Question Migrating Flies: Review Article

Migrating Flies: A review Article

Summary
Nearly 600 species belonging to the order Diptera have been found to display migratory behaviour by a researchers (incl. Dr Will Hawkes, Dr Myles Menz & Dr Karl Wotton ) from the James Cook University. These species play a role in pollination and the dispersal of plant genetic material.

A connected, unfragmented habitat is required for migratory Diptera species to complete migratory journeys and fulfil their ecological roles, highlighting the need for ecosystem restoration and protection.

Key Points from the James Cook University Media Release:
'Migrating flies vital for people and nature'
Published April 4th, 2025
James Cook University

‘Myles Menz, co-author of the new study and Senior Lecturer in Zoology and Ecology at James Cook University, said researchers found almost 600 of these species are likely to be migratory, and the true number is probably far higher.

‘Lead author Dr Will Hawkes, from the Centre of Ecology and Conservation on the University of Exeter’s Penryn Campus in Cornwall said migrating flies play an unparalleled range of ecological roles, making them a major force in ecosystems and the economy.’

‘Highly mobile species may also connect distant habitats, moving genetic material, such as pollen, back and forth, boosting the genetic diversity of plants.’
‘The researchers said that pollinating hoverflies alone have been found to visit 52% of major food crop plants globally (these crops have an estimated worth of around US$300 billion per year).’

‘Many of the species that benefit humans are under threat from climate change and other human impacts and many could disappear without ever being documented unless action is taken.’

‘With insect numbers declining worldwide, the review highlights the need for complex, connected habitats to support flies along their epic journeys.’

‘ “To conserve them, it will not be enough to protect or restore habitat at single locations; the entire migratory route must be capable of sustaining these insects,” said Dr Hawkes.’

‘ “Worldwide, factors including urbanisation, intensive agriculture and destruction of wetlands have made large areas unfriendly to insects, potentially creating damaging gaps on insect migration routes,” Dr Wotton said.’

‘Co-author, the University of Exeter’s Dr Karl Wotton, said the priority should be improving “connectivity” – linking insect-friendly areas across landscapes.’

‘ “What we’ve uncovered is that Dipteran migrants are vital to the planet’s ecosystems, yet they are hugely underappreciated. This review will hopefully inspire many more studies into this fascinating and important world of fly migration,” said Dr Menz.’

Key Points from the Review Article:
'Lord of the flies: dipteran migrants are diverse, abundant and ecologically important'
By Will L. Hawkes, Myles H.M. Menz, Karl R. Wotton
Published April 1st, 2025
Biological Reviews

Abstract:
‘Insect migrants are hugely abundant, with recent studies identifying the megadiverse order Diptera as the major component of many migratory assemblages. Despite this, their migratory behaviour has been widely overlooked in favour of more ‘charismatic’ migrant insects such as butterflies, dragonflies, and moths. Herein we review the available literature on dipteran migration to determine its prevalence, identify key migratory routes and elucidate areas that may prove fruitful for future research. Using 13 lines of evidence to determine migratory behaviour, we determined that species from 60 out of 130 dipteran families show evidence of migration, with Syrphidae fulfilling 12 of these criteria, followed by the Tephritidae with 10. By contrast, 22 families met just two criteria or fewer, underlining the need for more research into the migratory characteristics of these groups. In total, 592 species of Diptera were identified as potentially migratory, making them the most speciose group of insect migrants yet described. Despite this, only 0.5% of dipteran species were found to be migrants, a figure rising to 3% for the Syrphidae, a percentage mirrored by other migratory taxa such as butterflies, noctuid moths, and bats. Research was biased to locations in Europe (49% of publications) and while vast regions remain understudied, our review identified major flyways used by dipteran migrants across all biogeographic realms. Finally, we highlight an unsurpassed level of ecological diversity within dipteran migrants, including ecological roles of huge economic value. Overall, this review highlights how little is known about dipteran migration and how vital their migratory behaviour may be to the health of global ecosystems.’

Introduction
‘Studies of insect migration have focussed mainly on the larger, more ‘charismatic’ insects such as moths, butterflies, and dragonflies.’

‘Few have systematically analysed whole migratory assemblages. However, the studies that do exist have revealed a major group of migrants that remain hugely understudied and that are of great ecological importance: the Diptera.’

‘Recent systematic studies of insects passing through migratory bottlenecks have shown that Diptera can comprise nearly 90% of the individuals found in migratory assemblages in certain locations.’

Defining Migration:

‘A widely used definition of migration is one based on behavioural characteristics: Migratory behaviour is persistent and straightened-out movement effected by the animal's own locomotory exertions or by its active embarkation on a vehicle. It depends on some temporary inhibition of station keeping responses but promotes their eventual disinhibition and recurrence’

‘For dipteran migrants, and migratory insects in general, there are various viewpoints as to what constitutes migration.’

‘Herein, we use the broad behavioural definition of migration quoted above, while recognising that currently we can only be certain of migratory behaviour in a few species.’

Literature Search:
‘We searched Google Scholar, Web of Science and PubMed were searched to identify dipteran families with migratory behaviour based on at least one of the following 13 types of evidence.’

‘(i) seasonal back and forth movement; (ii) long-distance flight; (iii) seasonally appropriate directed movement; (iv) inability to develop in trapped habitat; (v) ability to choose favourable winds; (vi) mass arrival; (vii) capable of high-altitude flight; (viii) populations with a high rate of gene flow; (ix) strong flight capabilities (tethered flight mill); (x) orientation within a flight simulator; (xi) physiological/morphological changes in the migratory phenotype; (xii) seasonal appearance of a disease; and (xiii) unable to overwinter (in any state) in trapped location.’

‘Criteria 1–4 form the ‘core four’ most often reported migratory characteristics.’

Prevelence of Migration:
‘We found evidence for migration behaviour in approximately 47% of all dipteran families (60 out of 130) with data sourced from 344 papers.’

‘Syrphidae (hoverflies) were the most studied migratory dipteran family featuring in 48% of articles (recently reviewed by Reynolds et al., 2024) and meeting the most migratory criteria: 12/13.’

‘The Tephritidae (fruit flies) met the second highest number of migratory criteria. Culicidae (mosquitoes) were the second best studied with 17% of the articles and, together with Muscidae (house flies), and Calliphoridae (blow flies and screw worms) and Chloropidae (grass flies), were one of four groups that met nine migratory criteria.’

‘All these families, even the minuscule Chloropidae (~2 mm in length), have been recorded as showing the ‘core four’ criteria.’

Figure 1 (see attachment 1):
‘Migratory criteria (see Table 1) fulfilled by the 60 identified migratory families of Diptera. Heat map colours indicate the number of migratory criteria confirmed for each family, with orange being the most (12 criteria fulfilled) and dark blue the least (one criterion fulfilled). # indicates the total number of migratory criteria met. The ‘core-four’ criteria are: (1) Seasonal back and forth movement; (2) long-distance flight; (3) seasonally appropriate directed movement; and (4) inability to develop in trapped habitat. Families meeting the core-four are indicated by an asterisk (*). Other criteria are: (5) ability to choose favourable winds; (6) mass arrival; (7) capable of high-altitude flight; (8) populations with a high rate of gene flow; (9) strong flight capabilities (tethered flight mill); (10) orientation within a flight simulator; (11) physiological/morphological changes in the migratory phenotype; (12) seasonal appearance of a disease; and (13) unable to overwinter (in any state) in trapped location.’

Figure 2: (see attachment 2):
‘Dipteran phylogeny showing the number of migrant species, the percentage of migratory species in each family, and the number of migratory criteria met. Phylogeny based on Wiegmann et al. (2011). All photographs ©Will Hawkes, apart from Simuliidae (©Mehmet Akif Suna).’

Mechanisms of Fly Migration:
‘Studies performed at migratory bottlenecks suggest that warmer temperatures, dry conditions, and the presence of favourable winds are important factors influencing migration intensity in Diptera.’

‘In addition to using environmental cues, Diptera, like other migrants, undergo changes to their physiology during migration. These changes allow them to store energy and prepare for the long journey ahead. For example, flies will increase their fat stores before migrating, which provides them with the energy they need to fly long distances.’

Migration Paths or 'Flyways':
'We found a global distribution of migratory behaviour in Diptera (Fig. 3). Records were recovered from all continents.’

‘We focus on each geographic region where fly migration is known to occur and on routes taken, which we refer to as ‘flyways’

Figure 3: see attachment 3):
‘The geographic distribution of dipteran migration studies and migratory flyways. Red dots represent locations of the dipteran migration studies identified in this review. Black arrows represent hypothesised migration routes based on known migrations from the published literature. These arrows represent likely broad fronts of movement and may be further influenced by geography, innate compass senses, and wind patterns.’

Ecological Roles:
‘Diptera occupy an extraordinary breadth of terrestrial and aquatic habits associated with an impressive range of ecological roles, including as pollinators, decomposers, predators, parasites and pests, while exhibiting a highly diverse range of feeding habits.’

‘We identified a diverse range of dipteran migrants numbering 592 species from 60 families. The ecological roles of these species were determined from a variety of sources including guidebooks and scientific publications.’

‘62% of these species were pollinators, 35% were decomposers, 18% were pests, 16% were disease vectors, 10% pest controllers, and all played a role in the transfer of nutrients. The total sum is over 100% as individual species can play multiple ecological roles.’

Conclusions:
1. 'Our analyses of the literature on migrant Diptera reveal a highly diverse set of 592 suspected migrant species from 60 families, many of which appear to be highly abundant, and to migrate in huge numbers.'

2. 'Dipteran migrants, compared to other migratory insect orders, play an unsurpassed range of ecological roles, marking them out as a major contributing force to the functioning of terrestrial and aquatic ecosystems and the economy.'

3. 'Compared to other groups, very little is known about migratory Diptera and for many of the migratory families only a single study related to migration was found. We recommend a greater focus on the taxonomic diversity of dipteran migrants in future studies. This will help to understand the ecological roles of these insects as they connect distant landscapes.'

4. 'We recommend further study using techniques such as monitoring and trapping in migration bottlenecks, stable isotope and pollen analysis, trajectory analysis, flight simulators and flight mills, and NDVI measurements, along with emerging approaches such as radar networks, that can be used to infer behaviour, assemblages, origins, destinations, headings and numbers of mass movements of dipteran migrants.'

5. 'Many anthropogenically beneficial dipteran migrants are under threat from climate change and other anthropogenic impacts. It is possible that many migratory flies and their behaviour could disappear without being documented unless action is taken.'

6. 'To conserve these vitally important taxa, it will not be enough to protect or restore habitat at single locations; the entire migratory route must be capable of sustaining these insects, as for other migratory species (Runge et al., 2014). Refocusing agricultural, rewilding and conservation practices to ensure landscape connectivity could have the greatest impacts. Understanding the migratory cycles and pathways of these ecologically important species will be key to future conservation measures.'

Sources:

James Cook University. (2025, April 4). Migrating flies vital for people and nature.
www.jcu.edu.au/news/releases/2025/april/migrating-flies-vital-for-people-and-nature

Hawkes, W.L., Menz, M.H.M. and Wotton, K.R. (2025), Lords of the flies: dipteran migrants are diverse, abundant and ecologically important. Biol Rev. doi.org/10.1111/brv.70017