Royal Entomological Society HANDBOOKS FOR THE IDENTIFICATION OF BRITISH INSECTS Out-of-print handbooks may be downloaded from http://www.royensoc.co.uk/publications/index.htm This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 2.0 UK: England & Wales License. Copyright © Royal Entomological Society 2014 ROYAL ENTOMOLOGICAL SOCIETY OF LONDON Vol. IX. Part 4 HANDBOOKS FOR THE- Ii:>E,WTIFlCATION OF BRITISH INSECTS DIPTERA BRACHYCERA SECTION (a) TABANOIDEA AND ASILOIDEA By HAROLD OLDROYD LONDON Published by the Society and Sold at its Rooms .p, Queen's Gate, S.W. 7 12 December, 1969 Price £1 I)S. od. HANDBOOKS FOR THE IDENTIFICATION OF BRITISH INSECTS The aim of this series of publications is to provide illustrated keys to the whole of the British Insects (in so far as this is possible), in ten volumes, as follows: I. Part 1. General Introduction. Part 9. Ephemeroptera. , 2. Thysanura. , 10. Odonata. , 3. Protura. , 11. Thysanoptera. , 4. Collembola. , 12. Neuroptera. , 5. Dermaptera and , 13. Mecoptera. Orthoptera. , 14. Trichoptera. , 6. Plecoptera. , 15. Strepsiptera. , 7. Psocoptera. , 16. Siphonaptera. , 8. Anoplura. 11. Hemiptera. Ill. Lepidoptera. IV. and V. Coleoptera. VI. Hymenoptera : Symphyta and Aculeata. VII. Hymenoptera : Ichneumonoidea. VIII. Hymenoptera : Cynipoidea, Chalcidoidea, and Serphoidea. IX. Diptera: Nematocera and Brachycera. X. Diptera : Cyclorrhapha. Volumes 11 to X will be divided into parts of convenient size, but it is not possible to specify in advance the taxonomic content of each part. Conciseness and cheapness are main objectives in this series, and each part is the work of a specialist, or of a group of specialists. Although much of the work is based on existing published keys, suitably adapted, much new and original matter is also included. Parts are issued, separately paged and priced, as they become available. A second (revised) edition of A Check List of British ]Meets, by G. S. Kloet and W. D. Hincks, is being issued as an extra, eleventh, volume in this series. The Society is indebted to the Royal Society for a grant towards the cost of initiating this series of Handbooks. A list of parts now available appears on the back cover. Sole Agent :- E. W. Classey, 353, Hanworth Road, Hampton, Middlesex. DIPTERA BRACHYCERA SECTION (a) TABANOIDEA AND ASILOIDEA BY HAROLD 0LDROYD INTRODUCTION THE evolution of Diptera has clearly followed two main lines, one leading to Nematocera, and the other to all the rest, which are "Brachycera" in the broad sense (see Vol. IX (1) : 31). The second line of evolution led first to a sequence of more primitive families which have become stabilized, and (with few exceptions) are easy to separate from each other; and then to a complex of more advanced families (Cyclorrhapha) which appear to be much less stabilized, and which present many problems at the family level of classification. We are concerned here with the first group, Brachycera in the narrow sense, and not with all of these. The families Empididae and Dolichopodidae, especially the latter, foreshadow the Cyclorrhapha to some extent, and this fact, together with their smaller size and greater abundance of species, makes it convenient to leave them for individual treatment by specialists. The families dealt with in this part of the Handbook are those that were reviewed by Verrall in British Flies, V (1909), who dealt exhaustively with them, giving extremely detailed descriptions, as well as long discussions of synonymy and distribution. When the Handbook series was planned, therefore, it seemed unnecessary to cover these families until prior attention had been given to others, less well known. In the intervening 20 years, however, Verrall's book has become more difficult to obtain, while extensive work has gone on in some families, notably Tabanidae. The time now seems ripe for a Handbook on the "Verrall" families of Brachycera. These comprise:- T ABANOIDEA STRATIOMYIDAE, XYLOMYIDAE, XYLOPHAGIDAE, RHAGIONIDAE, TABANIDAE ASILOIDEA AsiLIDAE, THEREVIDAE, ScENOPINIDAE, AcROCERIDAE, BOMBYLIIDAE The segregation of families into two ~uperfamilies is partly structural but mainly . -biological. The fission · between Tabanoidea and Asiloidea is a fundamental one, which ought to receive more prominence than it does. In my book on The Natural History of Flies (1964) I stressed a dichot9my that runs through.Diptera. Starting from an ancestor which probably lived in 1 2 IX (4). DIPTERA: T ABANOIDEA and ASILOIDEA moss, or in damp, rotting wood, flies seem to have segregated into those which moved into more fully aquatic habitats and those which became more terrestrial. If we set aside Tipulidae as a basic group still showing great diversity of larval habitat, in materials of varying degrees of humidity, the rest of Nematocera fall into land midges which cluster round Bibionidae, Myceto­ philidae, etc., and water midges and gnats which include Culicidae, Simuliidae, Ceratopogonidae and Psychodidae. The latter group primarily suck blood, in females only, by use of stylet-like mandibles and maxillae, though the habit has been lost in Chironomidae and Psychodidae (except for Phlebo­ tominae). Coming now to Brachycera, we see that Tabanoidea invite comparison with the water midge group, though mandibles have again been lost in a few Tabanidae, the majority of Rhagionidae, and of course all Stratiomyidae. Many Rhagionidae are terrestrial (seep. 36). Asiloidea, on the other hand, are a definitely terrestrial group. They have lost all trace of mandibulate mouthparts, and although both sexes of adult Asilidae suck the blood of insects, they have evolved an entirely different equipment consisting of the sharpened hypopharynx ensheathed in the labium. The further development of specialized structures on legs and head which enable the Asilid to catch prey, and defend itself against retaliation, show that there is a long evolutionary history behind the Asilidae. Related families have not adopted this predatory habit, and have evolved along totally different lines; Bombyliidae, for example, have adapted not only the proboscis but the whole body to flower-feeding, as will be seen by comparing their legs with those of any Asilid. Larvae of this group, though carnivorous, are terrestrial, either living in earth, or being scavengers in nests, or internal parasites. From this point on, Diptera as a whole are terrestrial, and the few groups that have evolved aquatic larvae (e.g. Syrphidae, Ephydridae) have clearly done so by secondary return to the water. One or two anomalies occur. Acroceridae have a pulvilliform empodium (three-padded foot), and for this reason are often keyed out with Tabanoidea as "homeodactyla", while Asiloidea with two pulvilli and a bristle-like empodium are "heterodactyla". Biologically it seems much more reasonable to consider Acroceridae as being closely related to Bombyliidae, and therefore as Asiloidea which have retained the primitive pulvilliform empodium. Scenopinidae are always placed next to Therevidae, because their larvae are astonishingly similar: elongate, wormlike, with secondary subdivision of the abdominal segments. Even the head structure of the larva is similar in the two families, with a peculiar internal process which is spatulate in Therevidae and simple in Scenopinidae. Yet the adults of the two families are totally different. Therevidae are close to being the stem-group of Asiloidea, and it is possible that Scenopinidae form a link between them and Stratiomyidae, but at the present time there is little that can usefully be said about this. Finally, Stratiomyidae themselves are an enigmatic family. It is customary to place them at the beginning of the Brachycera, yet they are certainly not a primitive family. In head structure, wing venation, and larval habits they give evidence of having existed for a long time. It is my own view that Stratiomyidae are an ancient family, long past its prime, INTRODUCTION 3 possibly suffering from the competition of the more efficient Syrphidae. It is perhaps misleading to place Stratiomyidae in Tabanoidea, though they are linked with Rhagionidae through XylomyidaefXylophagidae, and possibly both groups might trace back to some ancestor living in decaying wood. It should be borne in mind, however, that Stratiomyidae stand apart from all the others. Volume IX, Part 1: Introduction and Key to Families discussed the struc­ ture of Brachycera, and only a few points need be mentioned h~re. SrzE.-One criticism of the Introduction in this series was that I did not give sufficient indication of the size of the flies mentioned. Size is an obvious characteristic of a species, but a deceptive one, and a measurement in milli­ metres gives an appearance of precision that is often not justified. Shape, surface structure and pilosity all produce an optical illusion of relative size and shape that may be difficult to substantiate by actual measurements. An example is Tabanus autumnalis, which in practice is distinctive because of its intermediate size, too big for the bromius-group, but not big enough for the bovinus-group; yet the range of variation in autumnalis spans the two. In the present Part of the Handbook, size is mentioned only when it is a positive help in identification, e.g. Sargus minimus, and the smaller Bombyliidae. GENITALIA.-One development in taxonomy since Verrall's time has been the use of genitalia, especially external genitalia of males, as a specific character. This is now such standard practice in all groups of insects that it may be thought surprising to mention it. Yet the use of genitalia as a specific-or even a generic~character is not as simple as it is often made to seem. The genital armature of male Brachycera is often bulbous, dark, and covered with hairs. It is difficult to draw in situ, and when dissected the separated parts cannot be mounted flat, nor accurately drawn in two dimensions. For the most part the male genitalia of Tabanoidea are simple and show few taxonomic differences. In the case of Tabanidae themselves, the females are so much more in evidence that female genitalia have been studied extensively, but only occasionally is a really significant difference to be seen: e.g.