(Diptera) Larvae: a Brief Review and a Key to European Families Avoiding Use of Mouthpart Characters
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1 Article Family-level keys to freshwater fly (Diptera) larvae: a brief review and a key to European families avoiding use of mouthpart characters Michael Dobson1 1 Affiliation: Freshwater Biological Association, The Ferry Landing, Far Sawrey, Ambleside, Cumbria, LA22 0LP, UK.* *Present address: APEM Ltd, The Technopole Centre, Edinburgh Technopole, Milton Bridge, Nr Penicuik, Midlothian, EH26 0PJ, UK. Email: [email protected] Received 3 October 2011; accepted 10 April 2013; published 18 June 2013 Abstract Identification of larvae of aquatic Diptera (true flies) is complicated by a range of factors, including reliance on examination of mouthparts at the very beginning of many keys, as well as an unclear distinction between aquatic and terrestrial habits for many species. Even at family level, these can cause problems. This review briefly introduces the history of keys to Diptera larvae, with particular reference to Europe, and identifies use of characteristics that may cause problems for the non-specialist, along with attempts made to mitigate these problems. It considers the validity of using identification features that do not require examination of mouthparts, giving examples of those used in previous keys. It then reviews Diptera families that are not normally considered to have aquatic representatives, concluding that in Europe six new families need to be added to the list of freshwater fauna: Cecidomyiidae (rivers and tree holes), Scatopsidae (tree holes), Anisopodidae (tree holes), Bibionidae (probably wetlands), Pachyneuridae (saturated dead wood) and Lonchopteridae (river edges). Elsewhere in the world, a further nine families have been recorded from fresh waters, all from submerged dead wood or from water-filled plant structures such as pitcher plants and tree holes. A dichotomous key to European families of aquatic Diptera is included, based purely on external morphology without reference to mouthparts, and incorporating the six families highlighted above. This key is copiously illustrated with line drawings, mainly of whole animals. Keywords: Aquatic Diptera larvae; diagnostic key; dichotomous key; external morphology; macroinvertebrate identification; synoptic keys. Introduction Diptera (true fly) larvae are by far the most diverse group, and the ones encountered in the widest variety Among the macroinvertebrates found in fresh waters, of habitats, from large lakes to tree holes and temporary DOI: 10.1608/FRJ-6.1.450 Freshwater Reviews (2013) 6, pp.1-32 © Freshwater Biological Association 2013 2 Dobson, M. pools, plus all types of running waters. Unfortunately, head so that it becomes much less obviously recognisable for the non-specialist they can also cause some of the as such. biggest difficulties in identification. For a few families The first group, the Nematocera, contains those families and geographical regions there are comprehensive keys to whose larvae have a fully developed head capsule, with species (e.g. Bass, 1998; Disney, 1999), but for most, keys mandibles that move opposably (in a horizontal plane, are out of date, incomplete or even simply absent. pincer-like – see Fig. 1a); most of its families have an affixed Furthermore, definitive identification even at the head but the Tipuloidea (containing the families Tipulidae, level of family can be problematic. The systematic Limoniidae, Pediciidae and Cylindrotomidae) are able to characteristics used to differentiate key groupings within retract their heads into the thorax. A clear exception to the the order, and therefore to produce a taxonomically general structure is the family Cecidomyiidae, whose head accurate key, require examination of the morphology of the heads, and particularly the mouthparts, which a) may not be possible without dissection. Probably the majority of people having to identify freshwater Diptera are those working for statutory water agencies in routine monitoring, and considerable experience by the author in providing identification training for such organisations has demonstrated that distinguishing different mouthpart types is a consistent issue, as few of their staff have the appropriate knowledge or experience of dissection techniques. Additionally, relatively few students embarking on ecological studies have such training, jeopardising effective identification and recording of aquatic Diptera. This review attempts to address this issue by developing an alternative method for identifying freshwater Diptera b) larvae. It initially considers the morphological problems encountered when producing a simple key to aquatic Diptera families, and then briefly reviews the families that occur in fresh waters, with particular reference to those that are not normally considered to be part of the aquatic fauna. Finally, it presents a key to European freshwater Diptera families, avoiding the morphological features that cause the non-specialist so much trouble when using most of the currently available keys to the group. Features of taxonomic importance in identifying larval Diptera The order Diptera is divisible into three groups, based on structure of the head and mouthparts. These represent an Fig. 1. (a) Pediciidae, dorsal view, highlighting opposing mandibles (m) of Nematocera, paired posterior respiratory evolutionary trend across the group of loss of sclerotisation spiracles (sp) and prolegs (pr); (b) Tabanidae: head and front of (hardening) of the head capsule, and modification of the thorax, ventral view, showing parallel mandibles of Brachycera. © Freshwater Biological Association 2013 DOI: 10.1608/FRJ-6.1.450 Family-level keys to Diptera larvae 3 capsule and mandibles are vestigial. The second group, Approaches used by keys to the Brachycera, contains families whose head capsule is freshwater Diptera families reduced but still clearly identifiable as such, along with mandibles that are parallel, and operate in a vertical Identification keys come in two basic types. Synoptic keys plane (Fig. 1b); almost all have heads that are retractable are those which distinguish taxa on the basis of characters into the body cavity, although there is one exception, that reflect evolutionary relationships and are therefore the Stratiomyidae. The third group is the Cyclorrhapha, used in taxonomic classification. Diagnostic keys use also with mandibles that are parallel, but in which artificial distinctions: those that are more straightforward the head capsule is so reduced as not to be obviously to identify, even if they have no role in determining identifiable as such. An important differentiation between evolutionary relationships. The advantage of synoptic Brachycera and Cyclorrhapha, albeit one that requires keys is that the features they use are definitive, but while high magnification and dissection, is the structure of they may be obvious they may equally be obscure or the pharyngeal skeleton, sclerotised rods that project difficult to see. Diagnostic keys are designed to be as from the head into the body; among Cyclorrhapha the easy as possible, but can be confusing when a specimen is branches are fused towards the front (Fig. 2), whereas among Brachycera they can move relative to each other. a) In practice, however, if you are dissecting a brachyceran, an identifiable head capsule, usually with obvious eye spots, will tell you that it belongs to this group, and further dissection to find the pharyngeal skeleton is not necessary. These divisions are based on the larval characters that were used by Brauer (1883), who first classified Diptera into suborders. His original classification saw the Cyclorrhapha as a suborder, while the Nematocera and Brachycera were divisions of a second suborder, the Orthorrhapha. The two names derive from a further differentiation identified by Brauer: the mechanism by which the emerging adult escapes from its pupal skin. Adults of Cyclorrhapha (meaning ‘circular seamed’) escape from a semi-circular aperture, while those of Orthorrhapha (‘straight b) seamed’) escape from a T-shaped aperture. Modern classifications continue to use these names, but have altered their relationships somewhat, with the Nematocera considered a suborder, while the Cyclorrhapha and Orthorrhapha are generally considered to be subdivisions within a second suborder, the Brachycera (Table 1). Fig. 2. Pharyngeal skeleton of Limnophora (Muscidae): (a) head and thorax, with the pharyngeal skeleton visible as a dark feature within the thorax; it is rarely this obvious without dissection; (b) the pharyngeal skeleton partially dissected; the length of this structure is approx. 1 mm. DOI: 10.1608/FRJ-6.1.450 Freshwater Reviews (2013) 6, pp. 1-32 4 Dobson, M. Table 1. Diptera families normally considered to have aquatic representatives. Families in bold occur in Europe. a) Nematocera Blephariceridae Exclusively aquatic Ceratopogonidae Peripherally aquatic Chaoboridae Exclusively aquatic Chironomidae Mostly aquatic Culicidae Exclusively aquatic Deuterophlebiidae Exclusively aquatic Dixidae Exclusively aquatic Nymphomyiidae Exclusively aquatic Psychodidae Peripherally aquatic Ptychopteridae Exclusively aquatic Simuliidae Exclusively aquatic Tanyderidae Exclusively aquatic Tipuloidea Cylindrotomidae Terrestrial and aquatic Limoniidae Mostly terrestrial; some peripherally aquatic Pediciidae Mostly aquatic Tipulidae Mostly terrestrial; some aquatic Thaumaleidae Exclusively aquatic b) Brachycera – “Orthorrhapha” Athericidae Mostly terrestrial; some aquatic Empididae Mostly aquatic or peripherally aquatic Rhagionidae Mostly terrestrial;