Bul. ent. Res. 78, 317-328 317 Published 1988 The parthenogenetic midge of water supply systems, Paratanytarsus grimmii (Schneider) (Diptera: Chironomidae) P. H. LANGTON 3, St Felix Road, Ramsey Forty Foot, Huntingdon, Cambs., PE17 1YH, UK P. S. CRANSTON ANIC, Division of Entomology, CSIRO, GPO Box 1700, Canberra, ACT 2601, Australia and formerly Entomology Department, British Museum (Natural History), Cromwell Road, Lon don, SW75BD, UK P. ARMITAGE Freshwater Biological Association River Laboratory, East Stoke, nr Wareham, Dorset, UK Abstract Chironomid midges have been known to include parthenogenetic species for over a century. One of these species, Paratanytarsus grimmii (Schneider), cited under several different names here shown to be junior synonyms, has attained some notoriety as a pest. Its occurrence as a supposedly paedogenetic (actually pharate adult parthenogenetic) inhabitant of water distribution systems is discussed and related to its more usual occurrence in a variety of small water bodies including aquaria. New synonymy is proposed and a lectotype designated. Introduction For over a century it has been known that some Chironomidae (non-biting midges) are able to reproduce parthenogenetically (Grimm, 1870, 1871; Zavfel, 1907; Johannsen, 1910). The phenomenon remained a scientific curiosity, albeit well studied, until the late 1930s, when a chironomid midge, widely distributed through the drinking water supply system of an unnamed German city, was found to be parthenogenetic. The pharate adult was able to lay fertile eggs within the pupa, a phenomenon sometimes erroneously referred to as paedogenesis. As a sanitary problem, this provoked intensive study, detailed by Kruger (1941), but the infestation disappeared only when the water froze within the mains pipes. Larvae and pupae of parthenogenetic midges within the water supply recurred in eastern England in the 1970s (Williams, 1974) and was recognized as widespread in southern England within the decade. In 1986, an infestation occurred in Cyprus and there is accumulating evidence that the problem is widespread in other countries. Taxonomic background Identification of the species involved has proved troublesome, partially because there are no males, the stage upon which traditional taxonomic judgements have been made. Additionally, it has proved difficult to trace specimens examined by previous workers. Furthermore, too great an emphasis was given previously to the failure of the nuisance 318 p, H, LANGTON, p, S, CRANSTON and p, ARMITAGE species to eclose, despite evidence of morphologically inseparable parthenogenetic populations with demonstrated facultative eclosure and variable proportions of eclosed and uneclosed egg-laying females, In the years since the first British reports of this nuisance, we have attempted to resolve the confused nomenclature of the chironomid species involved. This has entailed maintaining cultures derived from nuisance and natural populations, observing the behaviour over many generations and analysing the morphology of the species in relation to previously published and historic materiaL It is our conclusion that there is a single species involved, Paratanytarsus grimmii (Schneider), with relatively homogeneous morphology in all stages and with only minor variations throughout its almost worldwide geographic range and habitats, We therefore concur with Lindeberg's (1971) implicit view that "all these forms are perhaps strains of one species"; however, Lindeberg refrained from pursuing the nomenclatural conclusions, Despite the problems inherent in recognition of parthenogenetic species, we feel justified in viewing P. grimmii as one virtually world-wide 'weed' species, Abbreviations Repositories are abbreviated in the text as follows: ANIC Australian National Insect Collection, Canberra, Australia; BMNH Entomology Department, British Museum (Natural History), London, UK; CUIC Cornell University Insect Collections, Ithaca, New York State, USA (Dr J. Leibherr); DHDE Private collection of D. H. D. Edward, University of Western Australia, Nedlands, W. Australia; FBA Freshwater Biological Association River Laboratory, East Stoke, nr Wareham, Dorset, UK; PHL Private collection of P. H. Langton; ZSM Zoologische Staatssammlung, Munich, German Federal Republic (Dr F. Reiss). Paratanytarsus Thienemann & Bause Paratanytarsus Thienemann & Bause in Bause, 1913; 120. [For discussion of authorship see Reiss & Siiwedal, 1971: 73-74.] Type-species: Tanytarsus lauterborni Kieffer. 1909, des. Reiss & Siiwedal, 1981: 74. Stylotanytarsus Kieffer, 1921a: 276. Type-species: Tanytarsus bau.~els Bause, 1913: 97 [Kieffer, 1922: 96], here designated. The genus-group name Stylotanytarsus has a confused history arising from non-sequential publications. The first reference to Stylotanytarsus was in a key (Kieffer, 1921a) with no indication of included species, but later in the same year in another key (Kieffer, 1921b) two species were included in Stylotanytarsus, bauseellus "Kieffer" and boiemicus "Kieffer". These two names were not accompanied by descriptions, and it was not until the following year (Kieffer, 1922) that they were described from reared females as be\cnging to the genus Paratanytarsus "Bause", It is evident that the three publications (Kieffer, 1921a, 1921b, 1922) appeared in the reverse order to their production: Kieffer intended the genus Stylotanytarsus for bauseellus and boiemicus described in Paratanytarsus in an intended earlier publication. However, it is evident that Kieffer had earlier provided the names bauseellus and boiemicus to Bause who unintentionally made the names available by publishing them, accompanied by illustrations of the pupae. Thus the authorship and date of bauseellus and boiemicus must be Bause, 1913. We select Tanytarsus bauseellus Bause, 1913: 97 (with pupal illustration on fig. 73) as the type of Stylotanytarsus Kieffer. 1921a. Thus, although the immature stages were known, Kieffer based Stylotanytarsus on female adults. Further references to Stylotanytarsus after Kieffer (1922) relate to many of the species names listed under Paratanytarsus grimmii (Schneider) below and, almost exclusively, refer to parthenogenetic species of Paralanytarsus that lack a pupal thoracic horn, However, CHIRONOMID OF WATER SUPPLY SYSTEMS 319 the confusion over dissimilis originating from Johannsen (1905, 1937) (additionally detailed below) also impinges on Stylotanytarsus. Thus, when Kruger (1941) discussed Stylotanytarsus, his generic definition was based upon the pupal abdominal spine pattern, and in addition to the parthenogenetic 'boiemicus' group, he erected a second, 'securifer', species-group based upon Tanytarsus dissimilis Johannsen. In addition to dissimilis, Kruger included Tanytarsus securifer Goetghebuer, 1934, based upon pupae and adults named as this species, reared by Nietzke (1938). Examination of examples of this species in ZSM (but not Goetghebuer's type) shows this to be identical with dissimilis Johannsen. The larva and adult female of Stylotanytarsus cannot be differentiated from those of Paratanytarsus, and the pupa clearly belongs to the Paratanytarsus inopertus group of Pinder & Reiss (1986), differing only in lacking a thoracic horn. We do not believe that this loss of a thoracic horn warrants generic status and therefore treat Stylotanytarsus as a junior synonym of Paratanytarsus. This treatment is not new, but conflicts with the opinion expressed by Reiss & Sawedal (1981). Paratanytarsus grimmii (Schneider) (Chironomus-Art Grimm, 1870: 1) Chironomus grimmii Schneider, 1885: 301. Type(s): Not stated [? USSR] [believed lost]. Tanytarsus dissimilis Var. a Johannsen, 1905: 293. Syn. n. Tanytarsus boiemicus Bause, 1913: 97, fig. 74; Johannsen, 1937 (for T. dissimilis VaL a. Johannsen, 1905). Type(s): CZECHOSLOVAKIA: Hradec Kralove [believed lost]. Syn. n. Tanytarsus bauseellus Bause, 1913: 97, fig. 73. Type(s): GERMAN FEDERAL REPUBLIC: Westphalia, nr Munster [believed lost]. Syn. n. Paratanytarsus boiemicus Kieffer, 1922: 96 (as sp. n.). Paratanytarsus bauseellus Kieffer, 1922: 96 (as sp. n.). Paratanytarsus grimmii Schneider; Thienemann, 1929: 115. Tanytarsus virgo Goetghebuer, 1934: 291. Type(s): GERMAN FEDERAL REPUBLIC: Bremen [not examined]. Syn. n. Tanytarsus chlorogyne Goetghebuer in Goetghebuer, 1938: 113 (nom. nov. for virgo Goetghebuer, 1934 not Kieffer, 1917 [jun. homonym]). Stylotanytarsus inquilinus Kruger, 1941: 248. Lectotype (here designated): GERMANY: unnamed "mitteldeutschen Industrienstadt" [examined]. Syn. n. Stylotanytarsus luteola Goetghebuer in Thienemann, 1950: 162. Type(s): AUSTRIA: Lunz [not examined]. Syn. n. Lundstroemia parthenogenetica Freeman, 1961: 721; Edward, 1963. Holotype: AUSTRALIA: Western Australia, Lake Gwellup [Paratypes, examined, same locality]. Syn. n. Paratanytarsus parthenogeneticus (Freeman); Glover, 1973; Sasa, 1979. Lundstroemia agameta Forsyth, 1971: 137. Holotype: NEW ZEALAND [not examined]. Syn. n. Paratanytarsus sp. Anden Reiss, 1972: 60. The specific synonymy within this taxon is as complex as any in the notoriously confused nomenclature of the Chironomidae. As is often the case, types have not been retained or have been destroyed. However, excellent descriptions and figures of the pupae often facilitate recognition. The summary of our nomenclatural decisions below is followed by a discussion of the history and justification of the nomenclatural conclusions made. The taxon was first recognized by Grimm who described its morphology and biology as Chironomus-Art, first in German (Grimm, 1870) then later in English (Grimm, 1871). The species was named subsequently as Chironomus grimmii by Schneider, who added little further details, and no type has been found. Zavrel, who reared the parthenogenetic species whose pupae were subsequently described by Bause (1913) and adults by Kieffer (1922) as boiemicus, discussed partheno- and paedogenetic chironomids (Zavfel, 1907). Both he, Johannsen (1910) and Bause (1913: 17, in an extensive footnote) recognized the parthenogenetic chironomid described by Grimm (1870) as belonging to the Tanytarsini. However, Bause (1913), who used the immature 320 P. H. LANGTON. P. S. CRANSTON and P. ARMITAGE stages to describe boiemicus and bauseellus and to include them in his Tanytarsus lauterborni group (=Paratanytarsus), did not pursue the similarity to Grimm's parthenogenetic midge. Munsterhjelm (1920) and Thienemann (1929) did recognize the close relationship and Thienemann placed grimmii in the 'Paratanytarsus' group of genera. Despite Thienemann's virtual resolution of the problems in this 1929 paper, on many subsequent occasions when parthenogenetic Tanytarsini were encountered, further new names were proposed. For example, Goetghebuer (1934) described Tanytarsus virgo for a parthenogenetic midge from an aquarium in Germany without comparison with any other species. The immature stages were indistinguishable from bauseellus and boiemicus (Thienemann, 1935), as were those of Stylotanytarsus luteola described by Goetghebuer in Thienemann (1950). No types of these species have been examined, but the immature stages preserved in the ZSM confirm the identity. The most thorough attempt to understand the taxonomy (and some nomenclature) was that of Kruger (1941), who investigated the species causing nuisance in German water distribution pipes. Kruger believed that he could recognize several parthenogenetic taxa, including inquilinus that he described as new, based upon subtle differences in the pupa, particularly the numbers of filamentous setae (30--40) on the anal lobe. Syntypic slidemounted material of inquilinus exists in ZSM, and lecto- and paralectotypes are recognized here (see below). Re-examination of this material shows greater variability in setal counts and size than Kruger had published, and we are unable to confirm either the pupal separation of inquilinus or the existence of further species suggested by Kruger. The material falls within the range of morphological variation and biology that we recognize for grimmii. A great deal of subsequent confusion over this taxon, and the genus Stylotanytarsus erected for the parthenogenetic taxa related to Paratanytarsus, is due to Johannsen. He described Tanytarsus dissimilis with a variety a (Johannsen, 1905), subsequently recognizing this variety to be parthenogenetic and identical with T. boiemicus (Johannsen, 1937). In the latter paper, Johannsen (1937) redescribed the immature stages of dissimilis, recognized the species as being both parthenogenetic and paedogenetic, and placed it in the Paratanytarsus group. Unlike the European parthenogenetic taxa we now recognize as all belonging to grimmii, Johannsen's dissimilis was described as able to produce males and possessing a distinct pupal thoracic horn. Examination of Johannsen's material in curc shows that his rearing culture(s) contained two species: the holotype male of dissimilis belongs to Paratanytarsus and is identical to P. con/usus Palmen, 1960 (and therefore a senior synonym thereof) while dissimilis var. a (that he later recognized as boiemicus) is identical with P. grimmii. Johannsen's (1937) concept of dissimilis included both species; only the true dissimilis possesses a pupal thoracic horn. The confusion, further detailed under Stylotanytarsus above, probably arose through the abdominal spine patterns on the two species being extremely similar. More recently described species, P. parthenogeneticus (Freeman, 1961) from Australia and P. agametus (Forsyth, 1971) from New Zealand, were described without reference to earlier names. We concur with Sasa's (1979) recognition that the Japanese population he studied demonstrated the identity of Freeman's parthenogenetica and Forsyth's agameta. Type material of adults and examination and reference to descriptions of the immature stages by Edward (1963), Forsyth (1971) and Sasa (1979) convince us that, in turn. these are junior synonyms of grimmii. Description Female (n = 50+). The female conforms to the generic diagnosis of Saether (1977) and may be identical to Saether's Paratanytarsus sp. B (Saether, 1977: 145, fig. 68C, D). Body length 1·1-2·6 mm, wing length 1·3-2·1 mm, light green with pale brown thorax and legs, darker brown antenna and postnotum. Antenna with five fiagellomeres, lengths 92-112,56-63,59-72,53-69,59-89 ,um, exceptionally ftagellomeres 4 and 5 partially fused, fiagellomeres 2-4 spindle-shaped. Antennal ratio 0·22-0·29. Flagel\omeres 1-4 each bearing apically a pair of simple, narrow, sensilla trichodea of length 33-38 "m. 321 CHIRONOMlD OF WATER SUPPLY SYSTEMS Palp with segments 3 and 4 subequal, shorter than segment 5. Wing membrane with slight brown pigment, rather densely covered with macrotrichia. R2 + 3 absent. Vena rum ratio 1·2:1·3. Foreleg ratio 1·23: 1·50, Anterior tibial scale absent. Each mid- and hind-leg tibial comb usually bearing a strong spur, rarely one is missing, Combs nearly contiguous, occupying in total slightly more than half the circumference of the tibial apex, Genitalia as in Fig. 1. Fig, L-Paratanytarsus grimmii, <jl genitalia, ventral view, (Scale bar 100,urn.) 50+) (Fig. 2). The pupa of P. grimmii conforms to the generic diagnosis Pupa (n for Paratanytarsus given by Reiss & Sawedal (1981) and Pinder & Reiss (1986). Body length 2·8-4·0 mOl, yellow-green. Frontal seta weakly filamentous, 90-165 ,urn long, arising from slight tubercle. Thoracic horn absent. All dorsocentral (dc) setae ca 100,um long, dcl fine, arising very close to dc2, these widely separated from proximate dc3 and 4; dc2-4 about 4,um wide, stout and apically frayed. Abdominal setal pattern as in Fig. 2. Tergite I without shagreen. Tergite II with widespread shagreen and posterior hookrow of 60-110 hooklets. Tergite III with paired medio/postero-lateral groups of ca 40-50-,um-long translucent spines. Tergite IV with anteriomedian dark pigmented group of 25-35 spinules and two groups of longitudinal mediolateral rows of 25-35 ca 50-,um-Iong translucent spines. Tergite V with anterior paired dark pigmented groups of 18-26 spinules. Tergites VI-VII with median shagreen, tergite VIII with only antero-Iateral patches, tergite IX with weak anterior field of shagreen. C..omb on postero-Iateral margin of tergite VIII brown, 25-45,um wide, bearing 4--10 marginal teeth. Filamentous lateral setae on segments IV-VIII 2-3, 4 (3), 4, 4 and 5, respectively. 322 p, H. LANGTON, P. S. CRANSTON and p, ARMITAGE Fig. 2,-Paratanytarsus grimmii, pupal abdomen, dorsal view. (Scale bar 200 ,um, ) Anal lobe with 28-47 marginal filamentous setae (median of 41 in 33 British examples, probably a little lower in German examples). Fourth-(jinal)-instar larva (n = 50+) (Figs. 3-12). The larva of P. grimmii conforms to the generic diagnosis for Pararanytarsus given by Pinder & Reiss (1983). Body length 2·8-3· 7 mm, tinged with pink, with yellow head capsule 270--380 pm long; mentum, mandibular teeth and apical half of mola mid-brown. Antenna (fourth instar: Fig. 6; first-third instars: Figs 3-5) is five-segmented, segment lengths: 96--106,21-26, l(}-B, 3-7,3-6 pm. Antennal ratio 1·93-2·2. Antennal blade 3540 pm. Lauterborn organs small, 4-5 pm long, on short petiole of 2-3 pm. Premandible 66--82 pm long, with sharp apical tooth, broader subapical tooth and slight inner tooth. Mandible (fourth instar: Fig. 8; first instar: Fig. 7) 125-148 pm long; outer tooth much shorter than apical tooth; mola angled. Mentum (fourth instar: Fig. 12; first-third instars: Figs. 9-11) 81-89 pm wide, with slightly trifid median tooth 13-15 pm wide. Ventromental plate elongate, 105-112 pm long, maximally 20--23 pm wide; plates almost in contact medially. Abdomen: all claws on anterior and posterior parapods simple, approximately 15 claws on posterior parapod. Procerci about 45 pm high and20pm wide, subapically darkly pigmented, bearing apically eight setae of maximum length 450--500 pm. Anal tubules subovate. 80-100 pm long, much shorter than posterior parapods. CBlRONOMID OF WATER SUPPLY SYSTEMS 323 Figs 3-6.-Paratanytarsus grimmii, larval antennae; 3, first instar; 4, second instar; 5, third instar; 6, fourth instaL (Scale bars = 25 ,urn.) Diagnosis The genus ['aratanytarsus is known well only in the adult male and pupal stages; the larvae and adult females are rarely described, never adequately for discrimination from related species. For this reason, the descriptions given above contain some details that are of unknown diagnostic relevance. In this parthenogenetic species, only the pupa can be recognized with certainty; in Pinder & Reiss (1986), this stage keys to the inopertus speciesgroup, differing principally in lacking a thoracic horn and less distinctly in the tergal spine pattern. Our inability to discriminate females of P. grimmii from those of congeneric species means that we are unable to recognize un associated females with certainty. Glover's (1973) observation that P. grimmii (cited as P. parthenogeneticus) differs from Australian congeners in the reduction in macrotrichia at the base of wing cell ml + 2 cannot be substantiated even within the Australian fauna and the character cannot be used to discriminate elsewhere. Thus, we list unassociated females along with unassociated larvae after the next section. Type-material examined In the Zoologische Staatssammlung, Munich, there are many slides of P. grimmii bearing inadequate data labels that may relate to type material of some nominal taxa. Amongst these are two slide preparations labelled "Stylotanytarsus 10.6. (II)" and another "Stylotanytarsus 11.7. (III)" in one hand, with "inquilinus Kg" in different ink. Comparison with known examples of handwriting indicate that the original label was written by Kruger, the species name by Thienemann. Despite the lack of further data, these slides appear to be syntypes of inquilinus Kruger, and a lectotype is here designated and the slide so labelled on a red Zoologische Staatssammlung, Munchen, label. The slide comprises a female indicated under a coverslip containing two other females on the slide marked "11.7. (III)". Two further slides contain three females under one coverslip and two under an adjacent coverslip, and three pupal exuviae, respectively; these (and the remaining two on the slide containing the lectotype) have been labelled appropriately as paralectotypes of Stylotanytarsus inquilinus. All material has been returned to ZSM. Holotype male, a slide labelled "OAJ Lot 1519, Tanytarsus d dissimilis Johannsen, Elston Ice Pond, Ithaca, N.Y. Oct. Holotype No. 2363 Cornell University Dept. of Entomology". Returned to CUIC. 324 p, H. LANGTON, p, S, CRANSTON and p, ARMITAGE ~9' Il" ~ I I 12 ' I----i Figs 7 & 8.-Paratanytarsus grimmii, larval mandible; 7, first instar; 8, fourth ins tar. Figs 912,-Paratanytarsus grimmii, larval menta; 9, first instar; 10, second instar; 11, third instar; 12, fourth instar. (Scale bars 25 11m.) The only other nominal taxon discussed in which type material exists and has been examined is that of Lundstroemia parthenogenetica Freeman. The following have been studied: 10 female paratypes, pinned, Australia, Western Australia, Lake Gwellup, viii & ix.1958 (D. H. D. Edward) (BMNH). Associated pupae confirm the synonomy to grimmii. Other material examined (all slide-mounted in Euparal or Canada Balsam unless stated otherwise) AUSTRALIA: 1 ,?, 4 pupae, University of Sydney, McMaster Lab. "Dr J. Boray's aquarium", 22.ix.1966 (A. L. Dyce) (ANIC); 5 pupae, 2 larvae, 25.x.1961, 'ex-lab. culture' (D. H. Colless) (BMNH); numerous pupae, Western Australia, Lake Gwellup. viii & ix.1958 (D. H. D. Edward) (DHDE). CANADA: 1 S?, 2 pupal exuviae, New Brunswick, St Andrews Biological Station, 29,iii.1977 "in tremendous numbers blocking filters of water recirculating system for aquaculture" (M. 1. Dodswell) CH 3354 (CNC); 1 S?, 1 pupa, 1 pupal exuviae, 2 larvae, Ontario, University of Windsor, 8.iii.1983, "lab. colony" (P. Hebert). CHILE: many pupal exuviae and S? ,?, Punta ArenasIMagellanes, Rio Tres Brazos, 22.ii.1954 (L. Brundin); many pupal exuviae and s?,?, Rio Paine/Magelianes 15,ii.1954 (L. Brundin); many pupal exuviae and,? ,?, N.W. Tierra del Fuego, Rio Fortuna and Rio Oscar, 6-7.ii.1954 (L. Brundin) (all ZSM). CYPRUS: 1 ,?, 8 pupae, 15 larvae, Xylotimbou CHIRONOMID OF WATER SUPPLY SYSTEMS 325 Water Supply, "ex-mains" (G. Ophanides); 3 <?, 2 larvae, Satira village "water distribution system" (G. Ophanides) (BMNH). GERMAN FEDERAL REPUBLIC: numerous larvae and pupae with inadequate data but including: PIOn, "Thienemann's garden" & "algen kulture Kemforschungs-aulage lillich, x.83" (ZSM). ITALY: 2 pupae, 13.iv.1981 (B. Rossaro) (ZSM); PERU: pupal exuviae and <?<?, Puna Bay near Chucuito, 16.iv.1954 (L. Brundin) (ZSM). TURKEY: 2 pupae, Istanbul, xi.1943 (Bott) (ZSM); UK: 3 <?, Essex Water Co., 26.x.1973 "2nd generation adult" (P. H. Langton) (BMNH); many larvae, pupae, adult <?, same source, different dates (PHL); 2 larvae, 1 pupa, same source, 25.vii.1973 (P. S. Cranston) (BMNH); 5 larvae, 1 pupa, West Sussex, West Hoathly, "ex-water mains" (Stallybrass) (BMNH); 5 larvae, London, BMNH, "ex-water taps" (P. S. Cranston) (BMNH); 4 <?, 8 pupae, 13 larvae, Dorset, FBA "ex-aquarium" (P. Armitage) (BMNH), further material (FBA). USA: 1 larval & associated pupal exuviae, 15.ix.191O, Maine, Orono, OAJ Lot 1659; 1 <? with associated pupal exuviae, same source, 22.x.191O, OAJ Lot 1648; 1 pupal exuviae, same source, x.191O, OAl Lot 1646; 4 pinned <?, same source, ix.-x.1910 (All CUIC). Material probably P. grimmii but unconfirmed by pupa: USA: 3 larvae, Washington State, Mt. St. Helens, hot sulphurous pool near shores of Spirit Lake, 3.vii.l985 (P. S. Cranston) (BMNH). AUSTRALIA: 21 pinned <? <?' NSW, Sydney, 20.x.1966, "ex-culture" (A. L. Dyce); 3 pinned 9<?, ACT, Black Mt., l.ix.1960, "light trap" (I. F B. Common); Victoria, Ocean Grove, 12.xi.1962 (J. Martin) (ANIC). Distribution Australopacific Region: widespread in Australia from Western Australia to Queensland, sooth to Victoria (Glover, 1973, as P. pathenogeneticus). Neotropical Region: from Lake Titicaca in the north to north-western Tierra del Fuego in the south (Reiss, 1972, as Paratanytarsus sp. Anden). Palaearctic Region: widespread from Britain to Turkey; Japan. Nearctic Region: eastern USA and Canada, with a possible record from western USA (see above). Unknown from the Afrotropical Region. to Biology Accounts of the biology of P. grimmii have been given by Grimm (1870) and, under a variety of the names listed in the nomenclature section, by Thienemann (1935), Krilger (1941), Edward (1963), Forsyth (1971), Langton (1974) andSasa (1979). These accounts differ only in minor details and are summarized here with additional unpublished information from the authors. The eggs are laid as a single row in a gelatinous sheath, the egg-string, and lie at an angle of 40-45 0 to the longitudinal axis of the sheath. The first egg-string is laid within 2 h of eolosion (if ecIosion does take place), in some cases within a few minutes. Usually, a ' second string is laid later, within :;.t. h, and even eight strings have been reported from a single female of one clone ('parthenogeneticus'). The fly lays its eggs while resting on the water surface, the process taking about 30 s to complete, during which time the abdomen is angled away from the water surface. The tightly coiled fawn-coloured egg-string is formed on the end of the abdomen, which is then lowered to the water surface, where the string may float for a short time before sinking. On oeeasions, the egg string immediately contacts the water as it forms, as a slight undulate cord, dropping immediately to the bottom upon completion. Few adults are still alive two days after eclosion. The number of eggs in each egg-string depends probably as much on larval nutrition as on the clone. Most of the reported values lie in the range 13-80, with a total number of eggs laid by a single midge in the range 100-175, but a total of 13 for 'inquilinus' and of 352 for 'parthenogeneticus' are recorded. On contact with the water, the gelatinous sheath rapldly swells, displaeing the enclosed eggs from their orderly arrangement, uncoils and becomes transparent, the egg-string appearing milky or yellowish to the unaided eye due to the enclosed eggs. The initial 326 P. H. LANGTON. P. S. CRANSTON and P. ARMITAGE diameter of the egg-string is about O· 2 mm, but it may swell to 1 mm when it has absorbed water. The newly laid eggs are 0·18-D·35 mm long and 0·06-0·12 mm wide, larger adults laying larger eggs. Grimm (1870) reports that during embryonic development the eggs stretch to accommodate the developing larva, an egg of 0·22 mm reaching 0·27 mm before hatching. The young larvae hatch, tail first, 48-100 h after the eggs were laid. After a short period, during which they disperse by crawling, these first-instar larvae build silken tubes fastened to the substrate along their length, except for the mouth opening which may be raised. In these they live, extending the anterior end of the body from the tube to scavenge on a variety of organic debris. There are the usual four instars recognizable by head-capsule lengths of 9G-I05, 12G150, 18G-265 and 27G-380 ,urn. The fully grown larva has a body length of 3-4 mm and width of 0·5 mm, with pale brown head and greenish body, often with a pink tinge developing in older larvae. As the larva increases in size, it extends its tube, giving a final form of the tube as a long rambling hollow cone, about 0·5 mm in diameter at its origin, increasing gradually to about 0·75 mm at its termination, and as much as ten times as long as the larval body. The tube is embellished with small particles of organic or mineral matter, which are actively stuck to the circumference of the opening as the tube is extended. Often, however, a larva may leave its tube and start again elsewhere, so that complete tubes are rare even in the laboratory. The duration of the larval stage depends upon the temperature of the water and food availability. The first moult occurs 1-7 days after hatching, the second after 3-10 days and the third after 14-16 days. The shortest recorded period between hatching and pupation is 17 days. Prior to pupation, most of the tube is eaten to leave a section long enough to protect the pupa, about 6 mm long and 0·8 mm broad, the openings of which are narrowed to leave a small orifice through which a stream of water is maintained by undulations of the pupal body. The 'pupal' stage is of short duration; Kruger (1941) recorded 25 min to 4·5 h. How long the free pupal stage lasts is uncertain, if indeed it exists, for all 'pupae' are in fact pharate adults enclosed within the pupal exuviae. The pharate adults leave the pupal tubes and swim vigorously for 30 min to over an hour, after which they hang vertically from the water surface for about 20 s, when the thoracic region noticeably swells to the unaided eye. Within a minute from the final cessation of swimming, the eclosed adult is resting on the water surface or flying to rest nearby. If the pharate adult fails to eclose, it will lay its eggs while still enclosed within the pupal exuviae, the process erroneously referred to as paedogenesis by Zavrel (1907) and Johannsen (1910). The egg-string swells as it absorbs water and ruptures the exuvial abdomen, allowing the larvae to escape when they hatch. Generation times vary greatly for the reasons given above. The effect of temperature is given by Edward (1963): at median temperatures of 12·5, 16 and 25°C, the life cycle is completed in 32-40, 25-35 and 19-27 days, respectively. Temperatures above 27°C are progressively more lethal, and the larvae cannot survive freezing. Partly starved populations may take months to complete development, and then smaller adults are produced. In nature, the species inhabits shallow standing water; large populations occur frequently in small artificial garden fishponds, where the larvae are found in mats of filamentous algae or on the substrate close to the water's edge. They also occur in the shallow margins of lakes and reservoirs amongst emergent vegetation. Their pupal exuviae are rarely collected by skimming the water surface or by collecting flotsam, but examination of the filamentous algae reveals the trapped exuviae. The capacity to lay eggs without eclosion has enabled P. grimmii to colonize water distribution systems, and its frequent occurrence in aquaria may be due to its arrival in tap water. It has been observed that P. grimmii outbreaks in water distribution systems follow the replacement of old filters. This may be due to filter replacement dislodging immature stages of P. grimmii or to the new filters eXcluding the CHIROl"OMID OF WATER SUPPLY SYSTEMS 327 continuous supply of predators that cannot maintain populations within the water-filled pipes. Discussion A widely distributed parthenogenetic species which is a weak flier, living only for a short time in the most probable distribution phase and which has been in existence for a very great number of generations is likely to have been subject to minor mutations which affect its biology and morphology. One would therefore expect that P. grimmii occurs in a multiplicity of clones different to some extent and perhaps even coexisting in the same water body. Although certain of these clones may be preadapted for unusual habitats such as water mains, it would be a futile exercise to attempt to isolate and name clones on minute differences in structure and/or biology. The problems in doing so are compounded by environmentally induced variation during the life cycle, beginning perhaps even with the unlaid egg. Populations developing in small ponds, water distribution systems or carefully tended laboratory populations are more homogeneous in appearance, dimensions and biology. An example will suffice: 'parthenogeneticus' appears from the Australian material of Edward (described by Freeman (1961) and Edward (1963)) to be particularly recognizable in being larger and more fecund than other described 'species', including the New Zealand P. agametus (Forsyth, 1971). Sasa reported a similar clone to parthenogeneticus from Japan, which under different conditions produced the complete range of morphological variation between the two antipodean 'species'. Acknowledgements We are grateful to Dr F. Reiss (ZSM) for loan of material and taxonomic discussions and to Dr J. Leibherr for the loan of Johannsen material from CUIC. 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