In the heat of the night: an assassin bug's sensory journey

Natural History, July, 2005 by Graciela Flores

In rural areas of Latin America, adobe houses with thatched roofs are fixtures of the landscape. The houses are crisscrossed by crevices that serve as hideouts for insects. In the evenings, as the sun goes down, masses of intertwined heads, legs, and antennae start to disentangle. One by one, the insects come out and start their nightly search for blood. They are called kissing bugs or assassin bugs in English, but in Latin America they have many regional names. In my home country of Argentina, we call them vinchucas, a word derived from Quechua (a family of languages dating back to the Incan Empire), which means "those who let themselves fall."

These insects are bloodsuckers, and when searching for a blood meal they honor their Quechuan name: because they are such bad fliers, they let themselves drop from walls and ceilings. Once on the floor, or while crawling down the walls, the insects skillfully navigate the intricate micro-landscape of odors, shadows, and temperatures they encounter. When a vinchuca finds a host--a dog, say, or a sleeping human--it swiftly locates a blood vessel. Next, it extends its proboscis, a beak neatly folded under its head, and pierces the skin of its victim. As it feeds, the bug injects a cocktail of analgesics, anticoagulants, and vasodilators that help the blood flow smoothly and painlessly.

Slowly, the vinchuca grows, as Charles Darwin described with disgust after being attacked by one in Argentina, from "flat as a wafer to a globular form," as it becomes "bloated with blood." While still feeding, the insect releases urine and feces on the skin of the host, making room in its abdomen for the enormous amount of incoming blood. [See photographs on page 34.]

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The vinchuca's excretions can harbor a protozoan known as Trypanosoma cruzi--the cause of Chagas' disease. The T. cruzi parasites come, in turn, from another animal, and get sucked up when the insect feeds on that infected host. Thus the vinchuca acts as a carrier, or vector, for T. cruzi. When a healthy bite victim scratches at a vinchuca wound, any T. cruzi parasites in the droppings can slide inside the victim's open skin, and eventually move through the bloodstream to colonize heart muscle and other body tissues.

Symptoms of Chagas' disease vary dramatically, particularly in the early stages. Some people never develop side effects, whereas others experience fever, rashes, and fatigue a few weeks after being bitten. About a quarter of the infected population suffer from serious heart or digestive disorders, which only develop decades after the initial infection. Of the some 20 million people in Latin America now infected with T. cruzi, about 50,000 die every year. Those figures make Chagas' disease one of the most widespread and economically devastating tropical diseases in the world. And for poor countries, in which substandard housing is widespread, the total economic loss due to Chagas' disease is staggering.

Vinchucas are relatively unknown in big cities, simply because urban housing does not offer them appropriate refuges. As a city dweller myself, I did not encounter a vinchuca until I became a graduate student in the Laboratory of Insect Physiology, at the University of Buenos Aires. There I was part of a team trying to answer some of the many outstanding questions about vinchucas. As my colleagues and I immersed ourselves in the study of the insect and pooled our results, we were able to reconstruct the insect's sensory trip from its refuge to its host.

In common usage, the term "vinchuca" can refer to many species. We concentrated on one, Triatoma infestans, which thrives in the arid north of Argentina and lives almost exclusively in human dwellings. When we started our research, the behavior of T. infestans was not well understood. The director of our laboratory, Claudio R. Lazzari, had surveyed the species' behaviors during his own graduate work, with the insect physiologist Josue A. Nunez, also of the University of Buenos Aires. From Lazzari, Nunez, and a few other insect physiologists, we knew that T. infestans could home in on an animal's blood vessel even in a pitch-dark room. But we did not know how it accomplished the task. Odors, chemicals, and heat all clearly played a role, but we were clueless about exactly how the insects exploited that information. So we divvied up the various aspects of the vinchucas' biology and behavior among the members of our team. I focused on the thermal sense--to me, the most fascinating sense of all.

By day, the vinchuca remains in a sleeplike state known as akinesis. At dusk, however, the insect is activated by an internal biological clock, which regulates most of its behaviors. The newly awakened vinchuca raises its antennae--its sensing instruments--and, usually, grooms them: a mesmerizing process. Standing steady on its four rear legs, the insect lifts both front legs and unhurriedly draws first one antenna, then the other, through small combs on the inner sides of the two front legs. Now it is ready to go.