Claudio R. Lazzari

The spatial pattern of defaecation in Triatoma infestans and the role of faeces as a chemical mark of the refuge

Abstract Triatominae bugs aggregate around their faeces which carry an assembly signal. Here the biological role of the signal is studied for Triatoma infestans . We describe a Stereotypic behaviour of T. infestans where the insects exit from shelters in order to defaecate. A higher concentration of faecal depositions are found at accesses to refuges. In choice experiments where faeces were presented in association with artificial refuges to ascertain whether this affected election of shelters by the bugs, shelters with faeces, either inside or outside, were significantly preferred by bugs. We conclude that faeces help in the selection of refuges by these insects and the role of excrement as a chemical signal for a shelter under use is discussed.

Activity pattern in relation to refuge exploitation and feeding in Triatoma infestans (Hemiptera: Reduviidae)

The pattern of locomotion activity was studied in Triatoma infestans, in relation to the use of an artificial refuge offered to the bugs in an experimental arena. In assays performed with insects that had a post-ecdysis starvation period of 1 week, the activity was low and mainly related to refuge leaving or entering. Insects that had gone through a longer period of starvation (6 weeks) exhibited a high locomotory activity throughout the night. Specific temporal windows were detected for refuge leaving and entering after dusk and before dawn, which were not modified by different levels of starvation of the insects. T. infestans exhibits a higher motivation for feeding during the first night hours, in comparison to that observed at the end of the scotophase. The activity peaks previously reported for T. infestans are therefore related to host and refuge search.

Circadian control of photonegative sensitivity in the haematophagous bug Triatoma infestans

Abstract The negative phototactic response of Triatoma infestans (Hemiptera) was studied in an arena, half of which was kept dark and the other half illuminated with different light intensitites. For each intensity we measured the time the insects spent in the dark half, the time to reach the side opposite to that where they were released, and the number of passages through the middle line of the arena. T. infestans displayed a photonegative behaviour that was enhanced by high light intensities. Bugs maintained in 12:12 light-dark cycles responded differently to the same illumination levels when tested in their photophase and scotophase: sensitivity to light was higher during the latter. Bugs entrained to light-dark cycles, kept afterwards either in constant darkness or in constant light, and tested in their subjective night and day, showed the same responses as bugs from the light-dark group tested in their corresponding photophase and scotophase. Thus, phototactic sensitivity is under endogenous control. The behaviour shown by T. infestans may be understood as being composed of at least two different drives: an exploratory one, and a negative phototactic response that is under endogenous control and is particularly sensitive to light during the scotophase, when activity peaks occur.

Temperature and relative humidity affect the selection of shelters by Triatoma infestans, vector of Chagas disease

The preference of Triatoma infestans for shelters at different temperatures or relative humidities (RH) was tested in laboratory assays. The insects preferred refuges at a temperature of 34 degrees C rather than 26 degrees C, and chose to stay in refuges with an RH of 20% rather than 80%. Temperature and RH records made inside and outside experimental chicken-houses under natural climatic conditions showed that such habitats tend to maintain an inner climate favourable to T. infestans. Typical refuges in adobe walls showed strongly damped fluctuations in minimum and maximum temperatures, and delayed changes in these parameters when compared with an external reference. Similarly, the jarilla (Larrea sp.), a plant used for constructing roofing, damps fluctuations in minimum and maximum RH and delays these RH fluctuations when compared to an external reference. We discuss possible effects of environmental factors on the distribution of insects in human dwellings, and analyse the environmental characteristics of normally available refuges in materials used for house building in endemic zones.

The role of the antennae in Triatoma infestans: Orientation towards thermal sources

Abstract The behaviour of Triatoma infestans (Hemiptera: Reduviidae) in response to thermal stimulation was studied. We analysed the effect of a heat source on the locomotion activity, and antennal movements of tethered insects. Antennal movements were studied both in the horizontal and vertical planes, and related to whether locomotion was being displayed or not. In addition, we analysed the trajectories of intact and unilaterally antennectomised insects towards a warm cylinder in a closed-loop design. In the presence of a thermal source, triatomine bugs performed more antennal movements and walked faster than in its absence. Insects displayed a sequence of walking bouts and standing phases of variable length, concomitant with active antennal movements in both the horizontal and vertical planes. While walking, the antennae moved smoothly in co-ordination. When the insects stopped, the movements changed dramatically, displaying abrupt changes of direction. The trajectories of control animals were straight towards the thermal source. Unilateral antennectomy resulted in deviated trajectories towards the goal, with the deviation always occurring towards the side of the intact antenna. The trajectories were fairly straight up to the moment of the extension of the proboscis, at a fixed distance of around 15 mm between the tip of the head and the warm surface, when a sharp body turn occurred. Control animals probed the source at a position perpendicular to the warm surface. Antennectomised animals missed the goal and probed the air at a position tangential to the surface of the warm body. The function of the antennae during the approach to a potential source of food, and the mechanisms of orientation possibly involved, are discussed.

The role of water vapour in the orientation behaviour of the blood-sucking bug Triatoma infestans (Hemiptera, Reduviidae).

The behavioural response to water vapour of the haematophagous bug Triatoma infestans was analysed. Dry or humid discrete sources at different temperatures were used as stimuli for insects walking on a locomotion compensator. Humidity significantly increased the tendency of these bugs to orientate towards thermal sources. Furthermore, humid sources at room temperature were attractive to T. infestans, but this effect was limited to short-range distances. On the other hand, dynamic sources, i.e. airstreams carrying different water vapour contents did not affect the spontaneous anemotactic behaviour of this species, neither in sign (positive) nor in intensity. The anemotactic behaviour was also not influenced by the physiological water balance state of the bugs. Results are discussed in relation to the cues released by living hosts of triatomine bugs and in relation to their responses to air-currents.

The effect of relative humidity on the behaviour and development of Triatoma brasiliensis

Abstract The preference for relative humidity (RH) and suitability of different levels of this environmental parameter were investigated in the haematophagous bug Triatoma brasiliensis Neiva, 1911 (Hemiptera, Reduviidae). The hygropreference of T. brasiliensis was studied using a RH gradient and the effect of different RHs on the egg hatching, nymph mortality and moulting success was also analysed. The results show that egg hatching in first‐instar nymphs of T. brasiliensis was lower at extreme RHs and, particularly, it was lowest at 9.3% RH. The survival of starved nymphs was not affected by RH, but the percentage of engorged nymphs and the ecdysis success of these nymphs once fed was diminished strongly by high humidity. Fourth‐instar nymphs preferred to stay at the lowest RH during the first 5 days after feeding and during ecdysis. This preference changed markedly during starvation. Fifteen days after ecdysis, the bugs moved towards intermediate humidities, and 30 days after ecdysis they even preferred the most humid sectors of the gradient. Females preferred to lay eggs in dry environments, suggesting that they may not have a particular hygropreference for oviposition, but that they simply lay their eggs at the RHs where they prefer to stay.

Effects of relative humidity on the haematophagous bug Triatoma infestans: Hygropreference and eclosion success

Abstract The hygropreference of larvae and adults of the reduviid bug Triatoma infestans (Klug) was investigated along relative humidity (RH) gradients established by means of different salt solutions, as well as by H 2 SO 4 dilutions. Both larvae and adults preferred to remain at or near 0% RH. This preference was similar for bugs reared in the laboratory under different conditions, and for those collected in the field. Several assays to control for both experimental asymmetries along the gradient and the effects of thigmotactic behaviour permit exclusion of these factors as responsible for the behaviour of the insects. The preferred RH did not vary with the time of the day or fasting. Larval insects exhibited a significant tendency to remain close to 0% RH at the time of ecdysis. The same preference was exhibited to ovipositing females. The success of ecdysis and egg hatching were measured at different RH values ranging from 0 to 75%. No differences were found between groups; eclosion success ranged from 86 to 94% for egg-hatching and from 89.5 to 95% for ecdysis, at all RHs tested. Results agree with high saturation deficits found along the geographical areas exhibiting the highest prevalence of T. infestans . However, they are in contradiction with values usually recommended to rear this species in the laboratory. The adaptive value of the hygropreference is discussed.

The role of the ocelli in the phototactic behaviour of the haematophagous bug Triatoma infestans

In addition to compound eyes, most adult insects posses two or three simple eyes, the ocelli. The function of these photoreceptors remains elusive in most cases. Triatomine bugs posses two well-developed ocelli, located in a latero-dorsal position, behind the compound eyes. We tested the role of the ocelli in the phototactic behaviour of Triatoma infestans, by measuring the time spent by adult males in the dark half of an experimental arena, which had the other half illuminated. The occlusion of the ocelli or the compound eyes alone had little effect on the phototactic response of the bugs. Only those insects which had both their ocelli and compound eyes occluded showed a significant reduction in their negative response to light. The ability of the ocelli of T. infestans to mediate the phototactic response by themselves (i.e., not through the modulation of compound eyes sensitivity) constitutes the first report on this function in insects.

Comparison of disturbance stridulations in five species of triatominae bugs

It has been observed that Triatoma infestans and Rhodnius prolixus females stridulate to reject copulatory attempts performed by males. In addition, triatomines stridulate when disturbed or handled. In the present study, the temporal structure and frequency spectra of vibrational signals produced by mechanically disturbed T. infestans, T. sordida, T. guasayana, R. prolixus and Dipetalogaster maxima were analysed and compared. The inter-ridge distances of the prosternal stridulatory organ of the same species were also measured. The frequency spectra and repetition rates were similar, despite individuals of these five species have different sizes, their stridulatory grooves have different inter-ridge distances, and also their vibratory signals exhibited different temporal patterns. The hypothesis that disturbance stridulations are non-specific signals and could function to deter predators was discussed.