Larval types and Karyotypes

Integration of data of Jon Martin1 and Don Forsyth
1 Genetics, Genomics & Development, School of Biosciences, University of Melbourne, Victoria, 3010, Australia

When Freeman (1959) revised the New Zealand Chironomidae he recognized two species of the genus Chironomus (s.s.): C. zealandicus and C. analis. However, even at this stage it was known that adults of C. zealandicus could be produced from two distinct larval types, a salinarius-type (lacking ventral tubules) or a so-called "thummi-type" (with ventral tubules)(Forsyth, 1971), although in fact the latter are a bathophilus-type.  (The current classification system can be found here.)
Analysis of the polytene chromosomes further indicated that there were different chromosome number forms of both the salinarius- and the thummi-type larvae ( Lentzios et al. 1980), which appeared to be distinct species.  With the integration of the morphological and ecological analyses of Don Forsyth, the karyotypic studies of Jon Martin, and DNA studies with Ian Hogg, we have concluded that there are at least fourteen species of Chironomus in New Zealand.  (Note that this does not include the two species described by Sublette & Wirth (1980) from the subantarctic islands), but DNA studies are suggesting some of these are groups of closely related species as is common in other parts of the world.
Seven of the currently recognised species have a salinarius-type larva, the six others are bathophilus-type, although one is polymorphic for ventral tubule development, some having only slight development of the posterior pair of ventral tubules (halophilus-type).  The larva of the fourteenth species is unknown.

This listing is also available in pdf format (2.2 MB)(Updated 15 August 2014).

In general, the morphological terminology used in this document follows Sæther (1980), Webb & Scholl (1985) and Vallenduuk & Moller Pillot (1997).

Provisional Key to Adult Males
(based on data of D.J. Forsyth)

1a Anal point stout C. analis
1b Anal point narrower 2a
2a(1b) Abdominal tergites with dark saddle spots 5a
2b Abdominal tergites with bands or patches covering most of the segment 3a
3a(2b) Anterior tergites with a band over only about two thirds of the segment 4a
3b Anterior tergites with band over most of the segment C. 'thermarum'
4a(3a) Pigmentation of the anterior tergites narrowing to the posterior edge C. 'castaneum' (North Island)
4b Pigmentation of anterior segments covering whole width of segment 7a
5a(2a) Anterior LR generally above 1.5; only short sparse beard C. forsythi (part)
5b Anterior LR below 1.5; may or may not have beard 6a
6a(5b) Lacking a beard C. nr.'castaneum'
6b Dense short beard and sparse long beard C. zealandicus
7a(4b) Anterior LR generally above 1.5; only short sparse beard (BR <3) C. forsythi (part)
7b Anterior LR probably just below 1.5, at least some longer beard (BR 3-7) 8a
  note: *C. nr. antipodensis would also fit here, but LR not known
8a(7b) Dense short beard and sparse longer beard (BR 4-7) C. novae-zelandiae
8b Moderate beard (BR 3-4) (based on only 1 specimen) C. sp. NZ12

Note that the adults of some species are unknown.
The adults of C. 'spilleri', C.sp. NZ10 (not known), and C.sp. NZ12 are assumed to be similar to those of
C. novae-zelandiae since Don Forsyth included them all under that name.

SV typeIn the adult descriptions reference is made to the types of superior volsella shape as recognized by Strenzke (1959).  This is a helpful initial classification, but experience has shown that the types are not discrete, but are part of a continuum.  The three categories as described by Strenzke are:
S-type: The SV is shoe shaped, i.e. it is drawn out distal-medially into a broad, rounded lobe (Fig. a-c, below) (Strenzke's figure suggests the most distal point will be at the toe of the shoe).
D-type: The SV is ribbon-like: distally it may have a weakly thickened shoulder (Fig. d, below) (most distal point is not at the internal margin), or bent in a shallow sickle-shape (Fig. e-f, below).
E-type: The SV has the form of an elephant's tusk; distally it is sharply graded to a point, or with an expanded knob (Fig. g-i, below) (line from base to most distal point goes outside the limits of the SV).

Provisional Key to Fourth Instar Larvae

1a Larva a salinarius-type 2a
1b Larva with at least some development of ventral tubules 8a
2a(1a) More than 60 striations on Ventromental plate C. zealandicus (= C. species a)
2b Less than 60 striations on Ventromental plate 3a
3a(2b) Anal tubules relatively long sometimes with slight constriction near middle C. forsythi
3b Anal tubules relatively short, pointed or rounded 4a
4a(3b) Frontoclypeal region of head pale or only slightly darkened 5a
4b Frontoclypeal region very dark 6a
5a(2a) c1 tooth of mentum broad and flattened, c2 teeth relatively separated (not type I) sp. NZ9
5b c1 tooth of mentum narrower, c2 teeth only partially separated (type I) C. 'thermarum' (in part)
6a(4b) c1 tooth of mentum broad (type IIA) C. 'castaneum' group, 7a
6b c1 tooth of mentum generally narrower.(usually type 3) C. analis
7a(6a) Teeth of mentum rounded C. 'castaneum' and C. nr. 'castaneum'
7b Teeth of mentum sharp sp. NZ14
8a(1b) Slight development of posterior ventral tubules only C. 'thermarum' (in part)
8b Two pairs of ventral tubules present 9a
  note: for 8a, in thermal waters (but C. zealandicus, species 10 and sp. 12 can be also)  
9a(8b) Anal tubules relatively long, as in C. forsythi sp. NZ8
9b Anal tubules shorter and rounded 10(a)
10a(9b) Head somewhat narrower, ratio mentum width/VHL less than 0.65, ventral tubules sometimes longer than 1.0 mm. C. novae-zelandiae
  Note: Frontoclypeus generally darkened but sometimes pale or only slightly darkened (which group would include C. "spilleri"; C. sp. NZ10; and C. sp. NZ12). Variability possibly reflecting presence of more than one species.  
10b Head broader, ratio mentum/VHL above 0.65; with darkened frontoclypeus, ventral tubules about 0.5-1.0 mm. C. sp. NZ7
  Note: not in North Island  

Reference is also made to the mentum and mandible types originally devised by Webb & Scholl (1985) and Vallenduuk & Moller Pillot (1997) and Proulx et al. (2013), as well as a ventromental plate character.  The previous classifications were made for relatively small numbers of species, but they do not cover all the variability seen in these characters and so further modification has been necessary.

The mentum type is defined only by the degree of development of the 4th lateral teeth:

Type I - height in same line as the rest of the lateral teeth;
Type II - 4th laterals reduced, height about equal to that of the 5th laterals;
Type III - 4th laterals further reduced, height less than that of the 5th laterals.

The mentum may be further classified by the characters of the central trifid tooth:

Type IA - c2 teeth only partially separate from c1, i.e. as shoulders on c1 (figure a).
Type IB c2 teeth slightly more separated (figure b).
Type IIA - c1 broad, c2 teeth distinctly separated (figure c).
Type IIB c1 very broad, c2 less separated (figure d).
Type III - c1 tooth relatively narrow and much higher than the separated c2 teeth (figs e and f).
Type IV - c2 teeth well separated, not much lower than the relatively narrow c1 tooth (figs g and h).

Ventromental plate ratio (VMR) - ratio of the width of the marginal region of ventromentum (usually seen as a granular band under light microscopy) (a in figure below) to the distance from the anterior margin to the base of the striae (b in figure below).

The mandible type is defined by the degree of darkening and separation of the 3rd inner tooth:
It seems better to consider the two characters separately.
Type I - tooth fused
Type II - tooth partially free
Type III - tooth completely separated
Type A - tooth pale
Type B - some degree of pigmentation
Type C - as dark as other inner teeth

This figure shows IA; IIB; IIIC respectively

Chironomus zealandicus Hudson, 1892

There have been several attempts to relate C. zealandicus to one of the cytologically known species, and it has been suggested at different times that it was either species 1 (Forsyth, 1978 and Martin, 1998) or C. species a (Martin, 1996).
However, species 1 can be ruled out because there is an entry in Hudson's journal that he reared the specimens of C. zealandicus from larvae without ventral tubules.  Forsyth therefore thinks that C. species a is the species that corresponds to Hudson's C. zealandicus.  This is supported further by the fact that C. species a is the largest of the reared species, about the size of Hudson's C. zealandicus specimens.
The name is therefore used in this sense in this work.

Chromosome banding patterns have been published using the Australian standard system of Martin (1964), but are in process of being switched to the more universally used (although more difficult to uniquely specify bands) system of Keyl (1962) and Devai et al. (1989).

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