Medium sized, up to 9mm long, with ventral head length up to 650 microns, red colored including head. Head capsule evenly golden to mid-brown colored, with dark occipital margin and "collar" lying anterior to lateral occipital margin, giving impression of doubled margin.
Frontal apotome broad, without frontal fenestra; labral sclerite 1 and 4 disrupted, 2 and 3 complete.
Antenna six segmented, either with a short 4th segment or with each successive segment shorter than the preceding. Lauterborn organs small to moderately well developed and alternate on apices of 2nd and 3rd segments. Segment 3 with subapically inserted fine style or style absent. Ring organ in apical third of segment 1, seta absent. Blade extending beyond antenna apex.
SI plumose, with branching strongest on inner margin; SII long, curved, simple; SIII simple, short; SIVa small, SIVb strongly developed. Chaetae developed as 2 broad scales. Seta praemandibularis strong and simple. Labral lamellae broad, with slight indication of median division. Pecten epipharyngis of three separate scales, either simple, narrow, elongate and pointed or 3-4 toothed in a single plane. Chaetulae short, triangular or with 4-5 inner teeth; chaetulae basales weak. Premandible with 2 pointed teeth and strong brush.
Dorsal tooth absent (in one species perhaps represented by medio-dorsal hooked bulge); strong apical tooth and 3 small inner teeth. Pecten mandibularis absent. Seta subdentalis broad, sinuous, variably extended up to length of apical tooth. Mola and inner margin smooth. Seta interna absent.
Mentum with distinct cleft in mid-mentum, with cleft including either paired small teeth, fine serrations or smooth inner surface. Cleft and median teeth combined forming ventromentum faintly demarcated by antero-median extension of ventromental plate, dorsomentum of six teeth on each side, variously organized, sometimes directed antero-medially; all teeth brown. Ventromental plates separated medially by > 50% of mentum width, elongate with smooth margin, with striae either of regularly spaced broad lappets without anterior hooks or spines or variably reduced. Setae submenti very long, simple.
Maxilla broad, with exceptionally long maxillary palp.
Lateral and ventral tubules absent. Anterior parapods with dense, fine claws, some of which may be finely serrate apically; posterior parapod claws simple, some broad-based, with or without basal fine spinules. Procercus weakly pigmented, small, as wide as high, bearing 6-7 subequal anal setae. Supraanal setae as long as anal setae, procercal seta elongate, half length of anal and supraanal setae. Four unconstricted anal tubules.
Fissimentum is characterised by having a mentum with deeply sunken pair of median teeth, in combination with a 6-segmented antenna, with Lauterborn organs at apex of segment 2 only, long setae submenti that extend anteriorly past the mentum; and mandible lacking a pecten mandibularis and seta interna, but with large seta subdentalis. A cleft mentum is found also in Kribiodorum (=Stelechomyia) and extralimital Nilodosis Kieffer (which have a conventional 5-segmented antenna, Cranston and Spies, 1999) and the larva perhaps of another Afrotropical taxon, perhaps Zuluchironomus (Harrison, 2000).
First reported from the southern USA and the neotropics by Roback (1966) as “Tendipedini genus A”, Fissimentum encompasses the larvae of four species and one variety. At the time of the compilation of the first edition of this guide, the still-unreared taxon (referred to as “Chironomini genus A Roback”) was known to occur in marginal sediments of slowly-flowing, tropical, lowland rivers of South America, Florida and Texas. In describing this taxon as Fissimentum, Cranston and Nolte (1996) recorded the genus ranging from coastal plain drainages in southeastern USA: Lake Murray, S. Carolina (34°N), the Guadalupe River, Texas (29°N) and the Suwannee River, Florida (29°-30°N), Costa Rica, Puerto Rico, rivers in the foothills of the Andes (6°S, 11°S) and as far south as 30°S in the coastal plains of Rio Grande do Sul, Brazil. In Australia, two records spanned a range from 24° to 37°S. A larva has been found recently in sandy-bedded stream in southern Thailand. In Brazil, F. desiccatum is quite resistant to drought.