Species t. C. dilutus Shobanov et al., 1999

Previously placed in the subgenus Camptochironomus.

In BOLD Bin: BOLD:AAB4658.

Synonyms: Chironomus pallidivittatus Malloch 1915 (not sensu Edwards 1929 - see species 2l)

Adult:  Shobanov et al. (1999) describe the adult of C. dilutus as paler than the Palearctic C. tentans (hence Malloch's description of it as 'pallidivittatus'), with a pale green thorax with orange-grey mesonotal bands; abdomen grey-green, tergites uniformly colored or with a dark spot in the center.
Legs green-brown, with nearly 80 sensilla chaetica in ITa1; LR1 2.04 (1.79-2.27), BR1 1.30-1.46.
AR 3.36 (2.96-3.64).
Very similar to C. pallidivittatus, but males can be differentiated by characters of the hypopygium, viz.  Dististyle and inferior volsella longer and more tapered,  superior volsella longer,  anal point broader,  and indentation in tergite IX is more a U-shape.

Pupa:  Antennal sheath 1.89 (1.74-2.04) mm., wing sheath 2.80 (2.53-3.16) mm, abdomen length 9.51 (8.2-10. 7) mm., no. of hooklets on tergite 2, 92 (67-113); anal spur dark or black-brown with about 5-6 spines of varying length and width.

Larva a large (fem. 24.1 (20.8-28.9) mm; male 18.3 (15.3-24.9) mm; plumosus-type.  Lateral tubules long, up to 1/3 of segment length 640 (440-1070 µm; ventral tubuli longer than posterior parapods, posterior pair generally longer (ant. 2.16 (1.39-3.84) mm; post. 2.23 (1.39-3.94) mm.  Anal tubules well developed, about 2.5 times longer than wide.
Gular region darkened on post 1/2-2/3 and wider than the mentum, but occasionally pale, with frontoclypeus darkened particularly posteriorly at the centre.

Frontoclypeus (left) and gula (right) of C. dilutus. Photo courtesy of I. Proulx.

Mentum with generally rounded teeth; c1 tooth broad with short parallel sides, c2 teeth quite well separated (type II); 4th laterals only slightly reduced (type I).
Ventromental plates separated by about 35% of the mentum width, each with about 52-66 striae.  Pectin epipharyngis with about 10-18 sharp or broadly rounded or flat teeth (type C) (Hilsenhoff & Narf 1968), Shobanov et al. (1999)), presumably due to wear.
Premandible with inner tooth slightly longer and about 1.8x wider than outer tooth.
Basal segment of antenna about 3.7 times longer than wide; AR 2.1 (1.79-2.29); segment proportions (microns) 176 : 40 : 11 : 15 : 8.
Mandible of type II, variation of 3rd inner tooth shown in Figs 4L and M of Shobanov et al. (1999), about 19-25 striae on inner surface near the base.
Note that female larvae can be larger than the 25 mm limit indicated by Shobanov et al. (1999).

Cytology:  4 polytene chromosomes with the camptochironomus arm combination AB, DE, CF, G.
Arm G with 3 Balbiani rings but no nucleolus.  Position of Balbiani rings variable due to the polymorphisms; arms B and C with a nucleolus near the centromere, nucleolus also in arm D with an extra band.
Chromosome polymorphism in all arms, with 23 sequences known.
The male sex determiner is on arm C near the centromere, but the female sex determiner reported by Thompson (1971) does not actually exist (Martin and Lee 1984).
dil n'A1:    1a-b, 8c-7b, 3i-7a, 1g-c, 8de, 17-13, 1h-2c, 9c-8f, 10-9d, 11-12, 3h-2d, 17g-19
dil n'A2:
dil n'B1:
dil n'B2:
dil n'B3:
dil n'C1:    1-2d, 4g-6b, 9-11c,3c-2e, 11d-14c, 19-16, 7d-a, 6h-c,8, 15-14d,4a-f, 20-22
dil n'C2:
dil n'D1:    1-2b, 15-14, 10, 4-7, 2c-3, 13, 22-18e, 8-9,17-18d, 12-11, 16e-a, 23-24    as pD1, except for nucleolus and extra band in 10B
dil n'D2:    as pD2?, except for nucleolus and extra band in 10B
dil n'D3:
dil h'E1:    1-2b, 7h-8, 9-10b, 3e-2c, 7g-3f, 10c-13
dil h'E2:   
dil n'E3:    1 - 2b, 10a-c, 3f - 7g, 2c - 3e, 10b - 7h, 11 - 13
dil n'F1:    1 2, 7-9, 16, 6-3, 15-10, 17-23;           as pF3 in Europe
dil n'F2:
dil n'F3:    1a-d, 9b-12, 3b-2, 13a-d, 1i-e, 3c-5c, 16-14d, 7-9a, 6-5d, 14c-a, 2a-e, 17-23
dil n'F4:
dil h'G1:    BR1 and BR2 separated by about 1/3 of length, towards the centromeric end.
dil h'G2:    Inversion of virtually the whole arm, so the close BRs are now at the distal end.
dil n'G3:    Inversion including BR2, so that BR1 and BR2 are separated by over half the chromosome length.
dil n'G4:    Reported by Acton (1962) as rare, but limits not defined.

Found: Numerous localities across the northern U.S. and Canada.-
            Alberta (WR) - Elkwater; Edmonton; Lacombe.
            British Columbia (WC) - Chilcotin area; Williams Lake, Kamloops; Sawmill Lake, Westwick Lake, Lake Sorenson (A.B. Acton).
            Manitoba (WR) - Erickson; St. Alphonse; Churchill.
            Ontario (ER) - Hogs Back, Ottawa; Kars Creek, Kars; Toronto.
            Quebec (ER) - Montreal.
            Saskatchewan (WR) - Big Quill Lake; 6ml. n. & e. Colgate; Lake Waskesiu, Prince Albert Park;
              6ml. s. & w. Stoughton; 5ml. w. Theodore.
            Iowa (ER) - Lake Okoboji & Jemmerson Slough, Dickinson Co.; Little Wall Lake, Hamilton Co.
              Cerro, Gordo Co.
            Massachusetts (ER) - Longmeadow
            Michigan (ER)
            Minnesota (WR) - Badger Lake, Erskine, Polk Co. (47.67, -96.00); Lake Christina, Douglas Co.
            New Mexico - Taos, Taos County (36.394°N, 105.577°W).
            New York (ER) - Ithaca.
            North Dakota (WR) - Warsing Dam, Eddy Co.(47.83, -99.12); Braddock Dam, Emmons Co.; Fullers Slough;
            South Dakota (WR) - Lake Francis Case, Gregory Co. (43.27, -99.00); Gavins Point National Fish Hatchery, Yankton Co.(42.87, -97.47);
            Utah (WR) - Logan, Cache Co.
            Wisconsin (ER) - Stevens Pond & U.W. Arboretum, Madison, Dane Co.
            Wyoming (WR) - 6ml s Lander.

            In prairie sloughs, shallow eutrophic lakes and ponds, sewage oxidation lagoons.

Larva described by Johannsen (1938).  Chromosomes have been shown in a number of papers, e.g. Thompson (1971), Firling and Kobilka (1979), Martin (1979); full karyotype, using Beermann maps, published by Kiknadze et al. (1996a).  Keyl (1962) gives the sequence of arm F on his scheme, including IR-2 = p'F3 = n'F1 (Kiknadze et al. 1996a).  Other sequences from Kiknadze et al. (2004).

Martin et al. (2010) provided a physical map for the position of some genes on the polytene chromosomes.

Acton and Scudder (1971) consider North American populations to comprise three races - Alaskan, west Canadian, east Canadian.  Alaskan is considered here to be still probably C. tentans (see Sp3y).
The other two races are distinct in the West and East respectively, but tend to merge in the central area where the biogeographic barriers currently exist, although Gunderina et al. (1996) showed that samples from Minnesota and Saskatchewan were distinct from the eastern populations.  However this could just reflect an east-west cline.  Therefore inversion frequency data from all available populations were incorporated in a UPGMA analysis (see below), which still supported two groupings, with a break somewhere around the western borders of Wisconsin in the U.S.A., and Ontario in Canada.

UPGMA tree showing the distribution of the two races of C. dilutus

Eastern Race: Larval length - female 24.5 mm (21.5 - 28.9)(4); male 20.0 mm (16.9 - 24.2)(3).
Cytologically, this race is characterized by more frequent n'D2, h'E2, and n'F4, but lower frequencies of n'F3 and n'G3; with no n'G4 recorded.

Western Race: Larval length - female 22.5 mm (19.8 - 27.0)(13); male 19.1 mm (15.3 - 22.5)(14).
Cytologically, this race is characterized by more frequent n'F3, n'G3 and rarely n'G4 (known only from British Columbia), while frequencis of n'D2 and h'E2 are lower; n'F4 has not been recorded in this form.

Formerly considered a synonym of C. tentans Fabricius, the North American material clearly differs genetically from the Palearctic species and was renamed by Shobanov et al. (1999).
C. dilutus and C. pallidivittatus cannot be separated on the basis of the DNA "barcode" sequence of mtCO1, or CytB (Guryev et al. 2001) but can be separated by the sequence of the globin gene gb2β (Martin et al. 2002).

Molecular sequences:
mtCOI sequences in GenBank, Accession nos. AF110160 - AF110162.
mtcytB sequences in Genbank, Accession nos. AF109700 - AF109709.
gb2β sequences in Genbank, Accession nos. AF110173 - AF110174.

See also C. pallidivittatus and C. tentans

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Modified: 8 July 2021
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