The hunchbacked, small-headed, spider-killers | Curator of Diptera

For the past two days I have started, and finished recurating the British Acroceridae Collection. Wow, you must be thinking, that young Erica is fast! Recurating an entire family in two days; updating the nomenclature, bar coding and databasing the specimens, and then rehousing into modern museum standard unit trays. Well, a slight confession is that there are only three species found in the UK.

That being said there are only 520 or so species found globally. These have been split into three extant subfamilies Acrocerinae, Panopinae and Philopotinae encompassing 53 genera (Pape and Thompson 2011). The world collection at the Natural History Museum fits into just five drawers. All three of the British genera are found within the Acrocerinae subfamily, a confused group of species that are gradually being tackled by the experts as to whom/how they relate to the other species.

So, what are these flies? They are a family that fall within the group referred to as either lower (in reference to their evolutionary history) or larger (as it’s popular with British Dipterists!) Brachycera. There are 11 families in the UK that make up this ‘group’ and they are some of our more distinctive and charismatic animal species, and in relation to the global fauna we have relatively few species and so it’s relatively easy to get to grips with them.

And there are some fantastic resources such as the great book by Alan Stubbs and Martin Drakes ‘British Soldier Flies and their Allies’ and the recording scheme Soldier Flies and their Allies, which is organised by Martin Harvey. In Alan Stubbs and Martin Drake’s book they affixed the common names to the UK species, and our three species are called Top-horned Hunchback (A. orbiculus), Smart-banded Hunchback (O. gibbosus) and the Black rimmed Hunchback (O. pallipes).

Family Common name Global species (approx.) UK
Acroceridae the hunchback-flies 520 3
Asilidae the Robber flies 7400 28
Athericidae the water-snipe flies 120 3
Bombyliidae the bee-flies 5000 9
Rhagionidae the snipe flies 700 15
Scenopinidae the window flies 400 2
Stratiomyidae the soldier flies 2800 48
Tabanidae the horseflies 4400 30
Therevidae the stiletto-flies 1100 14
Xylomyidae the wood-soldier flies 138 3
Xylophagidae the awl-flies 136 3

Table 1. Global and UK species richness of the Larger Brachycera

Acroceridae are commonly either called hunchback flies, small-headed flies or spider killing flies, and the latter name is telling of what they do.

Figure 1. Sphaerops appendiculata Philippi 1865 (From Schlinger et al 2013) – a hirsute hunchback fly from South America


These amazing little flies are parasitoids of spiders. Indeed it is the turn of the flies, well their larval stage, to feed upon their nemeses. The adults have been recorded feeding at flowers and many species are thought to be specialised pollinators as many species have a large proboscis (tube-like mouthparts) to enable them to obtain nectar from tubed flowers.

However, as with all flies, don’t assume that it is one size fits all situations as some species have vestigial mouthparts, and presumably don’t feed at all as adults. All of the UK species have short mouthparts and are also thought not to feed at this stage, but some of their more exotic cousins, such as those in the genus Philopota, have very long mouthparts, often equal to the length of their body, and are considered important pollinators.

Three balls in a row

The adults are amusing in shape. The UK species have an abdomen that is described as globose in shape – that is they were nearly spherical. In fact, the three UK species look like a string off three balls that have been gently pressed together in a row, along a size gradient of very small to large(ish). People often think that the head has fallen off the specimen as they are often tucked under or just the diminutive size confuses them (Fig. 2).

Another very obvious feature is the calypters, which are the thin, membranous lobes at the base of the wings, which in these flies are huge! What exactly these pads are as yet unknown (considering the size of these and they presence on so many species of fly, this in itself is quite amazing) but suggestions have included either an aerodynamic function (Miyan and Ewing, 1985) or protection for the halteres against wind turbulence (Pringle 1948). Whatever these may or may not turn out to be, in Acroceridae they are big!

Figure 2. Acrocera orbiculus, one of the three Acroceridae species found in the UK

But it’s the mysterious larva that has me smiling. The role of adults for most species of animal is to find a mate and procreate (I say most as not all animals need to find a mate, some females of can produce offspring without the presence of males, a process called parthenogenesis!). The adult stage of these species is relatively short, depending upon species it can vary between a couple of days, to a month. But that does not hinder the extraordinary number of eggs that the females can produce. She emerges as an adult with her entire lifetimes supply of egg all ready and waiting, and over a period of two to ten days can squirt out around 5,000 eggs (Fig. 3)!!

Figure 3. A rather gravid female A. orbiculus (from Kehlmaier & Almeida 2014 publication) with her eggs

The tiny eggs of Ogcodes are deposited on dead branches whilst those of Acrocera are laid on grass stems (Schlinger 1987). The collection at the museum has several small twigs were the eggs have been discovered (Fig 4)

Figure 4. A collection of eggs of Ogcodes pallipes found on a small twig in the Museum’s Diptera Collection

So why does she need so many? The answer lies in the face that the larvae are parasitoids of quite active species – spiders. It seems almost inconceivable that a legless creature should actively seek out very mobile hosts! But it works (as the 520 species shows us), although the success rate is presumably quite low.

Finding their hosts

After three to six weeks and usually at night, a much specialised first instar (or stage) of larvae emerge, called a planidium emerges. These specialised early instar larvae are not confined to Acroceridae but are also found in other Diptera (and Hymenoptera and Coleoptera), and most are heavily sclerotized, with hairs and spines, to aid their movement to the hosts (Fig. 5). Rather entertainingly, many species have a caudal suction disk, with which they can sucker on to surfaces and then ping themselves at their hosts or move in a similar fashion to inchworm caterpillars. The exception is with the Acrocera species, which lack all of these features and can only crawl!

Figure 5. The larvae of the Acroceridae

And all but one species of this family are endoparasites. Most crawl up the legs of the spiders and penetrate the abdomen near the book lung (a series of folded plates containing haemolymph, over which air flows), from which the larvae obtain oxygen from. One species, Ogcodes adaptus, is so sneaky that when the larvae attaches to the spider it stays motionless till the spider starts moving. Picking up on this cue the larvae then starts to move up the leg, only stopping when the spider stops! This little piece of subterfuge carries on till the larvae reaches the body and makes its way to the start of the upper part of the abdomen, where it starts to cut a tiny entrance hole.

And what are the hosts? Well there is no complete list. Dr Evert Irving Schlinger, an American who dedicated his life to studying these flies, was a man obsessed with understanding these beasts. Not only concerned with the taxonomy of the ‘Acros’, as he called them, but he wanted to understand their entire life histories, to which end he was rearing 1000’s of spiders in captivity (no small task when some of these live up to ten years). But sadly, he appears to have been a lone wolf in terms of this level of dedication– what we need is more folks with such dedication.

Schlinger’s 1987 paper lists all known hosts and the hosts for the three UK species are giving in Table 2. There are experts working across the globe who are publishing on new host records, but we have very little information on the species that are found in the UK, the Kehlmaier & Almeida (2014) paper gives a summary of the most recent and up to date information. Another one of these experts is Dr Chris Borkent, who works on non-European material, but if you are really lucky, you can witness him doing his Acroceridae dance!

Acrocera sp Lycosa sp.(maybe now some Pardosa sp. That used to be classified as Lycosa)
  Eris militaris (not found in UK)
Acrocera orbiculus Amaurobius erberi (although not yet recorded in the UK?)
  Pardosa prativaga
Ogcodes gibbosus Enoplognatha ovata
  Trochosa sp.
  Elotes ep. (not found in UK)
Ogcodes pallipes Pardosa pullata
  Clubiona putris – nomem dubium, can’t be certain which species this actually refers to – maybe another Clubiona sp.
Clubiona sp.
Xysticus luctuosus
Aelurillus v-insignitus
Heliophanus sp.
Phlegra fasciata

Table 2. Known hosts of the three UK species of Acroceridae from Schlinger (1987), Neilson et al. (1999), Larrivee, M. & Borkent, C. (2009) & Kehlmaier, C & Almeida, J (2014).

Tidying up the collection

So back to my recuration project. I took our one drawer of British species, barcoded and transcribed the labels for our collections database (and to be sent to the Biological Records Centre via Martin Harvey, who runs the Soldier flies and their Allies recording scheme), and then moved the specimens into current museum standard trays and drawers (Fig. 6).

Fig 6. Before and after – the recuration of the British Collection of Acroceridae

The British Collection is just shy of 300 specimens that date back to the 1850’s. In fact, most of the collection is pre-1960. They are naturally rare, but their absence in the collection is also an artefact of how difficult they are to collect. What we need now are spider specialists to join us the task of understanding the life histories of these enigmatic creatures. We have relatively few images of the larvae, we have so much to learn about what all the hosts are, and we don’t know much about the UK distribution.

Ogcodes pallipes has been recorded from across England but there appears to be a massive absence in the middle and very few records from Wales (Fig.7). It would be great to get to the bottom of this British distribution pattern.

Fig 7 Ogcodes pallipes and its UK distributions.

So, my plea is to go forth and record these beasties. If you have any specimens, we would be grateful to have them for the British Collection at the Natural History Museum collections to ensure that we have a heritage for future generations. These would not only provide data for distribution maps, but if reared from spiders, they would provide host data. Maybe do as Chris Borkent once did and follow one of these minute larvae along the ground. Who knows where it will take you!


References and further reading

Edwards, M. (1984) A further observation of swarming behaviour in Acrocera orbicula (F.) (Dipt., Acroceridae). The Entomologist’s Monthly Magazine, 120, 236

Gilbert, F & Jervis, M (1998) Functional, evolutionary and ecological aspects of feeding-related mouthpart specializations in parasitoid flies. Biological Journal of the Linnean Scoeity. 63: 495-535

Harvey, M.C. 2018. Provisional atlas of soldierflies and allies. Unpublished draft version.

Kehlmaier, C & Almeida, J (2014) New host records for European Acroceridae Diptera with discussion of species limits of Acrocera orbiculus Fabricius based on DNA-barcoding. Zootaxa 3780 (1): 135–152

Larrivee, M. & Borkent, C. (2009) New spider host associations for three acrocerid fly species (Diptera, Acroceridae). Journal of Arachnology 37(Aug 2009):241-242 · July 2009

Schlinger, E.I. (1987) The biology of Acroceridae (Diptera): true endoparasitoids of spiders. In: Nentwig, W. (Ed.), Ecophysiology of Spiders. Springer-Verlag, Berlin, pp. 319–327.

Stubbs, A. & Drake, M. (2001) British soldierflies and their allies. The British Entomological and Natural History Society, Reading, 512 pp.

Winterton, S.L., Wiegmann, B.M. & Schlinger, E.I. (2007) Phylogeny and Bayesian time estimations of small-headed flies (Diptera: Acroceridae) using multiple molecular markers. Molecular Phylogenetics and Evolution. 43: 808-832

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