Small to medium sized larvae, 5-8 mm long, slender; length: width about 12:1.
Antenna with 5 segments; basal segment very slender, more than 5 x longer than broad at base; segment 3 very long, as long as segment 2, possibly with a non-annulated short apical portion; if so, segment 4 is extremely short. Ring organ situated below middle of basal segment. Two blades of equal length present, nearly reaching tip of antenna; inner blade dark. Style almost reaching tip of antenna.
Labrum SI and SII on moderately high, stout tubercles, that of SI lacking accessory seta; SIII represented by 2 slender setae between SII; SIV A and SIV B both slender. Pecten epipharyngis consisting of 5 long and slender spines. Spinulae well developed, ending in 2 or several spines.
Mandible with 11 teeth; 1 subapical outer tooth is followed by a large apical tooth and 9 smaller teeth on inner margin. Seta interna with about 17-30 branches (probably specific differences).
Mentum with 1 protruding median tooth and 15 pairs of lateral teeth; the 3 outer teeth very small and easily overlooked.
Body with procercus very long and slender, about 10 x longer than broad at base, hyaline anteriorly, blackish posteriorly; with 11-15 apical setae (possibly specific differences) that are comparatively short, all being shorter than procercus. Two small spiracular rings on eleventh body segment (as in Boreochlus).
Lasiodiamesa, a Holarctic genus, is distinguished by the elongate and bicoloured procercus bearing strong apical setae, and by the mentum with numerous (up to 14) teeth. The labrum is less laterally compressed than in other podonomines.
Lasiodiamesa seems to be confined to the north of the Holarctic region. Of the 9 known species (Brundin 1966a; Sæther 1967a, 1969a; Wirth and Sublette 1970a), 7 are known as pupae (Brundin 1966a; Sæther 1969a). The immature stages live preferably in more or less typical bog waters. The ecology of L. gracilis in the Netherlands is discussed by Verberk et al. (2003).