Introduction

Acroceridae is a species-poor family of orthorrhaphous Brachycera (Diptera) whose larvae develop almost exclusively as endoparasitoids in the body cavity of spiders, with the exception of the ectoparasitic Carvalhoa appendiculata Philippi (Gillung & Borkent 2017). Due to their larval development and their adult appearance, they are commonly known as spider flies, small-headed flies or hunch-back flies. Although the systematics of the family, with its five subfamilies, 60 genera and approximately 530 species has been largely revised in recent years (Winterton et al. 2007; Gillung et al. 2018; Gillung & Winterton 2019), only few ecological data and life-history traits are available for the family. The interactions between these parasitoid flies and their hosts have rarely been the focus of scientific studies, and most published host records are the result of accidental discoveries by arachnologists while rearing juvenile spiders to adulthood in individual containers. The currently known, geographically and temporally scattered host data were revised by Gillung & Borkent (2017), but there still remains a lot to be discovered. Here, we report nine new rearing records of Palaearctic acrocerid flies from spider hosts.

Material and methods

For details on specimen collection and identification, see the individual records below. If not otherwise stated, host spiders were identified by M. Schäfer and acrocerid flies by C. Kehlmaier. Figs. 1–2 & 19–22 were taken with a Canon EOS 5D Mark IV camera equipped with a Canon EF 100mm f2.8 L Makro IS USM or a Canon MP-E 65mm f/2.8 1-5x lens, without flash. Individual photos were stacked with Zerene Stacker. Figs. 3–6 were taken with a Zeiss Axio Zoom.V16 binocular equipped with a Zeiss Axiocam 712 Color camera using the Zeiss Zen software. Figs. 7–18 were taken with a Pentax K20D camera (ƒ/16.0, 100 mm, exposure time 1/180, ISO 200, flash).

Results

Acrocerinae

Acrocera (Acrocera) orbiculus (Fabricius, 1787) reared from Macaroeris sp. (Salticidae)
(Figs. 1–2)

Host spider collection data: juvenile, Spain, Canary Islands,Tenerife,LosOrganos,28.35932°N16.50188°W(WGS84), collected 6 March 2015 on Erica arborea (Linnaeus, 1753), leg. & coll. M. SCHÄFER (voucher: Te2015-94).

Larva eclosed from host: 7 April 2015.

Fly (male) eclosed from pupa: no exact date recorded; approximately one week after the larva emerged from the host.

Identification of parasitoid: following Kehlmaier & Almeida (2014), who demonstrated a broad morphological and genetic variation for the species. Identification of host: genus identification based on comparison with morphologically identical juvenile specimens obtained from the same locality and reared to adulthood.

Figs. 1–2.

Male Acrocera (Acrocera) orbiculus (Fabricius, 1787) reared from Macaroeris sp. from Tenerife (Canary Islands, Spain). 1. Dorsal view. 2. Lateral view. (Photos: MICHAEL SCHÄFER)

Acrocera (Acrocera) orbiculus (Fabricius, 1787) reared from Pseudicius picaceus (Simon, 1868) (Salticidae)

Host spider collection data: juvenile, Greece, Crete, Xyloskalo, 35.30748°N 23.91538°E (WGS84), collected 14 May 2018 on stones, leg. & coll. M. SCHÄFER (voucher: Cr2018-91).

Larva eclosed from host: 14 May 2018.

Fly (male) eclosed from pupa: no exact date recorded; approximately one week after the larva emerged from the host.

Identification of parasitoid: following Kehlmaier & Almeida (2014).

Identification of host: based on comparison with adult individuals from the same locality (Schäfer 2020), identified using Metzner (1999).

Figs. 3–6.

Female Cyrtus gibbus (Fabricius, 1794) reared from a female Eratigena saeva (Blackwell, 1844) from Spain. 3. C. gibbus in dorsal view. 4. C. gibbus in lateral view. 5. Vulva of E. saeva in dorsal view. 6. Epigyne of E. saeva in ventral view. (Photos: ANDRÉ REIMANN)

Figs. 7–14.

Cyrtus gibbus (Fabricius, 1794) reared from male Eratigena sp. from Spain. 7. Eratigena sp. in dorsal view. 8. Eratigena sp. in lateral view and C. gibbus larva (indicated by arrow) in dorsal view. 9. C. gibbus pupa in lateral view. 10–11. C. gibbus larva in lateral view. 12. C. gibbus pupa in dorsolateral view. 13. C. gibbus adult in ventrolateral view. 14. C. gibbus adult in lateral view. (Photos: ANGEL MARÍA ARES)

Figs. 15–18.

Larva of Cyrtus sp. reared from female Eratigena sp. from Spain. The fly did not reach adulthood. 15. Eratigena sp. in dorsal view. 16. Eratigena sp. in frontal view. 17. Cyrtus sp. larva in lateral view. 18. Cyrtus sp. larva emerging from Eratigena sp. in lateral view. (Photos: ANGEL MARÍA ARES)

Cyrtinae

Cyrtus gibbus (Fabricius, 1794) reared from Eratigena saeva
(Blackwell, 1844) (Agelenidae)
(Figs. 3–6)

Host spider (female) and parasitoid (female) collection data: Spain, Salamanca province, Puerto Seguro, Puente de los Franceses, 40.82707°N, 6.747936°W (WGS84), collected 27 May 1987 in between stones of a rubble wall, leg. H.-P. TSCHORSNIG, det. (host spider) A. REIMANN, coll. Stuttgart (SMNS-DIP-007715).

Note: at the time of collecting, the sluggish adult fly, which appeared to have eclosed from its pupa recently, was sitting on the spider remains.

Identification of parasitoid: following Sack (1936). However, the genus is in need of a taxonomic revision, as suggested by unpublished morphological and molecular data.

Identification of host: following the keys to the spiders of Europe by NENTWIG et al. (2023), using the structure of the female epigyne.

Figs. 19–20.

Male Ogcodes guttatus Costa, 1854 reared from Pseudicius sp. from Rhodes (Greece). 19. Dorsal view. 20. Lateral view. (Photos: MICHAEL SCHÄFER)

Cyrtus gibbus (Fabricius, 1794) reared from
Eratigena sp. (Agelenidae)
(Figs. 7–18)

Host spider #1 collection data (Figs. 7–14): adult male, Spain, Madrid, Galapagar, November 2011, leg. & det. A. M. ARES, no material preserved for further identification.

Note: the larva turned into a pupa a couple of days after eclosing from its host. The pupal period lasted for several days only. No additional information available.

Host spider #2 collection data (Figs. 15–18): adult female, Spain, Madrid, Galapagar, March 2013, leg. & det. A. M. ARES, no material preserved for further identification.

Note: the larva eclosed in March 2013 but did not develop into a pupa. No additional information available.

Ogcodinae

Ogcodes fumatus Erichson, 1846 reared from
Macaroeris nidicolens (Walckenaer, 1802)

Host spider collection data: adult female, Bulgaria, Batak Reservoir, 42.01151°N 24.19059°E (WGS84), collected 10 May 2018 in a dry and semiruderal habitat from underneath stones, leg. A. GRABOLLE, coll. M. SCHÄFER (voucher: BG2018-05).

Larva eclosed from host: 22 May 2018.

Fly (female) eclosed from pupa: no exact date recorded; approximately one week after the larva emerged from the host.

Identification of parasitoid: following Chvála (1980a, 1980b).

Identification of host: following Metzner (1999).

Figs. 21–22.

Female Ogcodes reginae Trojan, 1956 reared from Salticus sp. from Sardinia (Italy). 21. Dorsal view. 22. Lateral view. (Photos: Michael Schäfer)

Ogcodes guttatus Costa, 1854 reared from Pseudicius sp. Simon, 1885 (Salticidae) (Figs. 19–20)

Host spider collection data: juvenile, Greece, Rhodes, Kástro Monolithou, 36.12467°N 27.72628°E (WGS84), collected 2 November 2022 on Cupressus sp., leg. & coll. M. SCHÄFER (voucher: Ro2022-48).

Larva eclosed from host: 7 August 2023.

Fly (male) eclosed from pupa: 18 August 2023.

Identification of parasitoid: following Chvála (1980a, 1980b).

Identification of host: genus identification based on comparison with morphologically identical juvenile specimens obtained from the same locality and reared to adulthood.

Ogcodes reginae Trojan, 1956 reared from Heliophanus sp. (Salticidae)

Host spider collection data: juvenile, France, Corsica, Montemaggiore, 42.53636°N 8.873872°E (WGS84), collected 16 May 2016 next to a small, private swimming pool, leg. J. Guttenberger, coll. M. Schäfer (voucher: JG2016-10).

Larva eclosed from host: 20 June 2016.

Fly (female) eclosed from pupa: 24 June 2016.

Identification of parasitoid: following Chvála (1980a, 1980b).

Identification of host: genus identification based on comparison with morphologically identical juvenile specimens obtained from the same locality and reared to adulthood.

Fig. 23.

Adult Cyrtus sp. sitting on an egg-sac, presumably of Eratigena sp. (Photo: David Kohl)

Ogcodes reginae Trojan, 1956 reared from
Salticus sp. (Salticidae)
(Figs. 21–22)

Host spider collection data: juvenile, Italy, Sardinia, Gola di Su Gorroppu, 40.16298°N 9.507755°E (WGS84), collected 3 May 2013, leg. A. Grabolle, coll. M. SCHÄFER (voucher: Sa2013-7).

Larva eclosed from host: 21 June 2013.

Fly (female) eclosed from pupa: no exact date recorded; approximately one week after the larva emerged from the host.

Identification of parasitoide: following Chvála (1980a, 1980b).

Identification of host: genus identification based on comparison with morphologically identical juvenile specimens obtained from the same locality and reared to adulthood.

Discussion

Menge (1866) discovered the endoparasitic nature of acrocerid larvae over 150 years ago, but host-parasite data for these flies remain a scarce by-product of mostly arachnological studies. Gillung & Borkent (2017) summarised the available knowledge, comprising host records for at least 60 acrocerid species recorded from 25 spider families. Amongst their listings are 21 salticid hosts for at least 16 acrocerid species, and at least eight agelenid hosts for at least 13 acrocerid species. Tables 1 and 2 provide all known host spiders for the acrocerid species and all known acrocerid parasites for the spider species recorded in the current study (literature data from Gillung & Borkent [2017]). From the parasitoid's perspective, first host data could be obtained for Cyrtus gibbus. With the knowledge that the larvae of Cyrtus develop within Eratigena, an observation first made public in May 2015 on an internet platform dedicated to dipterology becomes most interesting (Kohl 2024). The thread shows an adult Cyrtus sp. sitting on the egg-sac of what is presumably an Eratigena specimen (Fig. 23). The photo was taken in June 2009 by David Kohl in the vicinity of Conques (France, Midi-Pyrénées), and raises the question whether female Cyrtus, stimulated by olfactory, actively seek out egg-sacs of suitable hosts in order to lay their eggs nearby or even on the egg-sacs. A similar behaviour was documented by Coyle (1971: 281f), who observed adult Eulonchus marialiciae Brimley, 1925 being attracted to freshly opened burrows of the trapdoor spider Antrodiaetus unicolor (Hentz, 1842) (Antrodiaetidae)—“… as if attracted by some chemical released by my digging”.

Winterton & Barraclough (2017) summarised the scarce documented knowledge on the oviposition behaviour of Acroceridae in their synopsis of the life history of spider flies in the following way: “Adult females oviposit large numbers of microtype eggs, either on branches and foliage, or the eggs are scattered during flight”. Koch (1872: 331) reported having found larvae of Acrocera sanguinea Meigen, 1804 within egg-sacs of the agelenid “Tegenaria agilis” in Tyrol: “Die Larve dieser Art findet sich in den schön gelben Eiersäcken der unter Steinen lebenden Tegenaria agilis Auss.” [The larva of this species can be found underneath stones in the beautiful yellow egg-sacs of Tegenaria agilis Auss.]. Furthermore, he stated, about Acrocera trigramma: “Ich fand die Larve dieser Art in einem Spinneneiersacke … Dieselbe entwickelte sich im Laufe einiger Tage zum vollständigen Insect.” [I found the larva of this species in an egg-sac … Within a couple of days it developed into a complete insect]. Subsequently, these observations were also cited by Brauer (1883: 61), König (1894: 163) and Kaston & Jenks (1937: 160). Acrocera trigramma is synonymous with A. sanguinea and Koch's “Tegenaria agilis” corresponds to Textrix denticulata (Olivier, 1769) in the current literature (Gillung & Borkent 2017). It has been documented that parasitized spiders produce silken sac-like structures (Montgomery 1903: 68), thick silk mats (Cady et al. 1993: 937) or irregular webbing (Kehlmaier et al. 2012: 284) shortly before the emergence of the acrocerid larva from their body, and that these are used by the parasitoid as a suitable site for pupation. Therefore, it might well be that Koch erroneously regarded such webbing as an egg-sac. However, the above documented contact of an adult Cyrtus with the egg-sac of a possible larval host species clearly depicts the limited knowledge of the life-history traits of Acroceridae in general and Cyrtus in particular.

Table 1.

Species of host spiders known for the acrocerid species recorded in this study. An asterisk marks host-parasite associations presented here. Other records taken from Gillung & Borkent (2017).

Table 2.

Species of acrocerid parasites known for the species of spiders recorded in this study. An asterisk marks host-parasite associations presented here. Other records taken from Gillung & Borkent (2017).