Medium-sized to large larvae, up to 10 mm long. Head pale to dark brown; body pink to brown. Head capsule rounded-oval; cephalic index 0.7-0.83. Dorsally, multibranched S5 far anterior to DP and multibranched S7, S8 setae. S8 posteromedial to S7 and anterolateral to DP, S7-S8-DP unaligned. Ventrally, S10 posterolateral and close to S9, far anterolateral (A. trifascipennis) to more nearly lateral to SSm; VP posterolateral to SSm and distant to S9 and S10. All setae multi-branched.
About as long as mandible. Antennal ratio 6.5-7.6. Basal segment about 5-6x as long as basal width, with ring organ about 0.8; segment 2 short, about 2-3.5x as long as wide; segment 3 scarcely longer than wide, shorter than apical segment; variably broad intersegmental band present between segments 2 and 3. Style large, as long as segment 2, from which it arises subapically, together with peg sensilla. Blade reaching mid-segment 4; accessory blade just reaching to segment 3.
Slender, moderately curved. Apical tooth 2.5x as long as basal width, almost 1/3 length of mandible. Mola shallow to protruding into apically-pointed tooth; small to large inner dorsolateral tooth. Seta subdentalis long, arising from pit between mola and dorsolateral tooth. Ventrolateral seta 1 simple; setae 2 and 3 with 1-3 branches; seta 1 and 2 close; Ring organ midway between seta 2 and 3.
Basal segment of palp very short, about 2.5x as long as wide, with ring organ near middle.
With fringe of swim-setae. With 4 rather long, conical anal tubules. Procercus 4-5x as long as wide, with 10-16 (A. trifascipennis) apical setae. Posterior parapod with 13-16 claws; some large claws with fine, appressed spinules on inner side; smallest claws simple, weakly curved.
Larvae of Apsectrotanypus differ from those of other Macropelopiini in the short antennal segment 2 with unusually deeply-set style, and the few, unusually large-sized, dorsomental teeth. The short basal segment of the maxillary palp will separate it from Macropelopia and Alotanypus, and the simple, small posterior parapod claws will separate it from Brundiniella.
Larvae of Apsectrotanypus live in small, cool, flowing waters. One species, A. trifascipennis, is known from the western Palaearctic: Vallenduuk and Moller-Pillot (2008) discuss the ecology in the Netherlands. Three species are known from the Nearctic, with the larva of A. johnsoni described. Another larva is known from North America but is not assigned to species (Roback, 1978). A larva from Nepal clearly belongs to Apsectrotanypus (Roback and Coffman, 1987). In Australia and New Zealand, species previously allocated to Macropelopia, as for example Anatopynia maculosa Freeman, belong to Apsectrotanypus. It remains to be seen whether species from South America also are best placed in this genus - larvae very similar to Apsectrotanypus are known from the region.