The role of antennae in the thermopreference and biting response of haematophagous bugs.

Insects sense thermal cues mainly through thermoreceptors located in the antenna. To analyse the impact of antennectomy on the thermal behaviour of the haematophagous bug Triatomainfestans, we studied the distribution of intact and antennectomised bugs in an experimental arena where a temperature gradient was established, as well as the biting response of insects with and without antennae to objects at the temperature of a potential host. Antennectomy did not abolish thermopreference, but modified the temperature at which the insects preferred to stay. In the arena, antennectomised insects chose to remain at a higher temperature (ca. 3°C higher in unfed bugs), and exhibited a larger dispersion around that preferred temperature, than intact bugs. In addition, ablated insects temporarily lost their ability to bite an object at the temperature of a potential host, but that ability was gradually recovered after the fifth day post-antennectomy. Results presented here show that thermoreceptors other than those located on the antennae can also guide thermal behaviours. We conclude that the function of antennal thermoreceptors can be taken over by other receptors located in different regions of the body. Those receptors have a different sensitivity and confer the insects with a different responsiveness.

The morphology and fine structure of the giant interneurons of the wood cricket Nemobius sylvestris.

The structural and ultrastructural characteristics of giant interneurons in the terminal abdominal ganglion of the cricket Nemobius sylvestris were investigated by means of cobalt and fluorescent dye backfilling and transmission electron microscopy. The projections of the 8 eight pairs of the biggest ascending interneurons (giant interneurons) are described in detail. The somata of all interneurons analyzed are located contralateral to their axons, which project to the posterior region of the terminal ganglion and arborise in the cercal glomerulus. Neuron 7-1a is an exception, because its arborisation is restricted to the anterior region of the ganglion. The fine structure of giant interneurons shows typical features of highly active cells. We observed striking indentations in the perineural layer, enabling the somata of the giant interneurons to be very close to the haemolymph. The cercal glomerulus exhibits a high diversity of synaptic contacts (i.e. axo-dendritic, axo-axonic, dendro-axonic, and dendro-dendritic), as well as areas of tight junctions. Electrical synapses seem to be present, as well as mixed synapses. The anatomical organization of the giant interneurons is finally discussed in terms of functional implications and on a comparative basis.

Orientation response of haematophagous bugs to CO2: the effect of the temporal structure of the stimulus.

Carbon dioxide is generally recognized as an important cue used by haematophagous insects to locate a food source. When the mammalian hosts of these insects breathe, they normally emanate considerable amounts of CO2 at discrete intervals, i.e. with each exhalation. In this work, we analysed the effect of temporally pulsing CO2 on the host-seeking behaviour of Triatoma infestans. We investigated the ability of T. infestans to follow continuous and intermittent air pulses of 0.25, 0.5 and 1 Hz that included different concentrations of CO2. We found that insects were attracted to pulsed airstreams of 0.25 and 0.5 Hz transporting 400 ppm of CO2 above the ambient levels and to continuous streams added with the same amount of CO2. On the other hand, insects walked away from streams pulsed at rates of 1 Hz regardless of the amount of CO2 they bear. The walking trajectories displayed by bugs to attractive CO2-pulsed streams were as rectilinear and accurate as those to CO2-continuous streams. Our results are discussed in the frame of the interaction between olfactory and mechanoreceptive inputs as affecting the behavioural response of bugs.

Repeated plasticization and recovery of cuticular stiffness in the blood-sucking bug Triatoma infestans in the feeding context.

Triatominae bugs experience changes in the mechanical properties of their cuticle prior to feeding. This process-plasticization-allows a rapid stretching of the unsclerotized abdominal cuticle of triatominae larvae and it is evoked by sensory inputs related to feeding. We tested: (a) whether the cuticle recovers its original mechanical properties after plasticization, (b) whether repeated stimulation would be able to evoke recurrent plasticization along the same larval instar, (c) the temporal course of recovering cuticular stiffness. We injected Ringer solution into the body cavity of the bugs at constant pressure, using the injection rate (ml/min) as a measure of the cuticle extensibility. To trigger plasticization, individuals were allowed to feed on blood from an artificial feeder at 32+/-2 degrees C. After plasticization occurred, the abdominal cuticle gradually recovered its original mechanical properties. Bugs were capable of plasticizing for a second time when repeatedly stimulated. The effects of plasticization vanished between 1 and 2 h after stimulation. Although one full meal could suffice to accomplish moult in other Triatomine species, Triatoma infestans is able to feed repeatedly during a single larval instar. Accordingly to this, their cuticle recovers stiffness in some hours and becomes able to respond repeatedly to sensory inputs associated with feeding.

Temperature preference in Rhodnius prolixus, effects and possible consequences.

The present work examines the thermal preference of adult Rhodnius prolixus along a temperature gradient. The mean preferred temperature differed slightly between sexes: 25.0 degrees C for males versus 25.4 degrees C for females. This preference was not constant, but varied daily by about 0.2 degrees C for both sexes, and reached its highest value at the onset of the dark phase and was lowest during the light phase. A change in the preferred temperature with the level of starvation was also observed (about 1 degrees C lower after 20 days of starvation). Changes in environmental temperature strongly affected the rate of weight loss for both sexes. When insects were maintained for 20 days in a chamber at 32 degrees C, they lost significantly more weight than when kept at 24 degrees C; both water loss and nutrient conversion processes are involved. This increase in weight loss rate with increasing temperature would cause a higher biting rate and consequently higher probability of Chagas' disease transmission. Females oviposit across a range of temperatures from 22 to 33 degrees C with a peak at 25-26 degrees C. These results are compared with patterns of thermopreference in other species of triatomine, as related to differences in their distribution and tolerance to starvation.

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 fine structure of the ocelli of Triatoma infestans (Hemiptera: Reduviidae).

The morphology and fine structure of the ocelli of Triatoma infestans have been analyzed by means of light and electron microscopy. The two dorsal ocelli of this species are located behind the compound eyes, looking dorsally and frontally. Externally, the ocelli are marked by the corneal lenses virtually spherical in form and limited internally by a cuticular apodeme. The lens focuses the incoming rays beyond the retina. A single layer of corneagen cells lies below the cuticular lens. The corneagen cells and photoreceptors are arranged in a cup-like fashion beneath the cuticular lens. A distal retinal zone comprises the rhabdoms, which are laterally connected in an hexagonal meshwork. A middle retinal zone comprises the receptor cell segment free of rhabdom, and a proximal zone their axons. In the middle zone, the oviform nuclei and spheroids are located. Screening pigment granules are present within the retinal cell. Spherical mitochondria are homogeneously distributed in the cytoplasm of the cell body. In the axonal zone, mitochondria are found in the peripheral region. Axons from receptor cells extend into the ocellar neuropile at the base of the ocelli, to synapse with second order neurons. The large axons of second order neurons are bundled by glial cells. The ocellar plexus exhibits a high diversity of synaptic unions (i.e. axo-dendritic, axo-axonic, dendro-axonic, and dendro-dendritic).

Dynamics of thermopreference in the Chagas disease vector Panstrongylus megistus (Hemiptera: Reduviidae).

The thermopreference of resting P. megistus (Burmeister, 1834) adults was studied in a temperature gradient. We also determined the thermopreference for oviposition and ecdysis. Thermopreference of resting individuals depended on the feeding state of the insects. Results demonstrated an initial resting preference of approximately 290 degrees C, which gradually changed to 26-27 degrees C with increasing starvation. A daily variation in thermopreference was also observed with this species. The insects showed a daily maximum preference at the beginning of the dark phase, and a minimum preference during the first half of the light phase. Ecdysis took place all along the gradient, but exhibited a maximum frequency at 30 degrees C. Most eggs were found between 25 and 29 degrees C. Results are discussed in relation to the ecology of P. megistus as compared with other related species, and the existence of mechanisms of behavioral thermoregulation in the group.

Aggregation behaviour in Panstrongylus megistus and Triatoma infestans: inter and intraspecific responses.

We tested the aggregation response to inter and intraspecific chemical signals in Panstrongylus megistus and Triatoma infestans. As previously described for T. infestans, larvae of P. megistus significantly aggregated on papers impregnated with their own excrement and on papers marked with cuticular substances deposited on surfaces on which these insects had walked. T. infestans bugs also aggregated on papers impregnated by faeces or by cuticular substances of P. megistus, and P. megistus aggregated on papers contaminated by faeces or by cuticular substances of T. infestans. The response of P. megistus to its cuticular substances was significantly stronger than that to its faeces. The non-specificity of the two signals is discussed in the context of the ecological relationship between both species.

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.

[Microclimatic properties of the Triatoma brasiliensis habitat]. / Aspectos microclimáticos del hábitat de Triatoma brasiliensis.

Vector-borne transmission of Chagas disease in Northeast Brazil is basically by Triatoma brasiliensis. It is thus crucial to determine this species' microclimatic preferences as limiting factors for its distribution and ability to infest domestic environments. We analyze the microclimatic properties of the shelters in which these insects are found in wild, domestic, and peridomiciliary environments in the State of Ceará, at Brazil. We measure temperature and relative humidity (RH) every 15 minutes for 3 days. Thermal variation was greatly dampened inside both domiciliary refuges and the more protected internal places in wild stony sites. For RH, we observed a similar dampening pattern, but mean RH was lower in both domiciliary refuges and wild ones inside stony sites as compared to reference levels in the surrounding environment. The results are discussed with regard to this species' microclimatic preferences in the laboratory and its potential as determinants of its geographical distribution.

The postembryonic development of the ocellar system of Triatoma infestans Klug (Heteroptera: Reduviidae).

Simple eyes or ocelli coexist with compound eyes in many adult insects. The change in the morphology of the ocelli along the five larval instars of Triatoma infestans was studied by light and scanning electron microscopy. Our analysis showed that the development of the ocelli of these bugs occurs gradually along the larval life. The photoreceptor layer is present from the second-instar onwards. The cornea appears first at the imaginal stage and grows up to the 18-20th day after the last ecdysis, associated to an increase in the retinal mass. Findings are discussed in a comparative fashion and in relation to the functionality of the ocellar system in T. infestans.

Temporal change of the aggregation response in Triatoma infestans.

Adults and larvae of Triatoma infestans spend daylight hours assembled in shaded places. An assembling factor has been demonstrated in the excrement of this species. We analysed different aspects of the dynamics of the response of bugs. Recently fed insects do not aggregate around faeces. They start to show a significant assembling response from the 8th hour after feeding onwards. Just deposited faeces do not evoke assembling, but a significant rejection instead. This reaction switches 3 h after deposition, when the faeces become attractive to the insects. The attractiveness of faeces persists for about 10 days and can be recovered after this time by rehydration. These findings are discussed in relation to the biological role of faeces and the dynamics of the use of refuges by T. infestans.

An ocellar "pupil" that does not change with light intensity, but with the insect age in Triatoma infestans.

The simple eyes (ocelli) of recently emerged adult Triatoma infestans exhibit a narrow elongated "pupil", surrounded by a ring of brown-reddish pigment, the "iris". This pupil does not respond to changes in the illumination, but varies in size after the imaginal ecdysis. This change corresponds, internally, with the growth of the corneal lens and the associated retina up to an age of about 20 days. This has not been previously observed in an insect. The use of this characteristic for recognising young adults of this species is suggested.

Performance of yeast-baited traps with Triatoma sordida, Triatoma brasiliensis, Triatoma pseudomaculata, and Panstrongylus megistus in laboratory assays.

The effectiveness of a trap for triatomines baited with yeast cultures has been previously demonstrated for Triatoma infestans in laboratory assays. We report here results from laboratory assays testing yeast traps for Triatoma sordida, Triatoma brasiliensis, Triatoma pseudomaculata, and Panstrongylus megistus. All assays were conducted in an open experimental arena 100 cm x 100 cm, with two traps placed at opposite sides of the arena. One of the traps contained a yeast culture, and the other trap contained a saccharose solution as a control. Two series of experiments were done, one without a refuge for the insects and one with a refuge. The results obtained clearly demonstrated that the yeast-baited traps were effective in the laboratory in capturing both T. sordida and P. megistus. For T. sordida, yeast-baited traps captured significantly more bugs than did the control traps (t test P value = 0.03). For P. megistus, when a refuge was provided during the assay, yeast-baited traps also captured significantly more bugs than did the control traps (t test P value = 0.006). In the experiments with T. brasiliensis and T. pseudomaculata, both traps captured some insects, but the yeast traps captured many fewer bugs than was true with the T. sordida and P. megistus bugs. These results indicate that, in the laboratory, yeast traps can capture considerable numbers of T. sordida and P. megistus in one night. We discuss the potential use of yeast traps for detecting and capturing both triatomine species.

The central projection of cephalic mechanosensory axons in the haematophagous bug Triatoma infestans.

The projections of mechanosensory hairs located on the dorsal and lateral head of the adult haematophagous bug Triatoma infestans were analyzed by means of cobalt filling. Axons run into the anterior and posterior tegumentary nerve and project through the brain to the ventral nerve cord. The fibres are small in diameter and run as a fascicle. Some branches run into suboesophageal and prothoracic centres; others run as far as to the mesothoracic ganglion. These sensory projections resemble that of wind-sensitive head hairs of the locust. The functional role of this sensory system in this species is discussed.

Interaction of visual and olfactory cues in the aggregation behaviour of the haematophagous bug Triatoma infestans.

We analysed how the assembling behaviour of Triatoma infestans is modulated by the convergence of chemical cues released by their faeces and the spectral quality of the light associated with refuges. Second-instar larvae were confronted with refuges associated with a visual stimulus (either blue, green or red lights having the same intensity, or darkness) and a chemical cue (presence or absence of faeces). In this context, faeces constitute a major attractant for bugs. In the absence of faeces, bugs always assembled in dark places. Green light was always rejected despite the presence of faeces, i.e. the assembling behaviour was controlled by a photonegative reaction to this light. In the presence of red light, orientation towards the chemical cue dominated over the bugs' photonegative reaction to this light. Such a light was avoided in the absence of faeces but not in their presence. Also, negative phototaxis to blue light could be counteracted by the presence of the chemical cue. Thus, a concrete interaction between visual and olfactory cues occurred in these experiments: the bugs' response changed depending on the specific combination of spectral light and faeces. Finally, bugs responded differentially to blue, red and green lights of the same intensity. They assembled preferentially on red, followed by blue and then by green. Thus, discrimination between lights of different spectral quality is possible, probably through an achromatic mechanism.

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.

Yeast culture volatiles as attractants for Rhodnius prolixus: electroantennogram responses and captures in yeast-baited traps.

Responses to air currents carrying volatiles from yeast cultures were measured by means of electroantennograms (EAGs) in the haematophagous bug Rhodnius prolixus and we tested yeast-baited traps in the laboratory. The volatiles liberated by yeast cultures generated a clear electrophysiological response, much higher than that obtained during stimulation with clean air. The addition of yeast cultures to the traps dramatically increased overnight capture. A modification performed on the traps was effective for this species, as the bugs were immobilised once they fell into the device. Our results demonstrate that yeast culture volatiles are attractive for R. prolixus. The results obtained by means of electroantennogram techniques represent the first electrophysiological evidence of the perception of yeast volatiles by the antennae of a triatomine bug. We discuss the relevance of our results in view of currently used sampling techniques and control strategies.

Triatoma infestans can be captured under natural climatic conditions using yeast-baited traps.

The effectiveness of a trap for triatomines baited with yeast cultures has been previously demonstrated in laboratory assays. We report results from assays testing yeast-traps for Triatoma infestans performed under natural climatic conditions. All assays were conducted at experimental chicken-coops colonised by bugs, situated at an endemic area for Chagas disease in Argentina. Two different models of traps were tested, for use either on the floor, or attached to walls. The results obtained clearly demonstrate that yeast-baited traps are effective for capturing triatomine bugs. Floor traps, when baited, captured significantly more bugs than their controls (t-test, P = 0.0008, k = 10). In addition, wall traps were also more effective, when compared to their controls (t-test, P = 0.011, k = 10). The maximum capture by a single yeast-trap was 39 bugs in one night, while for the same period the maximum capture by a control-trap was two bugs. Traps captured significantly more bugs at nights when maximum temperature was above 18 degrees C (t-test, P = 0.0002). Results indicate that yeast traps are capable of capturing significant numbers of free insects in one night. The potential use of yeast- traps for the detection of triatomine bugs is discussed.