Diagnosis: Nothobranchius insularis is distinguished from all species of the N. melanospilus group, except N. lucius, in possessing snout pointed in lateral view, jaws moderately long vs. snout blunt to weakly pointed, jaws short; caudal fin, in males, with broad dark grey to black band on the posterior margin vs. narrow; presence, in females, of dark dots over the whole flank vs. dark dots when present restricted to the posterior portion of the flank (Ref. 122074). It differs from N. lucius by having inner premaxillary teeth smaller than teeth of the outer premaxillary tooth row vs. larger; caudal fin subtruncate in males vs. rounded; in females, flank dark dots are vertically elongated and often arranged in oblique rows vs. dots rounded, arranged in horizontal rows; unpaired fins, in females, with dark grey dots when present restricted to their basal portion vs. extending over most fin; caudal, pectoral and pelvic fins shorter, caudal fin length 26.9-29.6% of standard length in males and 22.8-27.4% of standard length in females of N. insularis, vs. 31.3-34.9% of standard length in males and 30.3-32.9% of standard length in females of N. lucius; pectoral-fin length 17.1-21.8% of standard length in males and 14.2-19.3% of standard length in females, vs. 22.2-24.5% of standard length and 20.2-24.6% of standard length, respectively; pelvic-fin length 8.6-11.0% of standard length in males and 9.6-11.0% of standard length in females, vs. 11.6-13.1% of standard length and 11.5-13.0% of standard length, respectively; and three neuromasts in the posterior section of the anterior supraorbital series vs. two (Ref. 122074).

Description: Dorsal and ventral profiles slightly convex from snout to posterior end of dorsal and anal-fin bases, about straight on caudal peduncle (Ref. 122074). Body relatively slender, compressed; greatest body depth at vertical just in front of pelvic-fin base (Ref. 122074). jaws slightly elongated,snout pointed in lateral view; jaw teeth canine, numerous, irregularly arranged, outer teeth greater than internal teeth (Ref. 122074). Gill-rakers of first branchial arch 4-5 + 14-15; six branchiostegal rays (Ref. 122074). Dorsal and anal fins broad in males, extremity rounded, with short filamentous rays along distal margin, dorsal fin slightly longer than anal fin; in females, dorsal fin rounded, anal fin sub-triangular and slightly longer than dorsal fin; caudal fin subtruncate; pectoral fin rounded, posterior extremity between pelvic-fin base and anus; pelvic fin small, tip reaching between anus and urogenital papilla; pelvic-fin bases medially in close proximity; dorsal-fin origin on vertical between base of first and third anal-fin rays; dorsal-fin rays 15-16; anal-fin rays 16-18; caudal-fin rays 29-30; pectoral-fin rays 19; pelvic-fin rays 6; minute contact organs on first and second pectoral-fin rays in males; rows of papillate contact organs aong distal portion of middle dorsal-fin rays and two thirds of most rays of anal fin in males (Ref. 122074). Scales small, cycloid; body and head entirely scaled, except ventral surface of head; minute filamentous contact organs along posterior margin of scales on middle portion of flank and latero-ventral portion of head in males; body squamation extending over anterior 40% of caudal-fin base; no scales on dorsal and anal-fin bases; frontal squamation irregularly arranged in two longitudinal rows; longitudinal series of scales 31-32; transverse series of scales 9-10; scale rows around caudal peduncle 16 (Ref. 122074). Anterior supraorbital series of neuromasts arranged in two separate sections, each placed in shallow depression, the anterior section with two neuromasts, the posterior one with three; sometimes minute neuromast between depressions; posterior supraorbital series with four neuromasts placed in shallow depression; infraorbital series with 18-21 neuromasts, pre-opercular series 14-18, mandibular 17-18; one neuromast per scale of lateral line (Ref. 122074).

Colouration: Colouration in alcohol for males: flank, dorsum and head light brown, darker on posterior portion of scales of dorsal portion of flank, dorsum and opercle; venter pinkish grey; branchiostegal membrane dark grey; dorsal and anal fins hyaline with transverse series of grey spots, almost inconspicuous in anal fin; caudal fin pale yellow with dark grey to black stripe along whole fin margin, broader on posterior margin; pectoral fin hyaline, pelvic fin greyish hyaline with black tip (Ref. 122074). Colouration in alcohol for females: flank and dorsum pale brown, side of head and venter pale yellow; vertically elongated dark grey to black dots irregularly arranged in oblique rows on whole flank; sometimes few pale grey dots on opercular region; unpaired fins hyaline; elongated grey dots on basal portion of dorsal fin; anal and caudal fin often without dark marks, sometimes with almost inconspicuous pale grey dots on basal portion; paired fins hyaline (Ref. 122074).