Fourth-instar larva medium-sized, 3.6 mm long, head capsule 360-400 μm.
Antenna 5 segmented, apical 3 segments more or less subequal, segment 3 subequal to or slightly shorter than segment 4; blade bifid, inner branch a little shorter than outer, not extending to antennal apex; style slightly longer than 3rd segment, Lauterborn organs subequal to segment 3. Antennal ratio c 0.5. Ring organ at 1/4 of segment 1.
Labrum with SI bifid, with inner branch very weak, outer branch stout; remaining S setae simple, SII and SIII thickened, SIV single, very short; chaetae short, simple; premandible stout, with weak indication of 2 apical teeth, no inner teeth and without brush. Labral lamellae absent. Pecten epipharyngis of 3 unfused scales; with one pair of chaetulae laterales scale-like, other weakly pectinate. Ungula short and squat with few short and simple or pectinate chaetae. Basal sclerite spatulate.
Mandible with outer margin smooth, inner margin unevenly rounded. Apical tooth much shorter than combined 3 inner teeth, scarcely protruding beyond line of inner teeth. Large gap between innermost tooth and mola. Seta subdentalis short. Seta interna narrow with few simple branches.
Mentum with median tooth broad and either simple and with crenulate anterior margin, or perhaps comprising four indistinctly separated shallow teeth with median pair smaller and slightly recessed; 4 pairs of lateral teeth, the outermost protruding anterior to its inner neighbours. Broad median mentum and first two pairs laterals paler than mid-brown lateral-most pair. Posterolateral mentum highly complex and may or may not include a 'conventional' ventromental plate, but comprises at least 2 sclerotised layers posterolateral to base of outermost lateral tooth; with dense beard of fine simple setae extending from basal mentum to laterad of apex of outermost mental tooth. Setae submenti (and other central cephalic setae) stout.
Maxilla squat, bearing relatively elongate simple setae.
Body without any evidence of lateral setae. Anterior parapods separate, with crown of elongate, simple, yellow spines, slightly shorter highly pectinate yellow claws and simple shorter pale claws. Posterior parapods separate, with apical group of simple brown claws. Procercus very short, pale, as wide as high, without spurs, bearing laterally a short setae, and apically 4 medium-length anal setae. Anal tubules comprising one pair of elongate-oval tubules projecting posterolaterally from anal area posterior to parapods (as is conventional in Chironomidae); second pair modifed such that one protrudes antero-ventrally between posterior parapods and its paired partner apparently arises from a common base and extends in a more postero-ventral position. In contrast to Tonnoir (1923) all anal tubules are subequal in length.
The larva of Tonnoirocladius keys in Cranston et al. (1983) to Paracladius from which it differs in the shape of the outer mentum, the extent and density of the ventromental beard and the structure of the mandible. The larva of Tonnoirocladius resembles that of Naonella which also has strong ventromental beard but beneath a distinctive ventromental plate of different construction.
The immature stages of Tonnoirocladius live phoretically on New Zealand Blephariceridae. Courtney (in Cranston, 2007) observed that “nearly all associations appear to involve the Neocurupira hudsoni complex, although I occasionally found chironomids on other species of Neocurupira. I found no chironomids on Neocurupira chiltoni, Nothohoraia micrognathia and Peritheates spp. Levels of phoresy in some N. hudsoni populations (on instars 3 and 4) were probably close to 100%. The other extreme was 0%”.
Tonnoirocladius commensalis larvae were located either transversely or sometimes wound around the ventral suckers. Late instar larvae could be found on small Neocurupira larvae and several different instars of T. commensalis could co-occur on a single large host. Larvae could co-occur with pupae: pupae always were orientated transversely with their ventral surface appressed to the body of the host with the ventral suckers providing lateral support. As with larvae, multiple specimens could be associated with one large blepharicerid larvae: 3 or 4 were observed quite frequently.