Quite small (adult wing length 2-2.5 mm). Larva with light yellow head, short but distinct deep black neck, body light green. Head in dorsal view roughly trapezoid in form, somewhat tapering frontad; with neatly straight lateral margins, only slightly elongate. Eyes consisting of one larger and one smaller spot which are nearly fused.
Antenna 4-segmented; basal segment nearly 3 times longer than segments 2-4 together, with basal ring-shaped sense organ; segment 2 rather long and stout, nearly 2.5 times longer than the small and very short segments 3 and 4; the double antennal seta about as long as flagellum; segment 2 apically with at least two small sensillae (? Lauterborn organs).
Labrum with sensillae small and inconspicuous: S I flattened, lancet-shaped, inserting far anteriorly, near pecten epipharyngis and anterior chaetae; S II represented by pair of simple setae inserting far behind S I; S III very small, in an anterior position behind S I; bisensillum, S IV, situated postero-laterally of S II; its construction not clearly seen. Spinulae 4 in number, very large, erect, forming thin, more or less deeply fringed lamellae straightly cut apically. Elements of pecten epipharyngis lancet-shaped, comparatively small and hyaline and difficult to keep separate from similarly-formed median epipharyngeal chaetulae. Premandibles well developed.
Mandible rather similar to Protanypus; apical tooth long, pointed, strongly curved, reaching far beyond 4 very small inner teeth; seta interna fan-shaped, consisting of 3 very long simple and 5 shorter fringed branches.
Mentum mainly light yellowish; anterior margin consisting of broad, fairly smoothly rounded mesal portion and 7 blackish lateral teeth on each side, two mesal being obliquely cut.
Maxilla at labial margin with group of long lamellae possessing more or less fringed points; further laterad maxillary margin forms round projection carrying row of flat, simple lamellae. Palp very low, with some simple sensory pegs. Hypopharynx ventrally with two lateral groups and one middle group of close-set, more or less spine-shaped elements.
Abdomen. Posterior parapods long, each with 16 fairly large claws. Preanal segment with normal apical (procercal) setae, but procerci absent. Anal papillae very small.
Brundin (1966) placed Lobodiamesa in a separate tribe, Lobodiamesini. The larva of Lobodiamesa is easily recognized amongst New Zealand Diamesinae by the light colour of head and body and the broad, light-coloured and untoothed median portion of the mentum.
Known distribution is restricted to New Zealand. Larvae of Lobodiamesa live preferably in small streams with slow current. The intestinal content consists mainly of rests of small arthropods (chitinous fragments, setae); diatoms seem to play a very subordinate role.
Type location of L. campbelli: Blackball, Nelson, holotype, March 1920, J. W. Campbell (ex Brit. Mus., presumably lost); Ohakune, Wellington, Oct.-Nov. 1922-23, T. R. Harris (cf. Pagast, Freeman).
New material by Brundin. New Zealand: Waimarino Stream, near Erua, Wellington, 6.x.61, L. Brundin; small stream in Moncrieff Scenic Reserve, near French Pass, D'Urville Island, Marlborough, 1 f, 3 a pupae, 3 f pupae, 5 exuviae, 20 larvae of different size, 2.ii.61, V. Stout; Leabody Stream, D'Urville Island, Marlborough, 24.viii.60, V. Stout; small stream running into Porter River, Canterbury, 13.xii.59, V. Stout; small stream running into Lake Lyndon, just below Porter's Pass, Canterbury, 13.xii.59, V. Stout; Ribbonwood Stream, Canterbury, 7.x11.58, V. Stout; Henry Creek, running into Lake Te Anau, Southland, 21.x.61, Brundin.