Sand Flies

On a cloudy, damp afternoon, a hungry gecko was cautiously crawling over the trunk of an araucarian tree, looking for insects but at the same time watchful for enemies. In one area a portion of the bark was missing and several beetles had collected, all of which provided a potential meal. The lizard moved to grab one but they all darted away, and in the end, the reptile chose a small weevil for its next meal, one that was crawling along the tree and lilting its elbowed antennae to detect scents of the preferred host plant. In an instant, the gecko lunged forward and caught the insect, but lost his footing in the process and fell into a sticky pool of resin. After struggling for several minutes, the animal became hopelessly immersed and died even before his partially swallowed prey showed any signs of digestion.

Hungry sand flies that had been hopefully following the luckless gecko now were forced to look for other prey. These miniscule, gangly insects with their long, slender legs, hirsute wings, and extended, flexible antennae appeared to be quite fragile but in actuality were astonishingly resilient. They didn't have to search long because a group of sauropods had entered the forest and began feeding. Like ghosts, the sand flies flitted through the shadowy trees and settled on the dinosaurs. After a few hops on the pebbly backs of the victims to find feeding sites, the flies became immobile as they began to gouge away miniscule portions of dinosaur skin. While drinking the red fluid oozing from the wounds, the females were joined by some males of the species, which, while lacking the necessary cutting tools, were only too grateful to partake of blood exposed by the females. After engorging, the fe males were too heavy to fly and rested on the victims for several minutes, just long enough to excrete the liquid portion of the blood. They remained still while the expanding droplets of plasma issued from the tips of their abdomens and fell onto the skin of their hosts. With their stomachs now filled with concentrated blood cells, they laboriously flew to the overhanging branches of an araucarian tree, where they settled down to digest their meals. All, that is, except for one satiated fly, which by some miscalculation landed directly on a trickle of resin and would soon be inundated by another resin flow. In a few days, the surviving females would deposit their eggs and then be ready for yet another meal of blood.

Bloodletting is a medical practice used by humans for well over two thousand years. The procedure, known as phlebotomy, involved puncturing one of the larger veins and draining blood into a container. This process is reminiscent of the modus oper-andi of phlebotomine sand flies that have used the method for at least 100 million years. Sand flies are one of the earliest groups of biting flies that developed a taste for vertebrate blood. They had probably evolved by the Jurassic, and early forms may have used their mandibles to obtain sap from primitive plants, much as some sand flies penetrate and obtain phloem sap from higher plants today.143 Just when blood became the obligatory part of the diet is unknown, but by the Early Cretaceous, when the first fossils appeared in Lebanese amber,174 some had possibly acquired this habit. They may have initially taken blood from wounds, but certainly by the mid-Cretaceous, the hematopha-gous habit was well established. It is difficult to say whether their bites would have caused raised itching papules or extended swellings on dinosaurs as they do in hypersensitive humans.175

While sand flies are known to engorge themselves on amphibians, reptiles, birds, and mammals, some prefer the blood of snakes and lizards.176 177 Those that select reptiles have no problem inserting their mouthparts through the imbricate scales on their victims. Feeding on a snake takes about an hour, during which time an amount of blood equivalent to the insect's weight is ingested. This increased mass impedes flight until the serum portion of the blood is excreted. Then after taking three to five days to digest the meal, the female converts the remainder into eggs. Interestingly, males also crave blood even though they do not have the mouthparts to penetrate intact skin. Instead, they simply lap blood from wounds, including those made by biting females.178

Today, phlebotomine flies occur in a variety of climates from warm to tropical and habitats ranging from rain forests to semiarid deserts.175 179 Given Cretaceous temperatures,180 sand flies would have been globally dispersed in a variety of environs. Adults might have preferred dark, humid sites on tree trunks and leaves or tree hollows, caves, and other excavations. As the climate in certain regions became drier, they adapted to microhabitats such as rodent or turtle burrows that provided the humidity and temperature levels approaching those of their primeval homes. Phlebotomine larvae develop in concealed areas, like cracks and crevices in the soil, tree hollows and crotches, termite mounds, forest floor litter, and animal burrows.175 Fungi may have been the basic food for the larvae, since two Cretaceous individuals were found associated with the fruiting bodies of a club mushroom181 (color plates 8D, 12A). While small and fragile with a limited flight range, sand flies can travel up to a mile, and specimens found on islands suggest that wind currents carry the adults for much longer distances.175

Sand flies probably introduced trypanosomatids into dinosaurs (figs. 24, 35-37). This idea is based on the discovery of a specimen in Burmese amber that contains reptilian blood cells infected with the trypanosomatid Paleoleishmania proterus254 (color plate 8C). Today, those phlebotomines in the genus Sergentomyia feed and transmit the related Sauroleishmania to snakes and several families of lizards.183 The effect of trypanosomatid infections on reptiles has been little studied; however, when these protozoa were injected into four chameleons, they all died.184

Sand Fly Dinosaurs

Figure 24. A Cretaceous sand fly255 carrying a trypanosomatid parasite254 is feeding on the exposed skin between the scales of a dinosaur. Upper right, stages of the parasite that occur in the alimentary tract of the sand fly: A. Infected dinosaur blood cell with small spherical amastigotes of Paleoleishmania in the gut of the sand fly. Isolated amastigotes can also occur in the blood meal. B. Amastigotes become elongate flagellated promastigotes that multiply by simple division (asexually). C. Short para-mastigotes are formed that are infective to dinosaurs. Lower left, stages of the parasite that occur in the dinosaur: D. Two paramastigotes entering a dinosaur blood cell. E. Formation of a parasitophorous vacuole containing developing amastigotes. F. Dinosaur blood cell releasing mature amastigotes. Continue the cycle by going back to A. Dark oval objects in blood cells are nuclei. Not drawn to scale.

Figure 24. A Cretaceous sand fly255 carrying a trypanosomatid parasite254 is feeding on the exposed skin between the scales of a dinosaur. Upper right, stages of the parasite that occur in the alimentary tract of the sand fly: A. Infected dinosaur blood cell with small spherical amastigotes of Paleoleishmania in the gut of the sand fly. Isolated amastigotes can also occur in the blood meal. B. Amastigotes become elongate flagellated promastigotes that multiply by simple division (asexually). C. Short para-mastigotes are formed that are infective to dinosaurs. Lower left, stages of the parasite that occur in the dinosaur: D. Two paramastigotes entering a dinosaur blood cell. E. Formation of a parasitophorous vacuole containing developing amastigotes. F. Dinosaur blood cell releasing mature amastigotes. Continue the cycle by going back to A. Dark oval objects in blood cells are nuclei. Not drawn to scale.

If dinosaurs were as susceptible to visceral leishmaniasis as humans are, entire populations probably were decimated. Human infections, which include both a skin and internal (visceral) form of the disease, are centered in Asia, Southern Europe, Africa and South America. Cutaneous leishmaniasis infects an estimated 12 million people throughout the world, and 1.5 to 2 million new cases appear each year.153 The visceral form is called Kala Azar and was responsible for some 100,000 deaths in southern Sudan alone between 1989 and 1994.185 American soldiers in Iraq and Afghanistan are familiar with the hazards of being targeted by infected sand flies. Probably close to a thousand have been infected with the cutaneous form of leishmaniasis, and a number now suffer from the potentially lethal visceral type. The standard treatment is pentavalent antimony—which has a high incidence of adverse reactions. Although medication represses the infection, flare-ups can occur throughout life.159

Fossil evidence indicates that vertebrate infections of Pale-oleishmania were quite extensive. Of 21 fossil female flies examined, 10 contained trypanosomatids, an infection rate of nearly 50%. These figures indicate that Paleoleishmania had reached epidemic proportions in the Burmese amber forest 100 mya. Present-day levels of Leishmania-infected sand flies vary depending on geographical location and habitat. The causal agent of American visceral leishmaniasis occurred in only 0.05% of over 5,000 Colombian phlebotomines sampled and in just 0.23% of 860 Venezuelan specimens.186 Cutaneous leishmaniasis is endemic in the Jordan Valley, and infection rates in sand flies collected from burrows of infected rodents varied from 9.2%187 to as high as 56%,188 which is roughly equivalent to that observed in Burmese amber.

Trypanosomes are not the only pathogens transmitted by sand flies. In the Cretaceous they also probably carried malarial pathogens to dinosaurs, since these insects are vectors of lizard malaria today, a highly infectious disease often resulting in death.189 Sand flies also pass arboviruses to vertebrates. A human disease known as sand-fly fever is caused by a phlebovirus and produces flu-like symptoms.175 Other arboviruses carried by phlebotomines include a reovirus (Changuinola virus) in sloths and a rhabdovirus in humans, wild carnivores, monkeys, and ungulates in South American.175 There is evidence that Cretaceous sand flies were infected with polyhedrosis viruses,172 which were feasibly the precursors of arboviruses carried by these bloodsuckers today.

Present-day sand flies also transmit bacteria and nematodes.173 A disease called bartonellosis is caused by a bacterium that induces anemia (Oroya fever) or a chronic skin disease (Peruvian wart) in humans. If untreated, Oroya fever can kill up to 40% of its victims.175 The potential effects of sand-fly transmitted viruses, bacteria, and nematodes on dinosaurs are difficult to ascertain, but certainly they could have been quite debilitating, especially under conditions of starvation or in individuals with a compromised immune system.

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