PROTHCRACIC REGICN of ACALYPTERATE DIPTERA: A) MORPHOLOGY, B) TAXCNOMIC SIGNIFICANCE
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PROTHCRACIC REGICN OF ACALYPTERATE DIPTERA: a) MORPHOLOGY, b) TAXCNOMIC SIGNIFICANCE. A Thesis submitted in part fulfilment of the requirements for the Degree of Doctor of Philosophy in the University of London. by Vartin Christopher Dean Oneight, B.Sc., A.R.C.C.I. Department of Zoology and Applied Entomology, Imperial College, Prince Consort Road, London, S.W.7. June, 1960. ABSTRACT. The prothoracic morphology of a wide range of Acalypterates has been examined, and is described in some detail. Conflicting terminologies for the various sclerites are discussed and a comprehensive system of nomenclature is proposed for them. The possible uses of prothoracic sclerites in Acalypterate systematics has been surveyed, leading to a detailed study of the probasisternum (or prosternum). The variability of the prosternum has been analysed, in an attempt to arrive at an understanding of the evolution of its shape and the relation between this and the evolution of Acalypterates. The methods by which it has been attempted to derive such information from modern forms are presented in full. The status and probable relationships of 23 Acalypterate families are discussed, comparing deductions made using prosternal shape with deductions based upon other characters. In conclusion it is suggested that variation in prosternal shape provides a reasonably reliable guide to Acalypterate inter-relationships, and could be especially useful in unravelling relations between families. Attention is drawn to the possible applications of methods used here in other studies, and to the differing information content of dimorphic and polymorphic taxonomic characters. CONTENTS. SECTION 1 : INTRODUCTION 5 - 8 SECTION 2 : MORPHOLOGY OF THE PROTUCRACIC REGION 9 - 30 2a) INTRODUCTION 9 2b) PREPARATION OF VATERIAL 9 2c) PREPARATION TECHNIQUE 9 - 12 2d) EXAMINATION OF EATERIAL 12 2e) THE SCLERITES OF THE PROTHORAX 12 - 29 (1)Pronotum 12 15 (2)7ropleuron 15 - 18 (3)Sternum 18 21 (4)Basisternal apodemes and musculature 21 - 24 (5)Cervical sclerites 24 - 26 (6)Cervical organ 26 - 29 2f) CONCLUSIONS 29 30 SECTION 3 : THE PROTHCRACIC BASISTERNUM AND PHYLCGENETIC SYSTEMATICS 31 176 3a) INTRODUCTION 31 33 3b) THE PRACTICABILITY OF USING THE PROSTERHUV AS A TAXONOMIC CHARACTER 33- 35 3c) CLASSIFYING VARIATION IN PROSTERNAL SHAPE 36 er. 37 3d) PROBABLE FUNCTIONS CF THE BASISTERNUM 37- 41 3e) VLRIABILITY OF THE BASISTERNUM 41 - 45 42 (1)Within the species 41 - 42 ••• 43 (2)Within the genus (3)Within the family 43 45 (4)Within the superfamily 45 3f) EVOLUTION CF BLZISTERNAL SHAPE IN ACALYPTERATES 45 - 57 3g) SYSTEMATIC SURVEY 57 - 176 (1)Introduction 57 - 59 (2)Conopoidea 59 - 64 (3)Sciomyzoidea 65 - 93 (4)Pallopteroidea 94 - 113 (5)Milichioidea 114 - 141 (6)Drosophiloidea 142 - 168 (7)Chloropoidea 169 - 174 (8)Summary 175 - 176 SECTION 4 : GENERAL DISCUSSION 177 - 183 SECTION 6 : ACITOVILEDGEIMIITS 184 - 105 SECTION 7 BIBLIOGRAPHY 186 - 194 4 APPENDIX ONE : GENERA EXAMINED 195 - 214 APPENDIX TWO : FIGURES 215 •- 255 - 5 - SECTION 1: INTRODUCTION According to Oldroyd (1964), about 80,000 species of Diptera are known to science. One of the largest sub-groups within the Order is the Acalypterae (or Acalypteratae), a section of the Sub-order Cyclorrhapha, with perhaps 20,000 species. The vast number of species involved, coupled with the fact that many are of small size, not superficially very attractive, but fragile and difficult to handle, has resulted in this being one of the least understood of insect groups. In this respect the Acalypterates occupy a position similar to that of the rarasitica within the Hymenoptera, or the Staphylinoidea among Coleoptera. There have been few morphological studies of the Dipteran thorax. Except in the case of Crampton's work (1942), these usually occur as part of a description of the general morphology of single species. Discussion of comparative anatomy is then unavoidably based upon the work of a number of different authors and a variety of interpretations (Braula provides an extreme case: see under Braulidae). Interest in the modifications undergone by the pterothorax to accommodate specialised flight musculature, has produced detailed discussion of its structure, at the expense of discussion of the pro- thorax. The attitude of most authors is summed up by Ferris (1950), who dismisses the prothorax with the remark "Serving merely as a support for the prothoracic legs". Prothoracic morphology of Acalypterates has been mentioned by Ferris (1950), Hassanein and Abd- el-Salam (1962) Nayar (1962), and Pandey and Agrawal (1962). Apart from Nayar (Dacus and Sphyracerhala), each of these authors has examined only a single species. Prothoracic structures have also been referred to by taxonomists. This has resulted in the development of a system of terminology independent of that used by morphologists, but which incorporates terms that have morphological connotations. A number of names are now available for most sclerites, causing confusion. Through the combined effects of these various factors, Acalypterate taxonomy continues to be a very great problem to systematists. In order to grapple with the group at all, most authors have specialised in the study of individual families, and even then only ten authors (Aczel, Frey, Hendel, Hennig, Keiser, Melander, Morge, Shewell, Steyskal and Tonnoir) have published papers that deal with any family on a World basis. So far, little work has been done on the ways in which relationships between families can be elucidated, with consequent confusion over family definitions. As Sabrosky (1960) says, "Family classification in the Acalypterates will be one of the areas of ferment in the years ahead". As yet only six people have tried to discuss the major classification of Acalypterates to any extent. Not suprisingly„ each attempt has produced very different results. A final complicating factor in the study of Acalypterates results from their greater abundance outside the Holarctic region than within it. Of the sixty-two families currently recognised, twelve are completely absent from the Holarctic. Conversely, of important authors at present working on Acalypterates, some twenty are based in the -7 - Holarctic, four in the Ethiopian, three in the Australasian and one each in both the Oriental and Neotropic regions. Because of this, Acalypterate family definitions are based principally on European faunas. As tropical and southern Acalypterates become better known, family definitions are constantly having to be reviewed, producing great instability in the literature. Five new family names have been introduced by reputable workers during the past ten years. Despite the more restricted fauna of the Molarctic, it is still inadequately known. Britain, with some 1,000 species in 44 families, has never had any comprehensive account, in English, of half of them. There is an obvious need for more work on Acalypterates, embracing all aspects of their study. The account of prothoracic morphology presented here is directed at establishing the part which prothoracic sclerites can play in unravelling some of the current problems of Acalypterate systematics. It should also provide indications of other potentially useful lines of investigation. During the course of this work,. some 2,150 species from all over the World, belonging to 800 genera and between them representing 59 families, have been examined. This is in the region of a 1O/ sample of all known species and involves a much higher proportion of known genera. The family names used here are those of kennig (1958), since his is the most recent comprehensive account of Acalypterate classification. In addition, the family names Somatiidae and Pseudopomyzidae have been incorporated, as being names introduced since 1958, by authors with knowledge of Hennig's work. - 8 - It has not been possible to check the validity of all the generic names used in the text. They refer to the species at present found under those names in the British museum (Natural History) at South Kensington. When apparent anomalies have been encountered, the tax- onomic position of the specimens involved has been checked. A list of all the genera examined appears in the appendix. SECTION 2: MORPHOLOGY OF THE PROTHORAOIC REGION 2a) INTRODUCTION In the following account each of the prothoracic sclerites is considered in turn. Those of the prosternal region are treated in the most detail, because they come in for further discussion in sub- sequent sections. Special attention has also been given to the cervical organ, since it has been ignored by morphologists previously, and also because of its apparent association with prosternal sclerites. Prothoracic musculature, apart from that associated with the probasi- sternum, is not considered. 2b) PREPLRATION OF MATERIAL Apparent sutures in dry specimens are not always what they seem, and may also be camouflaged by confusing pigmentation of the cuticle. For these. reasons, and also to make illumination easier, the thoracic content of the specimens used for morphological work was removed, and the cuticle itself then bleached and stained. Some of the material for this work came from spirit collections (in this case 50-70% ethanol with a little glycerine added), but by far the greater proportion was acquired dry. The preparative methods used proved equally successAul for specimens from either source. 2c) PREPARATION TECHNIQUE (1) To facilitate removal of muscles and other internal structures, specimens were left in hot 10% potassium hydroxide solution for about two hours. To aid penetration of the Potash into the thorax, the head and abdomen of each specimen was removed. The head of a dry specimen - 10 - had to be removed with care, so that the cervical scierites were not pulled away from the thorax at the same time. (2) After treatment in potash, specimens were transferred to water. Here the disc of the thorax was cut away, so that the remains of its soft parts could be extracted. (3) Bleaching: Specimens were left in tubes of diaphanol out of contact with light, for about twelve hours. This was usually sufficient to render the cuticle thoroughly transparent.