Fowler (1940) created the genus Homalopterula for the new species Homalopterula ripleyi Fowler 1940 and distinguished it from other balitorids by the “peculiar shape of its jaws, in combination with its truncated caudal and entirely naked medial under surface of the abdomen” (Fowler 1940, p. 379).
Silas (1953) treated Homalopterula as a junior synonym of Homaloptera based on the illustration provided in Fowler’s type description and the variation that “exists regarding the nature of the caudal and presence or absence of scales on the ventral surface of the abdomen in species of Homaloptera.” Roberts (1989) followed this decision. Kottelat (1998) recognized four species (H. ripleyi, H. gymnogaster, H. heterolepis and H. vanderbilti as) as possibly forming a clade (Homalopterula) based on “a more cylindrical body, a relatively wide mouth, short paired fins, and a truncated or slightly emarginated caudal fin,” and using this description, Tan & Ng (2005) and Ott (2009) treated Homalopterula as a subgenus of Homaloptera.
Randall and Page (2012)[5] considered Homalopterula a putative subgenus within Homaloptera and also containing the four afore-mentioned species. All four are native to Sumatra and separated from congeners by their more cylindrical than compressed body shape, relatively wide mouth, short paired fins and truncate caudal-fin shape.
Randall & Page (2015) raised Homalopterula to full generic status, and it currently contains H. gymnogaster(Bleeker 1853), H. modiglianii (Perugia 1893), H. amphisquamata (Weber & Beaufort 1916), H. heterolepis(Weber & de Beaufort 1916), H. ripleyi Fowler 1940 [the type species], and H. vanderbilti (Fowler 1940). Homalopterula is only known to occur in Sumatra.
Homalopterula is distinguished by the following combination of characters: without reddish tints on fins in life; dorsal-fin origin posterior to pelvic-fin origin; 5½ and 7½, 7½ (M) branched dorsal-fin rays; 7 pelvic-fin rays; truncated or emarginated caudal fin; smooth scales, 57–75 total lateral-line scales, 28–56 predorsal scales; anus closer to anal-fin origin than to pelvic-fin base; adipose keel on caudal peduncle; small rostral cap; 2 thick and widely separated rostral barbels; thick crescentic upper lip; 2 fleshy lobes between lateral portions of lower lip; and presence of a central furrow at the isthmus.
H. gymnogaster has 60-73 lateral-line scales and its belly is naked anterior to the ventral fins.
The genus Homaloptera van Hasselt, 1823 has long been considered a polyphyletic assemblage (Kottelat, 1998), and following Randall & Page (2015) the majority of members have been moved into other genera as recognized therein:
Homalopteroides Fowler, 1905 with 11 species (Homalopteroides wassinkii, H. modestus, H. rupicola, H. smithi, H. stephensoni, H. weberi, H. tweediei, H. indochinensis, H. nebulosus, H. yuwonoi, and H. avii), is the most diverse and widely distributed genus of species formerly in Homaloptera. It is known from northeast India, Myanmar, Thailand, Laos, Cambodia, Vietnam (? see Kottelat 2012:51), Peninsular Malaysia, Sumatra, Java, and Borneo.
Homalopterula Fowler, 1940 is only known to occur in Sumatra and consists of six species, five described from Aceh Province (H. heterolepis, H. ripleyi, H. modiglianii, H. amphisquamata, and H. vanderbilti) and H. gymnogaster from Sumatera Barat province.
Balitoropsis Smith, 1945 contains two species, B. zollingeri and B. ophiolepis. The original type species B. bartschi (Smith, 1945) is now considered synonymous with B. zollingeri.
Pseudohomaloptera Silas, 1953, which has not been valid since 1953, now contains six species (P. tatereganii, P. sexmaculata, P. leonardi, P. yunnanensis, P. vulgaris, and P. batek) and is known to occur in southern China, Thailand, Laos, Cambodia, Peninsular Malaysia, and Borneo.
Ghatsa Randall & Page, 2015, contains the type species G. montana Herre, 1945, and four tentatively assigned species (G. pillaii, G. menoni, G. santhamparaiensis, and G. silasi).
Homaloptera van Hasselt, 1823, consists of six species (H. ocellata, H. bilineata, H. orthogoniata, H. ogilviei, H. confuzona, and H. parclitella) found in Myanmar, Thailand, Cambodia, Peninsular Malaysia, Sumatra, Java, and Borneo.
These loaches make fascinating aquarium inhabitants and are often referred to as 'lizard' loaches due to their behaviour and appearance. Like all balitorids they have morphology specialised for life in fast-flowing water, i.e., the paired fins are orientated and extended horizontally, head and body flattened, belly depressed. These features form a powerful sucking cup which allows the fish to cling tightly to solid surfaces. The ability to swim in open water is greatly reduced and they instead appear to crawl and hop their way over rocks and other surfaces.
The family Balitoridae as recognised by Kottelat (2012) is widely-distributed across much of the Indian subcontinent, Southeast Asia and China.
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