Chironomus samoensis Edwards, 1928

Edward's original description

Tokunaga's (1964) description of C. samoensis.
This is the most complete previous description of the adults.

The description of the upper basal appendage (SVo), and the list of species to which this species is compared, is important here - all of these species have a D type SVo.  Tokunaga (1964) notes the hypopygium is of the dorsalis type but illustrates it with a triangular apex which can be misinterpreted as an S-type.  The specimens described by Tokunaga from Micronesia are probably C. samoensis, and the illustration is presumably intended to depict the somewhat beaked SV seen in some specimens.  The misinterpretation of this illustration may be partly responsible for the identification of C. samoensis in other locations, which have an S-type SV. However, they also differ in other characters and are mis-identifications.
While the females are largely dismissed as "like the male apart from the usual sexual differences", the relative lengths of the fore leg segments appear to be useful in separating the species of this group.
Tokunaga makes the important point that the fore tarsus has Ta4 far longer than Ta3, and slightly longer than Ta2, although examination of a pharate female from Tutuila, American Samoa, suggests that Ta2 and Ta4 can be about equal in length.

Redescription based on specimens from Tutuila, American Samoa

AR about 2.4 - 2.9.
Frontal tubercles about 33 - 39 µm.
Palp proportions: 44 : 53 : 189 : 222 : 315
Wing length: 2.85 - 3.15 mm; wing width 0.30 - 0.67 mm.
VR about 0.95
Leg lengths (microns) and proportions as follows:


C. samoensis: Male hypopygium (left) and superior volsella (right) (note the "beak" of lower figure)

Abdomen pale, with darkening as described by Edward.  Hypopygium similar to that of C. dorsalis, with the SV of the D type, similar to fig. e of Strenzke (1959), but sometimes with development of a "beak".  The inferior volsella has mainly simple, curved setae, but a small number appear to have a small simple fork near the tip.  About 4-6 setae on the 9th tergite near the base of the anal point.

No females are available amongst the material, but some characters could be obtained from a pupa with a pharate female.  An important character is the relative proportions of the fore leg, particularly the tarsi, as Tokunaga (1964) notes that the Ta4 of specimens he assigned to C. samoensis was unusually long.  The approximate lengths of these segments were measured (in micron) as: Fe 900 ; Ti 750 ; Ta1 1020 ; Ta2 620 : Ta3 470 : Ta4 610 : Ta5 340; Ta4 about same length as Ta2, and about one third longer than Ta3.
Other characters:
Head - frontal tubercles - length 23 µm, width 13 µm.  Antennal segments (µm) 144 : 109 : 116 : 106 : 215.  About 24 clypeal setae.Thoracis setae: Acrostichal 14, dorsolateral 31, prealar 5, scutellar in two rows, ant. 14, post. 14.

Pupa: (Male) Exuviae length about 6.5 - 7.0 mm. (3); inner margin of wing case about 1.34 mm (1.27-1.42).  Pale, with darkened caudolateral spurs.  Cephalic tubercles about 87 µm long and 66 µm wide at the base.
Segment II of abdomen with about 67-77 hooks, and slight development of Pedes spurii B.  Progessive development of Pedes spurii A from segments IV to VI.  Spur of segment VIII about 180 µm long, with 1-3 spines.  About 78-88 taeniae on each side of the anal lobe.

Fourth instar larva: a medium sized plumosus-type larva (length female about 12.5-12.7 mm.;male about 10.8-11.8 mm.).  Lateral tubules about 280-360 µm; ventral tubules relatively long; anterior ventral tubules (1.24-1.84 mm.) shorter than posterior pair (1.40-2.28 mm.), posterior pair longer and coiled; anal tubules moderately long (about 1.6-2.6 times longer than wide), dorsal pair (240-410 Ám) slightly longer than ventral pair (215-370 µm).
Head capsule pale with darkening of the posterior half of the gula, frontoclypeus sometimes pale, but usually with slight darkening.  Distance between the antennal bases greater than that between the S4 setae.
Mentum (Fig. d) wider than usual, about 0.6 of ventral head length; with rounded teeth, c2 teeth of central trifid tooth well separated from relatively tall c1 tooth (essentially type IV), 4th laterals reduced to about the level of 5th laterals (type II), 6th lateral variable, sometimes arising at same level as other laterals but generally appearing to be at a slightly lower level, apparently due to wear.
Ventromental plates (Fig. e) separated by about 35-41% of mentum width, with about 32 - 35 striae, anterior margin smooth; VMR about 0.36.
Pecten epipharyngis (Fig. a) with about 13 (10-16) sharp pointed teeth.  Premandible (Fig. b) with sharp teeth, outer tooth shorter than inner tooth, which is about twice as wide as the outer tooth.
Antenna (Fig. c) with a moderately long basal segment, which is almost 4 times as long as wide, Ring organ between 0.4 and 0.5 up from the base of segment; AR about 2.03-2.30.  Antennal proportions (in µm): 110 : 24 : 6 : 11 : 7.
Mandible (Fig. f) about 208-228 mm long, with with third inner tooth relatively pale and only partly separated (Type IIA), and with about 13 (12-14, 8) furrows on outer surface at the base; Pecten mandibularis sparse, with about 8 (7-10, 5) setae.

Cytology:  4 polytene chromosomes with the pseudothummi arm combination AE, BF, CD, G.Nucleolus medial in arm G; two Balbiani rings distal to the nucleolus.
A further nucleolus at about group 20 of arm F and there is a large puff in arm C that might also be a nucleolus.
All chromosomes closely paired.  No polymorphism in the available specimens.
samA1:    1 - 2c, 10 - 12, 3 - 2d, 9 - 4, 13 - 19                as holomelas
samB1:    Puff of group 7 near distal end of the arm with dark bands proximal to it.
samC1:    Characteristic groups 3-4 just proximal to the large puff.
samE1:    Groups 11-13 near centromere.
samF1:    Groups 20-23 near centromere, with nucleolus about group 20.

The polytene chromosomes of C. samoensis also differ from those described for the others species in the group.  The arm combination is pseudothummi-cytocomplex, as in the other species, but the most obvious difference is that the nucleolus in arm G is near the middle of the arm, rather than almost terminal.  There is a second nucleolus near the diagnostic bands of arm F, and generally a large puff, which may be a nucleolus, near the middle of arm C.

Based on these descriptions, diagnostic features of the species are:
Frontal tubercles relatively long; LR about 1.50-1.52, fore Ta5 about one third of the length of the fore Ti, SV of the D-type, or "beaked"; in female fore Ta4 longer than Ta3 and about the same length as Ta2.
In larva, antennal segment 3 relatively short, usually shorter than A5.  In the polytene chromosomes, the nucleolus in arm G is median, and there is a further nucleolus about region 20 of arm F and usually a large puff in arm C.

Found: Type localities - Apia, Western Samoa; Faratogo, Tutuila (now American Samoa); Tonga.
            American Samoa - Mapusaga, Tutuila.
            Micronesia - (Tokunaga 1964).
            Specimens from other areas (Australia, Japan,and India) are related species, but not C. samoensis.

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Modified: 5 November 2018
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