Medium-sized to large larvae, up to 12 mm long.
Antenna 5 segmented with variable sclerotisation, basal segment longer than flagellum, 3-5 short. Ring organ situated in basal 1/2 of basal segment. Blade and accessory blade well-developed, arising on apex of basal segment. Lauterborn organs, if present, very small, located with style on apex of 2nd segment. Antennal seta absent.
SI and SII blade-like, SII twice length of SI, SI sometimes SI divided into 3-5 lobes (P. frequens group); SIII very small, seta-like; SIV small, 2 segmented. Labral lamellae absent. Pecten epipharyngis a wide plate with several transparent teeth. Premandible with 2-4 teeth, brush absent.
Mandible lacking dorsal tooth, with long apical tooth and 2 triangular or flattened inner teeth. Seta subdentalis quite short. Seta interna 4 branched, sometimes absent. Pecten mandibularis absent.
Mentum with median tooth simple or medially notched, 1.5-2.5 times width of first of 6-7 lateral teeth. Mentum uniformly pale or weakly pigmented. pale to brown. Ventromental plates rather wider than mentum, with crenulate anterior margin (sometimes strongly so). Setae submenti simple.
Posterior parapods claws simple.
Lenz (1938a, 1954-1962a) divided the genus into several species groups on the basis of larval and pupal morphology, viz. varus group, cryptotomus group, monochromus group and biannulatus group. Moller Pillot (1978) pointed out that these groups to a large extent were based on inaccurate observations and combined them within his arcuatus group. He added the vitiosus group and the longiforceps group (= frequens group), both of which have very distinctive features. Moller Pillot's solution is adopted here. There are, however, several Nearctic species with a very deviant menta (e.g. Sæther 1977c, fig. 87G).
The characteristic form of the pecten epipharyngis, which is a wide, transparent plate, carrying several teeth of various lengths, is sufficient to distinguish Parachironomus larvae from all other Chironomini.
Species groups of Parachironomus
- 1. SI divided into 3-5 lobes ... frequens group
- -- SI simple ... 2
- 2. Anterior edge of mentum weakly concave; median tooth double. Mentum and mandible both very pale ... vitiosus group
- -- Anterior edge of mentum convex; median tooth simple, rounded or pointed ... arcuatus group
Larvae of Parachironomus occur in standing and flowing waters of a wide range of types. According to Moller Pillot (1978a) P. frequens (Johannsen) (as P. longiforceps) live in association with Bryozoa, whilst some species are ectoparasitic on other invertebrates and others are leaf and stem miners in submerged macrophytes.
The genus has a worldwide distribution with at least 30 species from the Holarctic region. Problems of synonymy together with the fact that several species are only tentatively assigned to the genus make a precise estimate impossible. Most species are not described as larvae.