Royal Entomological Society HANDBOOKS FOR THE IDENTIFICATION OF BRITISH INSECTS To purchase current handbooks and to download out-of-print parts visit: 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 2012 ROYAL ENTOMOLOGICAL SOCIETY OF LONDON Vol. X. Part s (g) HANDBOOKS FOR THE IDENTIFICATION OF BRITISH INSECTS DIPTERA AGROMYZIDAE By KENNETH A. SPENCER LONDON Published by the Society and Sold at its Rooms ..p, Queen's Gate, S.W. 7 July 1972 Price £2.oo 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. , ll. 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: lchneumonoidea. 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 Insects, 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 so far published appears on the inside and outside back covers. DIPTERA Family AGROMYZIDAE By K. A. SPENCER SYSTEMATIC POSITION OF THE AGROMYZIDAE THE Agromyzidae belong to the section Acalyptratae of the suborder Cyclorrhapha Schizophora. Within this group the family occupies an isolated position. Hennig (1958) considered the most closely related family to be the Odiniidae; however, in a recent study of the relationships of the Cyclorrhapha undertaken by Griffiths (in press), he concludes that the sister­ group of the Agromyzidae is probably the Clusiidae, although the morpho­ logical gap between these two families is still considerable. The larvae of Clusiidae feed mainly on rotting wood, which could be considered as a link to the most primitive genus of the Agromyzidae, Phytobia Lioy, the larvae of which feed as borers in branches or trunks of young trees. GENERIC STRUCTURE OF THE AGROMYZIDAE The family consists of two well-defined subfamilies, the Agromyzinae and Phytomyzinae, which can be readily distinguished both in the adult and larval stages. In the Agromyzinae the sub-costa is fully developed and joins vein R 1 before this reaches the costa (fig. lA); most species are large and stout, with wing length of about 3 mm., and the costa generally extends strongly to vein M 1+2· In the larvae there is a third, upper arm of the cephalo-pharyngeal skeleton (fig. 2A) . In the Phytomyzinae the sub-costa is greatly reduced, frequently being little more than a fold, running parallel to the sub-costa and joining the costa independently (fig. lB); in several genera the costa is reduced and terminates at the apex of vein R4+5· The largest species in the family occur in the primitive genus, Phytobia Lioy, but many species are smaller, more slender and even minute, with wing length of less than 2 mm. The cephalo-pharyngeal skeleton of the larva has only two arms (fig. 2B). At the present time the family consists of 28 genera, of which 17 occur in Britain. Important changes in generic structure have been made in recent years. Hendel's conglomerate genus, Dizygomyza (1931), has now been split into a number of monophyletic genera and a further major advance was made when Nowakowski (1962) combined several genera and subgenera in the natural genus Cerodontha Rondani, to embrace the various groups feeding on monocotyledons, whose close relationship had not been detected by earlier workers. Finally, the genus Amauromyza Hendel has been en­ larged to include a number of groups whose relationship is clearly indicated by their male genitalia (cf. Nowakowski, 1962, 1964, and Spencer, 197la). Further splitting of the genera Agromyza Fallen and Phytomyza Fallen a 2 X (5). DIPTERA : AGROMYZIDAE seems probable in future but caution will be required when revising Phyto­ myza, as many of the disparate groups which can now be detected in associa­ tion with particular host families should probably at most be treated as sub­ genera or possibly deserve no more than species-group status. LIFE HisTORY All Agrornyzidae are exclusively phytophagous (despite some reports to the contrary) and virtually all parts of plants are used for larval feeding. The egg is inserted into green tissue of young trees, sterns, leaves or the in­ florescence of herbaceous plants and on hatching the larva eats out a channel in the carnbiurn, pith or parenchyma, frequently forming a characteristic feeding track or mine, which may provide a more reliable means of identifica­ tion than the external characters of the adult (see below). The young larva normally hatches after a few days and the time spent in this stage can vary from about 10 days to several months. Overwintering as a larva is unusual but occurs in some stem-borers and also in the common leaf-miner of holly, Phytomyza ilicis Curtis. A B A B 2 FIG. I.-Sub-costa of (A), subfamily Agromyzinae; (B) subfamily Phytomyzinae. FIG. 2.-Cephalo-pharyngeal skeleton of (A), subfamily Agromyzinae; (B), subfamily Phytomyzinae (after H ering). The pupal stage is normally 10-20 days in the early, summer generation but in species with a single generation or an additional autumn generation may be as long as 6-9 months. In most species the larva transforms into a pupariurn immediately on leaving the mine and falls to the ground. In a few cases the pupariurn can be loosely or firmly glued to the leaf near the enrl of the mine; in rather more species pupation takes place within the mine or, in the case of stem-feeders, beneath the epidermis or within the hollow stern. INTRODUCTION 3 DIAGNOSTIC CHARACTERS (a) Early stages Agromyzid larvae are generally whitish yellow but in some species more orange, varying in length from a few millimetres up to 2 cm. in the unusually narrow and elongate tree-boring species. The cephalo-pharyngeal skeleton, to which the mouth-hooks are articulated, provides an immediate indication of the subfamily (fig. 2A, B) but is not reliable for separating closely-related genera. The mouth-hooks consist of a rigid, paired structure, with the two halves normally arranged asymmetrically, so that in lateral view the mouth-hooks appear to alternate (figs. 241, 243). There is little specific variation in the mouth-hooks and many species can be reliably separated on this character. Both the anterior and posterior spiracles provide valuable diagnostic characters but the latter show greater differentiation. The spiracular processes may lie on the surface of the anal segment on scarcely defined plates or may be raised on conspicuous and characteristic stalks or protuber­ ances (figs. 13, 19). In the primitive arrangement there are three spiracular openings or bulbs on each process (fig. 13) but very commonly these may proliferate up to 10, 15, 20, 30 or even more. With the larger numbers there is inevitably some variation but different larvae, bearing for example 3, 10, 20 or 40 bulbs will certainly represent distinct species. De Meijere (1925 et seq.) has described and illustrated larval structures for many European species in a series of papers in the Tijdschrift voor Ento­ mologie, Amsterdam. The essential larval characters are detectable in the puparium, although it may not be possible to make such an accurate count of spiracular bulbs. The mouth-hooks and cephalo-pharyngeal skeleton are also preserved in the puparium, attached within the first segment (if this has not been lost on emergence of the adult). Important additional characters are provided by the colour of the puparium, which may be white, grey, brown, orange, dull or shining black. The shape is also significant--flattened, cylindrical or ovoid (figs. 331, 341). (b) External characters of adult The main characters used in this Handbook are indicated in fig. 3, showing an adult fly in side view, together with the wing. Identification of adults, apart from the male genitalia, is dependent on an analysis of three main groups of characters, namely, the chaetotaxy, particularly of the head and mesonotum; the colour of the frons, orbits, antennae, palps, mesonotum, scutellum, legs and, less frequently, abdomen; finally, the size and shape, particularly of the head and third antenna! segment, and length of the wing. HEAD.-There are normally between three and six orbital bristles divided into two groups. The upper orbital bristles (ors) are generally inclined up­ wards or slightly outwards; the lower orbital bristles (ori) may be inclined partly upwards but also frequently inwards. The strength and number of these bristles has great taxonomic significance, although there can be slight variation within species. There are usually two upper orbital bristles, which 4 X (5). DIPTERA : AGROMYZIDAE may be equal in length, or the upper one is frequently shorter or sometimes entirely lacking.