Exploring the Origins of Diptera
- Debbie Browning
- Jan 23
- 4 min read
Updated: Mar 5
In this post we will discuss the origins of Diptera and their amazing adaptation to life with two-wings.
Darwin correctly hypothesized that species change over time. He theorized that different species may share a common ancestor and gradually diverged from one another through generations. He explained this through his theory of natural selection published in 1859. (Sadava 2017)
The ancestors of all flies would have had four-wings, and its through evolution that the hind-wings of diptera shrank and transformed into specialised structures that perform a very different but beneficial function. This re-purposing of an existing structure through evolution is known as exaptation or co-optation termed by Gould and Verb (1982) see McLennan (2008) for more on this (link in references).
How did this change occur in a physical sense?
A particular gene called the Ultrabithorax (Ubx) was heavily involved in this adaptation. This is a ‘hox’ or homeobox gene, producing proteins that bind to DNA and regulate how much they are expressed. They are present in all multicellular animals (Mark et al. 1997). Right from diptera embryo stage, Ubx is present within cluster of genes in chromosome 3, determining the development of the third thoracic segment or T3 (Sadava 2017). (Shown in red within the diagram below).
Further genes are both regulated and activated to encourage the alternative growth pattern of halteres. Sensory receptors and proteins are produced, including campaniform sensilla . These are dome-shaped motion sensors that detect the bend and flex (or mechanical stress) of the exoskeleton. (Agrawal et al. (2017)
Mechanosensory signals from halteres are relayed to the nervous system via neurons. This allows the fly to determine both where is it in space, and what steering forces would be needed to follow the intended flight path. Receptor signals then relay to motor-neurons that control wing steering and head movements so that both flight path and gaze are controlled. (Yarger et al. 2016)
The sensory hairs on insect legs are the likely source of the adaptation of the growth and emergence of haltere sensory receptors. These senor-coding alleles would have been present in the wing-discs before the development of halteres, as wings are thought to have first originated from leg-flaps or leg excites of distant ancestors of the fly. (Dickinson et al. 1997).
When and how did it happen?
Scientists trace the origins of the order Diptera to the Jurassic period between 200 to 150 million years ago (lower diptera). Higher diptera, including Brachycera, developed around 180 million years ago (a sub-order of flies including house flies, fruit flies, robber flies and horseflies) and by the late Jurassic (150-65 mya), flies with halteres underwent huge diversification. (Wiegmann et al. 2011).
Hindwings of fly ancestors were replaced with halteres over millions of years, with benefits from the new functionality causing a Darwinian genetic natural selection. In essence, fly species with haltere-like adaptations had survival and reproductive advantages over those that didn’t, called a directional selection of a particular trait or phenotype. (Holsinger, 2001)
This occurred as;
The genes existed to be mutated (sensory genes and hox genes) and trait-variation existed among the early ancestors of flies.
Flies produce many offspring (r-strategists) meaning only the best suited to their ecosystem pass on genes and...
The ancestors to flies had a ‘struggle’ or pressure for survival stimulating adaptation.
What were the pressures for survival in the Jurassic that stimulated evolution?
Radiation pressures that led to this adaptation may have included:
Predator avoidance
Increased ecological opportunities
Increased competition
Extinction events
These are described below;
Predator avoidance was crucial. Manoeuvrability meant avoidance of being eaten and therefore survival, with better manoeuvrability traits (halteres) passed to offspring. During the Jurassic, huge dragonflies, spiders (webs), mantids, amphibians and early birds predated on early flies and their ancestors. Spiders webs had been around since the Carboniferous era (Mario-martins et al. 2020). This created a survival 'struggle' benefiting those that could fly dynamically to avoid webs. True birds developed in the late Jurassic (around 150mya) (Brussate et al., 2015), and bats at 50 mya, an increased predator pressure which may have been the catalyst for the devleopment of halteres.
Ecological opportunities provided by the emergence of flowering plants in the cretaceous meant landing accurately on plants was an advantage. Traits favouring accuracy and stability in flight (halteres) would mean better survival and reproduction. (Brian et al. 1994). The growth of dense large forests would have also led to advantages in those hexapods with agility, with early flies navigating through complex three-dimensional environments.
Increased competition for food, habitat and breeding areas would have occurred as early flies lived alongside insects holding similar niches such as dragonflies and damselflies (Odonata) lacewings antlions (Neuroptera) and beetles (Coleoptera). Increased aerial performance for capturing food (such as robber flies that feed on small invertebrates) or defending a breeding space (e.g. over water such as some hoverfly species) would have ensured beneficial genes for manoeuvrability (i.e. haltere development) were passed on.
Extinction events create ecological opportunities to be exploited; the Permian-Triassic extinction occurred around 252 million years ago and reset life on earth, opening up ecological niches (Sadava 2017). It led to the development of the ancestors of flies, taking advantage of feeding on decaying matter. The Cretaceous-Paleogene extinction (around 66 million years ago -Sadava 2017) happened around the time flies had fully developed halteres. They would benefit again from the abundance of rotting organic material left behind (a food source for larvae) and were able to survive and further diversify.
