North American Chironomus: C. (Chaetolabis) ochreatus

Species 3e. Chironomus (Chaetolabis) ochreatus Townes, 1945.

Tendipes (Chaetolabis) ochreatus - Townes (1945)

Adult:

Male:
Wing length 3.9-4.6 mm, width 0.93-1.03 mm.  VR about 0.93-0.96.
Head yellowish green; antennae and palps dark.  AR about 3.9-4.1.  Frontal tubercles small, about 5-10 x 8 µm.
Palps (micron): 80 : 72 : 277 : 304 : 420.  About 30-45 clypeal setae.
Thorax greenish, mesosternum and mesonotal vittae brownish yellow, posterior part of postnotum brown. Setae: Acrostichal - 12; Dorsocentral - 25-26; Prealar - 7; Scutellar, anterior (2 rows) 4+10; posterior 13 (total Scutellar 27).
Legs yellowish green, anterior legs darkened at the knees, tibia and tarsi dark; other legs with tibiae yellow brown shading to black on distal tarsal segments.  Anterior tarsus without a beard.

Leg lengths (in microns) and proportions as below:

	Fe	Ti	Ta1	Ta2	Ta3	Ta4	Ta5	LR	F/T	BR
PI	1690	1580	2480	1225	900	780	355	1.55-1.70	1.07-1.08	2.06-2.11
PII	1810	1740	1062	610	435	283	192	0.57-0.61	1.03-105
PIII	1950	2105	1428	850	583	350	228	0.68	0.92-0.94

ForeTa5/Ti about 0.21.

Abdomen: Basal segment yellowish green, the rest blackish brown. Superior volsella club shaped, which is distinctive. About 9 setae on tergite IX in individual pale areas. Inferior volsella extending beyond the anal point and to about midpoint of the gonostylus where it begins to narrow markedly.

Male terminalia of C. (Chaetolabis) ochreatus.
Note the club shaped superior volsella.

Female:
Median mesoscutal vitta dark brown and divided longitudinally by a pale line.  Otherwise similar to the male except for the usual sexual differences.  Further data based on one reared specimen:
Wing length 4.98 mm, width 1.40 mm.  VR 0.90.  3-4 Scf on brachiolum, 23-26 setae on squamal fringe.
Head: Antennal segments (micron)(fraction of neck in brackets): 230 (0.28): 160 (0.47): 175 (0.46): 135 (0.43): 295, segments not sufficiently swollen to give obvious necks (see figure below).  AR 0.42, A5/A1 1.28.
Frontal tubercles small, about 9-10 x 12.5 µm.
Palpal proportions (microns): 75 : 60 : 293 : 333 : 426; P5/P4 1.28; P5/P3 1.45.
Width of clypeus 1.9 times the diameter of the antennal paedicel; about 54 clypeal setae.
Thoracic setae: Acrostichal - abt 13; Humeral 5 in a patch + 2 linear and 6 in a loose patch; Dorsolateral - 38-41 (Humeral + Dorsolateral 43-46); Prealar - 8-9; supraalar - 1; Scutellar, anterior row 14, posterior row 21 (total 35).
Leg lengths (in microns) and proportions as below:

	Fe	Ti	Ta1	Ta2	Ta3	Ta4	Ta5	LR	F/T	Ta4/Ti
PI	1920	1850	-	-	-	-	-	-	1.05	-
PII	2000	1940	1100	590	420	300	230	0.56	1.03
PIII	2120	2340	1580	820	610	360	260	0.68	0.91

Setae on GcIX - 4; segment X long and narrow (see figure below), about 239 x 27 µm, 8.6-9.4 times longer than greatest width, with 7-8 setae; cercus (below) a rounded square with a small basal extension on the dorsal margin.

Antenna (left) and ventral abdomen (right) of C. (Chaetolabis) ochreatus female.

Pupa:  Length of exuvia about 10.3 mm (male), inner margin of wing case about 2.04 mm; color yellowish brown; with well developed narrow cephalic tubercles (70 x 20 µm) arising from a broad base, with subapical seta about 56-58 µm long; also a large secondary tubercle or frontal wart about 56-68 µm long and 43-48 µm wide at the base.
Basal ring kidney shaped, about 177 x 82 µm, i.e. l/w about 2.1-2.2; 2 vacuoles slightly anterior to it. Abdomen with a central patch of shagreen on segments II-V, grading from posterior 2/3 on seg. II, to whole length of segment on segment V; on segment VI the shagreen is restricted to the anterior third.
Pedes spurii B on segments II and III small; PsA on segment IV 243 µm long ((0.25 of segment length) and 134 µm wide.  Numbers of L-setae on segments II-IV also as usual, the posterior two on segment IV arising reasonably separated.
Hook row on segment II with about 93 hooks, occupying about 42% of width of segment.
Posterolateral spur of segment VIII with 1-2 large spines with 0-3 small spines at their base, all spines closely applied and main spines extended to a fine curved point.  About 142 taeniae in multiple rows on each side of anal lobe.

Fourth instar larva: In two of the Wisconsin sites there were two different sized larvae in samples taken in June: common large larvae and a single much smaller larva.  Samples taken later in the summer contained only larvae of similar size.  The obvious answer that they were 3rd instars was not acceptible as these smaller larver were able to be sexed, which is not possible for 3rd instar larvae.  A number of characters indicate that these are C. ochreatus: thummi-type larva with pale head capsule, curvature of the basal antennal segment, long segment A2 and similar morphology of other head characers:
Small larval type (both males): Length not measured, but larva from Littlejohn Jr. Lake was smaller than the specimen from Mud Lake,: VHL 240-320.5 µm; mentum of Ty. 1B. width 140-142 µm; VPA about 0.27-0.29 of the mentum width; Width of FC between the antennal bases 107-109 µm; that between the S4 setae 94-109 µm.  SAL about 58-66 x 20 µm wide and about 3 times wider than deep.
Ventromental plates about 150-152 µm wide; 2.6-3.3 times wider than deep and 0.8-1.07 times the mentum width; IPD about 80% of mentum width; with about 40-42 striae.
Premandible with relatively sharp inner tooth which is 3.5-5.2 times wider than the narrow, sharp outer tooth (type B2).  PE with about 17-18 irregular teeth, with 3-4 small interstitial teeth (type D).
Antenna with A1 about 41-42% of the Ventral head length and slightly curved; A2 long, about 39% length of A1, which is about 3.3-4 times longer than wide; ring organ about 40% up from the base; AR 1.43-1.47; antennal segment proportions (micron) 104 : 40 : 10.5 : 14 : 6.  i.e. A5 shorter than A3.
Mandible with 3rd inner tooth partly separated but only moderately pigmented (type IIB); with 16-19 furrows on outer surface at base; PecM 13-14 taeniae; Mdt-Mat about 20-24, MTR 0.34-0.35.

The larger specimens are a medium sized thummi-type; length about 12.8-16.8 mm (Female 15.5 (14.8-16.8) mm; male 14.1 (12.8-15.3) mm); posterior pair of ventral tubules longer and coiled (ant. 1.34 (1.21-1.57) mm; post. 1.50 (1.41-1.81) mm), those of the Quebec specimens much shorter than those from Wisconsin.  Anal tubules very large, about 4.5-6.4 times longer than wide, with a constriction about one third from the base (dors: 1120 x 200 µm, vent: 1180 x 170 µm).
Gula region and frontoclypeus usually pale, but some specimens have very slight darkening of both.  Width of frontoclypeus between the antennal bases (ASA of Vallenduuk & Moller Pillot, 1997) about 186-190 µm, greater than the distance between the S4 setae (about 160-175 µm).  Oesophageal opening about 83.5 µm wide and 3.7 times wider than deep.
Mentum (Fig. d) with relatively pointed teeth; c1 tooth moderately broad with short parallel sides, c2 teeth moderately well seperated (type IB); 4th laterals hardly reduced (type I); 6th laterals arising at about the same level as other teeth.
Ventromental plates (Fig. e) about 248-250 µm wide and 3.06-3.38 times wider than deep and 1.08-1.09 times the mentum width; separated by about one third of the mentum width (IPD); with about 39-45 striae (Webb et al. 1987).
Premandible (Fig. b) with relatively sharp inner tooth, about 4-5 times wider than the narrow, sharp outer tooth (type B2).  Pecten epipharyngis (Fig. c) with about 23.2 (21-25) irregular teeth, with small interstitial teeth (type D).
Antenna (Fig. a) with A1 about 35-44% of the VHL; A2 long, over 1/3 length of A1, which itself is about 3.3-4.3 times longer than wide; ring organ a third to almost half way up from the base; AR 1.95 (1.50-2.27; antennal segment proportions (micron) 154 : 44 : 11 : 18 : 8. i.e. A5 about same length as A3.
Mandible (Fig. f) with 3rd inner tooth partly separated but only moderately pigmented (type IIB); with about 19 (17-21) furrows on outer surface at base; Pecten mandibularis of 13.2 (12-15) taeniae; Mdt-Mat about 28, MTR 0.31-0.47.

Cytology: 3 polytene chromosomes which may have the modified thummi arm combination, AB, CD, GEF; but Keyl pattern very difficult to recognize.
Nucleolus near the junction of arm G with E, arm G generally unpaired; 2 Balbiani rings between nucleolus and end of arm, with another in arm E, found in some cells of one larva only. No nucleolus in the long chromosomes.
Polymorphism in arms B (simple inversion) and F Limits uncertain and region unpaired).

Found: Ontario – Cambridge (43.40°N, -80.30°W) (Telfer et al. 2015)
  Quebec – Lake Opasatica, Rouyn-Noranda (48.17°N; -79.33°W).
  Arkansas – Galloway (Townes); White River National Wildlife Refuge, Arkansas (Chordas et al. 2004.)
  Georgia - roadside pond (B. Caldwell, in Epler 2001)
  Maine – Lincoln Co. (Townes)
  Massachusetts – Holliston (Townes)
  Michigan – Empire (Townes)
  New Jersey – Medford Lakes (Type locality); Chesilhurst; Glassboro (Townes)
  New York -– Lake Sebago, Bear Mountain State Park (Townes)
  Rhode Island – Westerly; Wickford (Townes)
  South Carolina – Greenville, Table Rock State Park, Pickens Co. (Townes)
  Virginia – Four-mile Run; Norfolk Co. (Townes)
  Wisconsin – Little John Jr Lake, Vilas Co. (46.00°N, 89.63°W); Mud Lake, Vilas Co. (46.02°N, 89.62°W).

At depth of about 4 m amongst Drepandocladus exannulatus, and in pools.

Wiederholm (1979) considered Ch. ochreatus to be a synonym of Ch. atroviridis, but did note that this was subject to confirmation by further analysis. The presence of the large secondary tubercles in the pupa (above) shows that it is clearly distinct. Detailed features of the larval ventromental plates are given by Webb et al. (1987).
The shared chromosome banding patterns and the extremely similar COI DNA sequence to that of C. ochreatus (see below), suggests that the fusion of arm G to the end of arm E may be a relatively recent occurrence. The difference in chromosome number would imply that any hybrids would be sterile or inviable due to chromosomal imbalance.

DNA analysis: Sequence for the mitochondrial CO1 and the nuclear gb2β genes are available.
CO1: Gene bank accession numbers KF278327; KF278328; KF2788351. Also Barcode sequences in BOLD database.

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