Chironomus dilutus Shobanov et al.

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

Previously placed in the subgenus Camptochironomus.

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

Adult:

The cytology shows that there are two races of this species – the Eastern Race and the Western Race. However the original description does not distinguish between them and there is not enough data to detect whether there are any morphological differences. It is noticeable that most of the data in Shobanov et al. (1999) comes from the Western Race and specimen size is smaller than that in the Eastern race, and indicates that the supposed size difference from C. tentans in mature larvae is not supported.

Male: 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).
Leg lengths and proportions (micron) (from Shobanov et al. (1999):

	Fe	Ti	Ta1	Ta2	Ta3	Ta4	Ta5	LR	F/T	BR
PI	1920	1910	2340	1200	910	760	380	1.79-2.27*	1.02-1.03	1.30-1.46
PII	2030	2030	880	540	430	330	250	0.44-0.45	0.98-1.01	-
PIII	2300	2350	1010	700	590	380	280	0.43-0.51	0.97-0.99	-

*these figures do not seem to be compatible with their data in Table 4

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.

Additional data including further specimens:
Wing length abt 4.55-5.40 mm, width 1.14-1.32; VR 0.95; 4 Scf on brachiolum; 25-41 setae on squamal fringe.
Head: AR 3.34 (2.72-3.65); CT 29.7 (21-35) µm long and 1.90-2.0 longer than wide; 61-65 setae on clypeus, which is about 0.85-0.94 of the diameter of the antennal pedicel; palp proportions (µm) 82 : 75 : 278 : 273 : 328 ; P5/P4 1.15-1.33, P5/P3 1.23-1.56.
Thoracic setae: Acrostichal - abt 12-16; Dorsocentrals 32-36; Prealars - 1; Supraalars 6-9; Scutellar 8-20 in anterior rows, 11-22 in posterior row (total 22-42).
Leg lengths (micron) and proportions:

	Fe	Ti	Ta1	Ta2	Ta3	Ta4	Ta5	LR	F/T	BR
PI	2104	2074	2418	1259	994	828	420	1.14-1.32	1.01-1.02	1.10-1.48
PII	2410	2142	995	752	460	357	272	0.44-0.47	0.98-1.01	-
PIII	2400	2483	1242	757	626	424	303	0.43-0.56	0.94-0.99	-

One specimen had 3 setae in a small clear area right at the base of the anal point. Inferior Volsella reaching to about 2/3 the length of the gonostylus, with simple setae along the distal 2/3. Gonostylus narrows gently along its length usually to more of a point than figured by Shobanov et al. (1999).

Female: The female has not been described, but it is likely that the genital regions will be quite similar to those of C. tentans as described by Shilova (1978).

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).
Go to Immatures and Cytology.

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 (52.00°N; -122.17°W);
Lake Sorenson (A.B. Acton).
Manitoba (WR) - Erickson; St. Alphonse; Churchill.
Ontario (ER) - Hogs Back (45.35oN, -75.68oW), Ottawa; Kars Creek, Kars (45.13°N, -75.63°W); Mud Lake, South March
(45.40°N, -75.87°W), all Carleton Co.; Cambridge (43.40°N, -80.30°W) (Telfer et al. 2015); Kelly Lake, Sudbury
(46.45°N, -81.07°W); Toronto.
Quebec (ER) - Montreal.
Saskatchewan (WR) - Big Quill Lake, 1 ml. s. Dafoe (51.73°N, -104 53°W); 6ml. n. & e. Colgate; Lake Waskesiu,
Prince Albert Park; 6ml. s. & w. Stoughton; 5ml. w. Theodore (51.75°N, -102.95°W).
Colorado - Walden Pond, Boulder.
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°N, -96.00°W); 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°N, -99.12°W); Braddock Dam, Emmons Co.; Fullers Slough;
Hankinson.
South Dakota (WR) - Lake Francis Case, Gregory Co. (43.27°N, -99.00°W); Gavins Point National Fish Hatchery,
(42.87°N, -97.47°W); 4 ml N Yankton, both Yankton Co.; Su-Falls; Missouri R., Running Water.
Utah (WR) - Logan, Cache Co.
Wisconsin (ER) - Stevens Pond & U.W. Arboretum, Madison (43.03°N, -89.42°W);, Dane Co.; Theresa Marsh.
Wyoming (WR) - 6ml s Lander.

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

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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