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Trans. Suffolk Nat. Soc. 41 (2005)

STURMIA BELLA (MEIGEN) (DIPTERA: TACHINIDAE) AN INTERESTING ADDITION TO THE SUFFOLK LIST

DAVID R. NASH

Tachinids are extensively and strongly bristled two-winged true flies (Diptera) closely related to our familiar “blue-bottles” and house flies with over 260 species currently recorded from Britain. They are popularly known as “Parasitic Flies” because their larvae feed on the internal body tissues of immature or adult invertebrates obtaining their oxygen by tapping into their host’s respiratory system with the chosen host eventually dying when the last of its vital organs are consumed. Most tachinids attack the larval stages of their chosen host with the caterpillars of butterflies and moths being especially favoured although some species attack other groups such as beetles and true bugs.

Tachinids lay their eggs in a variety of ways. Some deposit their eggs on the host’s food where the eggs are either ingested by the host or else hatch into tiny larvae which then clamber onto a host when it comes into proximity. Others either inject eggs directly into the host’s body or else lay them on parts of the host where they are difficult or impossible to remove e.g. the head.

It was originally thought that all tachinids choosing the latter method merely buzzed around their victims and then, when opportunity arose , darted in and rapidly deposited their eggs with the ovipostor protruding in a simple fashion from the apex of the abdomen. It was subsequently discovered however that, for some species at least, egg-laying was an extremely precise, delicate and unhurried process. Sharp (1899, p. 507) quotes from a fascinating account of how one North American tachinid was observed sitting on a leaf facing a caterpillar feeding about 5 mm away and which had already got five fly eggs stuck on one of its eyes.:

“Seizing a moment when the head of the larva was likely to remain stationary, the fly stealthily and rapidly bent her abdomen downward and extended from the last segment what proved to be an ovipostor. This passed forward beneath her body and between the legs until it projected beyond and nearly on a level with the head of the fly and came in contact with the eye of the larva upon which an egg was deposited”.

The term “Parasitic Flies” is somewhat of a misnomer; they should really be termed parasitoids since true parasites (e.g. fleas and roundworms) do not kill their hosts. In butterflies and moths, the pupa (chrysalis) develops inside the body of the full-grown larva (caterpillar) and is revealed when the last larval skin is sloughed; in the Tachinidae and related fly families, the last larval skin hardens and the true pupa is formed inside this elongate-oval, seed-like, brown/black casing, with the whole structure being known as the puparium (plural, puparia).

On 23 August 2004 I found two puparia under the basal leaves of a isolated weed growing close to my bungalow wall in an expansion joint in my drive at Brantham, East Suffolk (O. S. grid reference TM1134). Suspecting they might be those of a tachnid and knowing that the biology of many of our species is still poorly known, they were retained. On 26 August, a medium

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sized (ca 8 mm) black tachinid fly with a grey dusting and orange abdominal side patches hatched from one of the puparia (Fig. 1) and a few weeks later the other produced a host of tiny, ca 1 mm long, parasitic ichneumon wasps.

Figure 1. Sturmia bella (Meigen) Male. © The Natural History Museum.

As the fly did not resemble any other tachinid which I had encountered, it was sent together with a puparium (and a sample of the wasps) to the Natural History Museum, London where it was identified as Sturmia bella (Meigen)(det. N. Wyatt) a well-known, common parasitoid of vanessid butterflies on the continent. The species was first noted in this country in July, 1998 when it was reared from pupae of the Peacock (Inachis io L.) at Southampton, Hampshire (Ford, Shaw & Robertson, 2000). It is probably a recent arrival in this country rather than an overlooked one and may have been released here following the importation of caterpillars from abroad for breeding purposes. As can be seen from the accompanying map showing the currently known distribution, S. bella has spread quite rapidly in the southern half of the country. The dot shown for SX97 is based upon previously unpublished data viz.:

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1 female, Dawlish, Devon (SX9777); host Comma (Polygonia c-album) coll. 25. viii. 2003 on Elm (Ulmus); parasite larva emerged 30. viii. 2003 – one of two ex host pre-pupa; fly emerged 12. ix. 2003.

1 male, Dawlish, Devon (SX9577); host Small Tortoiseshell (Aglais urticae) coll. 31. viii. 2003; parasite larva emerged 8. viii. 2003 – one of two ex host pre-pupa; fly emerged late viii. 2003 (both specimens leg. A. A. Allen, det. N. P. Wyatt; in coll. Nat. Hist. Mus.).

S

S

F

Map showing current distribution of Sturmia bella (Meigen) (F = First UK record; S = Suffolk records).

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It is almost certain that the two Brantham puparia originated from pupae of Small Tortoiseshell (Aglais urticae L.), several of which were suspended from the eaves of the bungalow almost directly above the collection site. Extrapolating from the Dawlish data, which seems to indicate that normally two S. bella larvae develop within a single host caterpillar, it is highly probable that both Brantham puparia also had a similar common origin.

Chandler, Ford, & Falk (2001) reported the discovery of S. bella at Center Parcs, Elveden [TL8080] on 28 May 1999 so the fly is now known from both of our vice counties.

Of equal interest but of greater significance than the fly itself, were the parasitic ichneumon wasps (34 females, 13 males) which exited through a single hole in the second puparium (Fig. 2). These proved to be Trichopria nigra (Nees )(= inermis Kieffer) (Diapriidae) (det. D. Notton), a common species of the western Palaearctic (including the U.K.) with a range extending at least as far east as Iran and Kazakhstan. It is a relative of the well-known Apanteles glomeratus (L.) whose larvae develop inside caterpillars of the Large White (Pieris brassicae L.) and which when mature bore out of the fully grown caterpillar (usually as it is spinning up on a fence etc. in preparation for its change into a chrysalis) and pupate inside individual, yellow, silken cocoons spun on and about the remains of the host. According to Nixon (1980), T. nigra is a gregarious endoparasitoid of the pupae (within the puparium) of a range of cyclorrhaphan Diptera including the common “Green-bottle” Lucilia sericata (Meigen). By analogy with most other Trichopria, it probably attacks its host at the point when the puparium is forming i.e. whilst it is still soft and easy for the female to drill into to lay its eggs. Records of the wasp from tachinids are rare with this appearing to be the first from Sturmia bella (see Nash, 2005).

Figure 2. Trichopria nigra (Nees) showing antennal difference between the sexes and size range in females. Adrian Chalkley

♂ ♀ ♀

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The fly as well as voucher specimens of the wasps have been deposited at the Natural History Museum.

Nixon (op. cit.) points out that “information about the biology of the Diapriinae and their host-preferences is meagre in the extreme”. A similar comment might almost equally well be applied in relation to the biology of many of our tachinids. Although there are good modern keys and an excellently organised British Tachinid Recording Scheme with website (www.tachinidae.org.uk), the family has not attracted the same attention as, for example, the hoverflies (Syrphidae) and our list of known Suffolk species is woefully out of date. It is hoped that this account may stimulate someone in the society to undertake a special study of this fascinating but neglected family.

Acknowledgements I thank the following: Howard Mendel (Collections Manager, Natural History Museum, London ) for arranging identification of the specimens; David Notton and Nigel Wyatt for identification, helpful information (including the Dawlish records) and for organising the photograph of S. bella; Chris Raper and Matthew Smith (Tachinid Recording Scheme) for permission to reproduce the map of S. bella and additional details of the Elveden capture; Adrian Chalkley for his photographs of my mount of the wasps.

References Chandler, P., Ford, T., & Falk, S. (2001). Blepharipa schineri (Mesnil) new to

Britain and notes on recent additions to the British Tachinidae (Diptera). Dipterists Digest 8: 11–17.

Ford, T., Shaw, M., & Robertson, D. (2000). Further host records of some West Palaearctic Tachinidae (Diptera). Entomologist’s Rec. J. Var. 112: 25–36.

Nash, D. R. (2005). Trichopria nigra (Nees) (Hym.:Diapriidae) reared from Sturmia bella (Meigen) (Dipt.:Tachinidae) – a new host record. Entomologist’s Rec. J. Var. 117: 167–168.

Nixon, G. (1980). Handbooks for the Identification of British Insects . Diapriidae (Diapriinae) Hymenoptera, Proctotrupoidea part 8 (3di). Royal Entomological Society of London.

Sharp, D. (1899). The Cambridge Natural History vol. 6 – Insects. Part 2. London: Macmillan and Co.

David R. Nash 3 Church Lane Brantham Suffolk CO11 1PU