Horseflies and Deerflies

A rather large horsefly rested on a ginkgo leaf while the early rays of the sun dried its new brown-spotted wing membranes. Just a few days before, this insect had been a white legless grub crawling through mire on the shore of an extensive lagoon, sucking the lifeblood from any luckless invertebrate it could overpower. Now a second phase in life was beginning. The large, iridescent eyes searching the landscape for prey gave the illusion of beauty and innocence but masked a rather sinister passion—a lust for vertebrate blood. Multiple lenses in the eyes suddenly registered the movements of an iguanodon feeding on ferns along the edge of the marsh. The horsefly immediately became airborne and Hew directly toward the dinosaur. Initially delaying contact, the ravenous fly circled around the animal's head while looking for a safe landing spot. The flight pattern shifted into smaller and smaller circles until, almost effortlessly, the insect settled on the large vertebrate's shoulder. Exposing the powerful mouthparts, the fly began to gash an opening through the victim's skin. The searing pain caused the herbivore to momentarily stop feeding and turn her head. But she couldn't reach the irritation and twitching the skin failed to dislodge the fly. No amount of the jerking could disturb this determined predator, and soon a sanguineous fluid began to trickle down between the tuberculate scales as the insect's abdomen took on a crimson hue. The smell of fresh blood attracted non-biting flies that welcomed this free meal of concentrated protein.

Then, almost as silently as the landing of the horsefly, the first sign of an even more sinister danger approached from the water, disguised in a series of bubbles and small whirlpools surrounding two protuberant eyes and a set of nostrils. Without a second horsefly bite on the hind leg, the agitated dinosaur might not have thrown caution to the wind and splashed into the brackish waters to dislodge the intruders. While the soaking Anally dislodged the fly on her leg, the plunge took her further out than she would have normally ventured. The attack by the lurking giant crocodile was swift and deadly. Vise-like teeth clamped down on the young iguanodon's haunch and the helpless animal was dragged into deeper waters. If horseflies hadn't badgered the animal, thus changing her normal course of cautious behavior, the iguanodon's life might have played out differently

Horseflies and their smaller cousins deerflies of the family Tabanidae were widespread throughout the Cretaceous and because they now feed on both warm- and cold-blooded ani-mals,210-213 they certainly took blood from dinosaurs (fig. 22). Their painful bites and persistence behavior would have greatly irritated the huge reptiles, just as they do many animals today, from reptiles to humans. At least four species of horseflies are known to prey on crocodiles and anacondas in the Amazon.214 The onslaught begins with the flies circling rapidly around the victim before alighting. Feasting commences as soon as they land, and each species appears to have a preferred feeding site, either the head or back.

The wounds are obviously unbearable since caimans become extremely agitated and wave their tails in a vain effort to rid themselves of the pests. Because that action is usually unsuccessful, the reptiles resort to diving into the water, a recourse also used by anacondas who are attacked. While this gives some temporary relief, the insects just wait around for the reptiles to emerge and then resume the assault. Even when the victims are partially submerged, the bloodsuckers will attack any exposed body parts. One wonders if many of the large sauropods that are depicted as feeding in water may have adopted this behavior to keep biting flies away from their ventral surfaces.

Turtles, including the large Galapagos Island tortoises, are also victimized by horseflies.215 The flies seem to know when leather-backs are going to come out of the sea to lay their eggs. At least six species of tabanids feed on marine turtles in Suriname and French Guiana, attacking the new arrivals on necks, shoulders, and legs from sunrise to sundown.216 Freshwater species are likewise ravaged during the summer months, and to add injury to the insult, these flies also transmit turtle malaria.217 Perhaps the most vulnerable animals are mammals. Not only man but also his domestic animals can be savagely bitten by these persistent creatures. Just moderate to heavy feeding can result in weight loss in beef cattle, reduced milk production in dairy cows, and hide damage (for leather products) from the resulting large punctures.

Tabanids infect victims with pathogens by a simple, direct method called mechanical transmission. Microbes picked up on the flies' mouthparts after feeding on a diseased animal are subsequently transmitted to a healthy animal via the wounds. The mouthparts function like a pair of scissors that were dipped into a solution filled with germs. Using this simple method, they are able to transmit viruses, rickettsia, bacteria, and protozoans to a range of animals. Among the pathogens transmitted are anthrax bacteria, spirochetes such as those responsible for Lyme disease, and viruses causing leukemia and cholera.218 This method of transmission is probably how many pathogens were originally spread to vertebrates by insects.

If a horsefly's mouthparts were contaminated with bacteria from the last meal and some of the cells entered a wound, they might have started to multiply in a dinosaur's bloodstream. Whether the infection would continue to spread, eventually causing a lethal septicemia, would depend on the nature of the bacteria and the state of health of the animal and its immune system.

The only parasites carried by tabanids that actually develop inside the fly are filarial nematodes. Loiasis, a potentially debilitating disease of humans and simians in Africa, is just one of these. The adult worms live under the skin near the head of the victim, sometimes passing under the conjunctiva of the eye, which is how they became known as eyeworms. Another filarial nematode lives in the heart and blood vessels of elk and moose, clogging their arteries and causing blindness and even death.173,218

Thus bloodsuckers could have left tiny parasitic nematodes in dinosaur wounds. In time, the roundworms would develop, reproduce, and bear their young in the tissues of iguanodons and others. They probably would not kill their host, but they would become yet another physical burden the dinosaur would have to endure during its lifetime.

So a young iguanodon might not have just borne the feeding scars of horseflies, but could have been infected with any number of pathogens they introduced. Most Cretaceous dinosaurs would have been fair game, especially the large, slow-moving sauropods that had no real protection against biting flies. Since tabanids are strong fliers, they probably occupied the same wide range of habitats during the Cretaceous they do now.218

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  • thomas
    Do horseflys inject their eggs into human skin?
    9 years ago

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