The Bizarre Reason Tsetse Flies Don’t Lay Eggs

Many people are familiar with the tsetse fly by name. Fewer people, however, know about the unique way that tsetse flies give birth to their young. Most flies produce dozens, if not hundreds, of eggs, but the tsetse fly’s birthing process is more similar to that of a mammal.

This video shows a female tsetse fly giving birth to one larva at a time, nearly the size of her body. Whereas other insects and flies produce large batches of eggs that mature externally, female tsetse flies nurture their young individually inside their uterus. This process is rarely seen by humans in the wild. But laboratory environments have allowed scientists to witness this rare reproductive process. Let’s learn more about tsetse flies and how they reproduce.

What is the Tsetse Fly?

Tsetse flies (Glossina morsitans) are a biting fly of the Glossinidae family. As an obligate parasite, they survive by feeding off the blood of larger vertebrate animals. Scientists have studied tsetse flies extensively because of their outsized role in transmitting disease. While they look similar to other large flies, tsetse flies tend to fold their wings across their abdomens while resting. They also have an elongated appendage called a proboscis, which protrudes forward from their heads. Tsetse flies are tough little creatures, too. Other large flies like horseflies are easily killed by a flyswatter, but tsetse flies are more difficult to eliminate.

They are found in large numbers in Africa between latitudes 15° north and 20° south. There, they mostly stick to woodlands but will venture into open grasslands to feed on host animals. Blood is their main source of sustenance, and tsetse flies feed nearly every day. While different biting fly species are active at various times throughout the day, in general, tsetse flies wind down after sunset. Male tsetse flies prefer a human snack, while females feed on larger game. This may be because they require more sustenance for their unique birthing process.

How Do Tsetse Flies Reproduce?

Female tsetse flies typically give birth eight times during their life. Each birth comes one at a time, with the developing young accounting for much of the mother’s body size. That’s because tsetse offspring develop internally during the first three larval stages in a process called adenotrophic viviparity. While in the ‘womb,’  the larva feeds on a milky substance produced by a modified gland in the mother’s uterus. This development lasts between four and ten days, during which the larvae share resources with their mother.

Once they reach a considerable size, sometimes even exceeding their mother’s mass, they are born, as shown in this video. The larvae then burrow into the ground and grow a hard outer shell called a puparial case. This protects them from predators and the elements until they emerge as adults. Technically, tsetse flies develop like other flies, passing through three larval stages and a pupal stage before reaching maturity. However, thanks to the tsetse fly’s blood-rich diet, the larvae endure this process inside their mothers.

What Diseases Do Tsetse Flies Spread?

The tsetse fly’s blood diet relies mostly on warthogs, bush pigs, and red river hogs. They also feed on humans, which has had a profound effect on the transmission of disease. While feeding, tsetse flies can transmit parasites called trypanosomes. These parasites cause sleeping sickness in humans and a variation known as nagana in livestock.

If infected with sleeping sickness from tsetse flies, humans develop fevers, joint pain, and headaches within the first few weeks after being bitten. As the months progress, the infection produces numbness, confusion, coordination problems, and insomnia. Sleeping sickness affects hundreds of people each year and can result in death if left untreated.

As shown in this video, tsetse flies undergo a unique gestation and birthing process. Their blood-rich diet allows larvae to share resources with their mother. This parasitic diet, however, has severe consequences for human and livestock populations.