The gland responsible for producing poison is found at the end of the bristles that cover the back of stinging caterpillars; this poison is a cocktail of toxins that can cause serious health problems and even death when it comes in contact with human skin. The venom-producing cells in the caterpillar Lonomia obliqua were identified by researchers at the Butantan Institute in São Paulo. Led by the biologist Diva Denelle Spadacci-Morena of the Pathophysiology Laboratory, they analyzed the caterpillar’s bristles using a scanning electron microscope capable of enlarging the image tens of thousands of times and identified the existence of a cell responsible for producing the poison.
For some time it has been known that other caterpillars, like the genus Automeris, have a poison gland in their bristles. But there was doubt about the more dangerous stinging caterpillars, which have already been dubbed deadly after fatal accidents. Before the Butantan Institute team, other researchers also investigated the bristles of Lonomia. Since they did not find the gland, they inferred that the poison was produced within the caterpillar’s body; as far as they knew, the bristles were only tubes that carried the secretion out.
In addition to identifying the toxin-producing cell, the researchers from the Butantan Institute reported that after the poison is produced, it is stored at the end of the hairs. “Brushing past them slightly is enough to break the tip and release the liquid,” says Spadacci-Morena, lead author of the article that described the finding in the September 2016 issue of the journal Toxicon. Interestingly, not all the bristles produce the venom, which is an orange liquid. Some release hemolymph, a greenish fluid responsible for transporting nutrients through the caterpillar’s body.
These caterpillars are common in rural properties in southern Brazil, and can reach 6 centimeters in length. They are the immature phase (larvae) of a brown moth, which has wings that resemble a dry leaf when they are open. The caterpillars are born from eggs deposited on tree trunks and leaves, and go through six developmental stages before becoming adult moths. As larvae, they live in groups of more than 80 individuals; despite their green spines, they are often confused with tree bark. These situations cause the most serious accidents.
Within minutes, small amounts of the poison cause skin burns, local swelling, and slight malaise. But in large quantities, some days later the poison can trigger bleeding disorders and even kidney failure. “In these cases, if left untreated, the individual begins to bleed from the nose and gums and loses blood in the urine,” explains entomologist Roberto Moraes, one of the authors of the Toxicon study. A specific protein in the venom (lopap) alters blood clotting and facilitates bleeding.
Reports of accidents with Lonomia began to emerge in the late 1980s, especially in the north of Rio Grande do Sul. At the time, about 300 people were injured in the city of Passo Fundo alone, where the caterpillar is more common. In total, around 500 accidents are recorded every year in Brazil. Because of the severity of these cases, in 1994 researchers from the immunochemistry laboratory at the Butantan Institute (headed by the immunologist Wilmar Dias da Silva) began to produce a serum to neutralize the poison. The physician Fan Hui Wen, who leads the arthropod laboratory at the Butantan Institute, currently coordinates the production of this antidote, which is made from an extract of the bristles from the caterpillar which is purified and then injected into horses. Every year the Butantan Institute receives about 3,000 caterpillars from farmers, which allows it to produce 10,000 vials of serum, enough to treat accidents that occur in the country.
Identification and characterization of the hemolytic toxins of the Lonomia obliqua caterpillar and the toxin-producing gland: biochemical and morphological study (nº 01/07643-7); Grant Mechanism Regular Research Grant; Principal Investigator Ida Sigueko Sano Martins (Butantan Institute); Investment R$127,423.13.
SPADACCI-MORENA, D. D. et al. The urticating apparatus in the caterpillar of Lonomia obliqua (Lepidoptera: Saturniidae). Toxicon. Sep. 1, 2016.