Kitchen Sprite — The Remarkable Fruit Fly (Part I)

As winter arrives, the activity levels of many animals decline noticeably. Wildlife in nature slows down, and even domesticated pets respond to the seasonal change. Yet in everyday life there are always a few small creatures that seem unaffected by the cold. Insects, for instance, can still be seen even in winter. Among them are the familiar household pests that appear throughout the year. Today, Rodrigo will introduce one such resident of our homes: the fruit fly.

Black-bellied fruit fly（圖片來源：André Karwath aka Aka，採用 CC BY-SA 2.5 授權）

The fruit fly, scientifically known as Drosophila melanogaster, is an insect belonging to the order Diptera, family Drosophilidae, and genus Drosophila. It is also one of the most thoroughly studied insects in the entire history of biology. Just how thoroughly it has been studied is a story worth telling.

Why Is It Called the “Black-bellied Fruit Fly”? A Brief Look at Fruit Fly Life History and Anatomy

Fruit flies receive their common name largely because of their feeding habits. In the wild, they mainly rely on decomposing fruits as a food source. In human environments they feed on leftover fruits and starchy foods. The Latin genus name Drosophila directly reflects this lifestyle: “droso” means moist, and “phila” means loving, together describing an organism that prefers damp environments. The species name melanogaster, meaning “black abdomen,” refers to the distinctive dark abdominal pattern characteristic of this species.

Because of these habits, fruit flies are often treated as pests and blamed for a variety of problems, some of which they do not actually cause. People sometimes claim that fruit flies lay eggs in unripe fruits and damage crops, or that food touched by fruit flies spreads serious diseases. In reality, the insect responsible for many agricultural losses is not the fruit fly but another species known as the oriental fruit fly, Bactrocera dorsalis. Compared with many other flies, fruit flies are also rarely associated with serious human diseases. At most they may contribute to mild digestive discomfort if contaminated food is eaten. Their reputation in popular media is therefore somewhat exaggerated. (This does not mean wild fruit flies can never carry microbes. In hot weather they accelerate food spoilage, and eating such food may lead to gastrointestinal discomfort.)

Oriental fruit fly laying eggs（圖片來源：嘎嘎昆蟲網，採用 CC BY-NC-ND 4.0 授權）

If fruit flies do not spontaneously appear from fruit, then where do the fruit flies in our homes come from? Like all other organisms, fruit flies do not arise from nothing. They reproduce sexually. Their tiny body size allows them to land on humans or objects and easily travel in and out of houses. Once inside, they are quickly drawn to the kitchen, where food scraps and moisture create an ideal environment. After locating a suitable habitat, male and female fruit flies mate, and within one to two days the female lays the first batch of eggs, each equipped with a small respiratory filament.

Fruit flies are insects with complete metamorphosis, and their life cycle consists of four main stages: egg, larva, pupa, and adult. Embryonic development inside the egg takes roughly one day. The egg then hatches into a larva, which spends about four to five days feeding and growing. At this stage the larva resembles a miniature maggot and undergoes three molts. After this period, the larvae leave the moist feeding substrate and search for a vertical surface where metamorphosis can begin.

At this point their appearance changes dramatically. Two slender respiratory tubes develop on the head, and the larval skin hardens to form a protective casing known as the puparium. This intermediate state is called the prepupa. The prepupa gradually darkens and transforms into the pupal stage, which also lasts about four to five days.

Schematic diagram of the fruit fly life cycle（圖片來源：bananflue，採用 CC BY-NC-SA 3.0 授權）

Inside the pupa, an extraordinary transformation occurs. Many larval tissues undergo programmed cell death, effectively dissolving much of the larval body. Only a special group of structures called imaginal discs remains intact. These discs contain the blueprint for the adult body. As development proceeds, they grow, differentiate, and reorganize into the structures of a complete adult fruit fly. Once development is finished, the emerging fly secretes enzymes that soften the pupal case and uses pressure from its head to push open the shell. A newly formed adult then emerges. Within about half a day, the young fly expands its wings and begins flying.

Cartoon illustration of imaginal discs（圖片來源：John.Tuthill，採用 CC BY-SA 4.0 授權）

Fruit fly wing disc after immunofluorescence staining。感謝陳文華提供的照片

Adult fruit flies are typically yellowish-orange with black bands on the abdomen and bright red compound eyes. Their bodies are covered with fine hairs of varying lengths. Like other members of the order Diptera, fruit flies possess only one pair of wings. The hindwings are reduced to small knob-like structures called halteres. These organs are filled with sensory nerves and function as gyroscopic stabilizers, allowing the fly to detect body rotation and maintain balance in flight. This remarkable sensory system is one of the main reasons fruit flies and other flies are so difficult to swat.

Male and female fruit flies also differ in appearance. Adult females are slightly larger and possess six abdominal segments, each marked with dark stripes, with the terminal segment forming an ovipositor used for egg-laying. Males have only four visible abdominal segments, and the end of the abdomen is broader and almost entirely black. Their terminal segment forms specialized copulatory structures. In addition, male fruit flies possess a distinctive structure called the sex comb on their forelegs. This comb is a cluster of modified bristles that helps the male grasp the female during mating.

Cartoon diagram of male and female fruit flies（圖片來源：Madboy74，CC0 1.0 公共領域）

Differences between male and female adult fruit flies under a dissecting microscope; male on the left, female on the right（圖片來源：Martin Hauser，採用 CC BY 3.0 授權）

Where Did Drosophila melanogaster Come From? A Brief History of Fruit Fly Evolution

To answer this question, we must first consider the evolutionary history of the insect order Diptera. The name Diptera literally means “two wings.” It originates from the ancient Greek description of flies given by Aristotle (Aristoteles, 384–322 BC). Today the term refers to a large group of insects including flies and mosquitoes.

The earliest known dipteran fossils date back to the Late Triassic, when these insects represented only a small lineage within the class Insecta. Their first major diversification occurred during the Carnian stage of the Triassic, a time marked by the Carnian Pluvial Event. Dipteran larvae depend heavily on moist environments, and the widespread rainfall of this period transformed much of the planet into ideal habitat for them. As a result, dipteran insects diversified rapidly, eventually splitting into many lineages, including those that would give rise to modern flies and mosquitoes.

An early dipteran fossil (crane fly), age unknown（圖片來源：Chris Light，採用 CC BY-SA 4.0 授權）

Later, during the Cretaceous period, the appearance of flowering plants triggered what scientists call the Cretaceous Terrestrial Revolution. Insects adapted to the newly abundant flowers by evolving pollination behaviors. Dipteran insects were part of this transformation as well. The high-energy resources provided by pollen and nectar encouraged some lineages to shift from decomposers to pollinators. Hoverflies, which many people recognize today as pollinators, separated from scavenging fly lineages during this period.

A piece of Eocene fly preserved in amber, whose appearance is already very similar to modern flies（圖片來源：Manukyan Andranik，採用 CC BY-SA 4.0 授權）

At this point one might wonder where fruit flies fit into this story. With flowering plants producing fruits, shouldn't fruit flies have appeared at the same time? Surprisingly, most members of the family Drosophilidae appeared later, largely after the Eocene epoch. During the Eocene, flowering plants evolved a new innovation: fleshy, sugar-rich fruits. These structures allowed plants to enlist animals as seed dispersers. However, they also created a new ecological niche — the decomposition of rotting fruit. Some fungus-feeding flies began exploiting this resource, and over time this lineage evolved into the fruit fly family. This marked the true emergence of fruit flies as we know them today.

Ephydridae (shore flies), the closest relatives of fruit flies genetically, still retaining the ancestral scavenging feeding habit（圖片來源：嘎嘎昆蟲網，採用 CC BY-NC-ND 4.0 授權）

As millions of years passed, fruit flies continued to diversify and adapt to a wide range of environments. Species of Drosophilidae can now be found in mountains, coastal areas, polar regions, and deserts. Among them, the species discussed here, Drosophila melanogaster, is one of the most widely distributed. Unsurprisingly, this global distribution is closely tied to humans.

Molecular biological studies suggest that Drosophila melanogaster likely originated in tropical and subtropical forests of Africa. Around five thousand years ago, it began spreading into Europe and Asia alongside expanding human activity. Researchers believe this shift may be connected to the rise of fruit cultivation. Compared with natural forests, human settlements provided a far more consistent supply of fermenting fruit. As agriculture and trade expanded, fruit flies followed humans into their living environments. By the eighteenth and nineteenth centuries, these flies had traveled on human ships and eventually airplanes, reaching the Americas and Australia and completing their global expansion.

A hand-drawn illustration of a steamship from 1888; the rise of steamships may have facilitated the spread of fruit flies（圖片來源：Of the photography : ComputerHotline，CC0 1.0 公共領域）

For humans, however, the spread of fruit flies has rarely been considered good news. For centuries, dipteran insects have had a poor reputation. Their buzzing flight, their role in accelerating decay, and their association with disease have made them targets of pest control efforts. When Drosophila melanogaster began appearing in the United States in the early nineteenth century, the U.S. Department of Agriculture began tracking and attempting to control these insects, funding extensive research into their biology. In hindsight, those control efforts ultimately failed — fruit flies remain common in kitchens around the world. Yet the data gathered during those studies proved invaluable in another field: genetics. As the saying goes, when one door closes, another opens.

In 1901, a geneticist named William Ernest Castle (1867–1962) first took serious notice of this remarkable insect. At the time, genetic research focused mainly on mammals and plants, organisms that scientists considered more familiar and convenient to study. Yet these model organisms had drawbacks: they were relatively large, costly to maintain, and reproduced slowly. Castle sought an organism that was small, inexpensive, capable of producing large numbers of offspring, and able to pass through generations quickly. The fruit fly fit these requirements perfectly.

A modern fruit fly culture incubator; the tubular containers shown are fruit fly culture media（圖片來源：Image Editer，採用 CC BY 2.0 授權）

Through Castle's work, Drosophila melanogaster rapidly emerged as a new model organism. Many of the basic experimental practices used in fruit fly research today originated in Castle's early studies. At the time, however, his work was still somewhat niche. The scientist who would later elevate fruit fly research to global prominence was Castle's friend, Thomas Hunt Morgan (1866–1945).

(To be continued...)

Author: Rodrigo

Reference:

1. Drosophila (Sophophora) melanogaster Meigen, (1830). Catalogue of Life. Species 2000: Leiden, the Netherlands. Retrieved May 2, 2024.

2. Wei D, Feng YC, Wei DD, Yuan GR, Dou W, Wang JJ (2015). "Female remating inhibition and fitness of Bactrocera dorsalis (Diptera: Tephritidae) associated with male accessory glands". Florida Entomologist. 

3. "FlyBase: A database of Drosophila genes and genomes".(2009) Genetics Society of America. .

4. Blagoderov, V. A.; Lukashevich, E. D.; Mostovski, M. B. (2002). "Order Diptera Linné, 1758. The true flies". In Rasnitsyn, A. P.; Quicke, D. L. J. (eds.). History of Insects.

5. Andreas Keller. (2007)“Drosophila Melanogaster's History as a Human Commensal.” Current Biology, .

6. "Thomas H. Morgan – Biographical". nobelprize.org.(2018) Les Prix Nobel. T.H. Morgan's Nobel Prize biography mentioning C. W. Woodworth's suggestion and W. E. Castle's use of Drosophila.