locomotion activity shown by these bugs.