largely by fatty acid metabolism and the tricarboxylic acid cycle. Consequently, lipid metabolism is more important than carbohydrate metabolism in triatomines. Their use of fatty acids as a source of ATP production in substitution of carbohydrates has many advantages including large fuel stores and high ATP yield per molecule.
Triatomines vary in color and size, according to species. Colors vary from light yellow to black; some animals have different patterns of grey, green, orange, white, or yellow spots, principally on the connexivum. Size varies within the subfamily from 5 to 45 millimeters (about the size of an average cockroach) and is a key characteristic in distinguishing species as well as sex, with adult males being larger than females.
Triatomines make a squeaky shrill sound by rubbing their suction tube against ridges in its protective sheath under their bellies. Their front wings are leathery and their back wings are membranous, at best serving to glide several hundred meters. Although there is great variation between species in the use of wings for flight, some are capable of flying considerable distances, which is important in the colonization of new habitats and in the spreading of Chagas’ disease.
Nymphs are wingless and remain in the area where the eggs are deposited. Nymphs, as well as adults, are superb crawlers, with three pairs of highly developed legs. Because the nymphs are small, they can more easily hide, enter through bedding, and take a blood meal than the larger adults. Mosquito netting is ineffective against nymphs, which can crawl underneath the netting or arise from under the mattress.
Triatomines search out warm-blooded animals in rooms and corrals by means of temperature gradients detected by their antennae. Carbon dioxide produces increased activity of the bugs. Another factor attracting them are feces left by a previous bug. Triatoma infestans and Rhodniusprolixus habitually defecate after a blood meal, and pheromones have been found in their feces. (Pheromones are hormones whose presence communicates certain biological activity response between members of the same species; a good example is a bitch in heat attracting male dogs.) Pheromonal attraction can be greatly lessened by house and personal hygiene, thus eliminating fecal matter for the insects’ sensors.
Pheromones not only attract unfed bugs to a blood source, their aggregation toward fecal matter may also be important in the maintenance of the correct gut fauna by coprophagy (Molyneux and Ashford 1983:79). As is the case with many hematophagous insects, Triatoma infestans and Rhodnius prolixus need bacterial and fungal symbionts for digesting blood.
Instar and Life Stages of Triatomines
Using Rhodniusprolixus as an example, the sex life of female triatomines begins one to three days after the bug’s emergence into the adult stage (Molyneux and Ashford 1983:81). Males develop a bit more slowly and inseminate females five to nine days after their emergence as adults. As mentioned, wings are sprouted and used only at the adult stage, primarily for copulation and reproduction purposes. Flying necessitates large amounts of energy. The male flies on top of the female to embrace her with his legs, and then slides alongside, inserting his reproductive organ into her. Females are inseminated shortly after molting and can produce viable eggs for one year after separation from males (Perlowagora-Szumlewiecz 1969). Multiple copulation does not affect egg production. Although blood meals are necessary for viable egg production, unfed mothers may lay fertile eggs if they maintained a high nutritional status as nymphs.
Within ten to twenty days, the female deposits her eggs. In laboratory conditions, the eggs of T. infestans are clearly visibleivory colored and about half the size of a grain of rice. Laboratory-fed females lay anywhere from 80 to 150 eggs; in natural environments the crucial variable for egg production is the availability of blood meals. Egg production also varies among species, with certain species producing up to 2,000 eggs (Molyneux and Ashford 1983:80). Triatoma infestans adults live from eight to sixteen months and lay an average of 240 eggs (Zeledon and Rabinovich 1981). The life span of T. infestans in Bolivia has been estimated at three years (Jemio Alarico and Ariel Sempertegui, interview 6/18/1991).
Females lay their eggs in their microhabitat, in such places as cracks or crevices in the homes, burrows, or nests of the mammalian or avian hosts of the bugs. Some eggs are laid loose and others are glued in clumps to a surface. The eggs hatch in from one to two weeks, depending on the temperature. The emergent triatomine will pass through five nymphal instars on its way to growing wings and becoming an adult. An instar is the period between the nymph stages in the life of an insect. During the nymph stages, triatomines crawl. The nymph stages can take anywhere from four to forty-eight months, depending on the temperature, humidity, and frequency and volume of blood meals. Nymphs can imbibe between six to twelve times their own weight in blood.
Triatomines need an adequate blood meal for growth and development during each of the five immature stages and for oviposition by the adult females (Marsden 1983:266). The content of blood meals increases with each nymph stage, adult bugs drinking about 100-300 mg of blood, if possible. Among certain species, fifth-instar nymphs can consume more blood than adults, and female adults more than male adults (Molyneux and Ashford 1983:81). Instars from the third to fifth stage are frequently used in xenodiagnosis. At all stages they take many times their weight in blood, and the molt from a fifth-stage larva to an adult is particularly dependent on a large blood meal.
Although triatomines can go for months without blood meals, many take their meals after seven days, which provides sufficient time for T. cruzi to have multiplied in the bug with its six-to- fifteen-day reproduction cycle. When their hindgut becomes extended with feces, triatomines seek out blood meals, which is a distinct advantage for the parasitic T. cruzi.
Triatomines can live a long time without blood; T. infestans is able to tolerate longer periods of starvation throughout the later instars. During the fourth and fifth instars the organism can survive up to seven months without food. Younger nymphs need blood meals more frequently in order to pass through the stages to adulthood. Long periods of starvation don’t affect the persistence or viability of trypanosomes in the insect’s gut (Molyneux and Ashford 1983:80). T. cruzi have no known pathological effects on vinchucas, and to what degree this parasitic relationship negatively affects a triatomine’s metabolism is not known. Conversely, T. cruzi have adapted to the gut of vinchucas, an environment equally as hostile as that of their human hosts.
Of epidemiological significance, vinchucas are capable of being vectors for T. cruzi throughout all their nymph stages. However, they have to become initially infected from ingestion of the parasite in a blood meal from a host; they are not infected from birth. It follows that there will be lower degrees of infection among early nymph stages, because some have not taken as many blood meals. Significantly, the proportion of infected bugs rises with age, and the adult infection rate can be used as a convenient measure of transmission risk in the field (Minter 1978b; Marsden et al. 1982).
In Tarija, Bolivia, for example, vinchucas within the nymph stages had an infection rate of 25 percent, whereas within the adult stage it was 37 percent (Valencia 1990a). The infection rate of vectors, however, greatly depends on the degree of infection found in hosts. In certain regions of Bolivia, as high as 90 percent of adult vinchucas are infected, as are 50 to 60 percent of the nymphs. Thus, variables affecting the rate of bugs infected with T. cruzi are its developmental stage, access to blood meals, and the incidence of infected hosts. In a household colonized with vinchucas which has one person with Chagas’ disease, it is only a matter of time until all members will become infected.
Although the feeding activities of domestic vectors are not well known, Rabinovitch et al. (1979) have calculated that within a house with a typical insect population of Rhodnius prolixus, an adult would be bitten an average of nine times each night and in exceptional circumstances up to fifty-eight times. Bolivians can take some encouragement from the fact that Rhodniusprolixus is much more voracious than Triatoma infestans, whose populations average only two bites per adult per night. The least bothersome is Panstrongylus megistus; Marsden (1983:258) sat for two nights in a house with many P. megistus present on the walls and was approached by only a single bug. Some bugs remained on the same spot of the wall for as long as forty-eight hours.
