Males of four new species of caddisflies from Kampong Speu and Siem Reap provinces, Cambodia [Psychomyia kimcheangi n. sp. (Psychomyiidae), Macrostemum siemreapensis n. sp. (Hydropsychidae), Leptocerus angkorwatensis n. sp. (Leptoceridae), and Trichosetodes kampongspeuensis n. sp. (Leptoceridae)] are described and illustrated. Psychomyia kimcheangi n. sp. can be distinguished from other species by characters of the inferior appendages. The main body of each inferior appendage has two branches that are separated near its base but are parallel and close together until the end. In Macrostemum siemreapensis n. sp. the dorsal tip of the phallus has no curved hooklet but instead a knot-like protruding structure in ventral view and looped-like structure in dorsal view. Leptocerus angkorwatensis n. sp. can be distinguished from others by the characters of long needled-like preanal appendages. Trichosetodes kampongspeuensis is distinguished from others by the characters of segment IX in ventral view, which is asymmetrical, and inferior appendages that are not divided and asymmetrical.

Males of four new species of caddisflies, Polyplectropus hofmaierae n. sp. (Polycentropodidae), Eoneureclipsis chinachotiae n. sp. (Psychomyiidae), Hydropsyche khaonanensis n. sp. (Hydropsychidae), and Lannapsyche tairomyenensis n. sp. (Odontoceridae) are described and illustrated. Polyplectropus hofmaierae n. sp. is distinguished from other species by the shape of the apical end of its inferior appendages and its sharp intermediate appendages. The posterior edges of their inferior appendages run slanting to the ventrodistal point and are densely covered by short and stiff bristles. Eoneureclipsis chinachotiae n. sp. is differentiated by characters of its phallus, as the first two thirds of its length are slender and slightly curved. The distal part has a dorsal hump with a very slender thread on its caudal edge and is slightly bent downward and dilated. Hydropsyche khaonanensis n. sp. can be differentiated from the other species by its phallus, in which the two apicolateral lobes in ventral view have rounded outer edges. Lannapsyche tairomyenensis n. sp. can be separated from the others by characteristics of the dorsal view of segment IX and the harpago of its inferior appendages. Segment IX is triangular with distinctly convex preanal appendages along the posterior edge of this segment, and the harpago and subapical lobes are relatively similar in shape and length.

The Tapee River is one of the most important rivers in southern Thailand. The river crisscrosses from Khao Luang National Park, Nakhon Si Thammarat province to Surat Thani province and flows to the Thai Gulf. To study the biodiversity and distribution of Trichoptera species along the Tapee River, insects were collected by light trapping every 3 months from April 2004−February 2005. The results show that 105 species were found in 35 genera of 15 families. The most abundant families of Trichoptera in the Tapee River were Hydropsychidae, Leptoceridae, and Ecnomidae. The more widespread Trichoptera species included Ecnomus atevalus Malicky & Chantaramongkol, Dipseudopsis knappi Schmid & Denning, D. robustior Ulmer, Aethaloptera sexpunctata (Kolenati), Amphipsyche meridiana Ulmer, Cheumatopsyche cognita (Ulmer), Macrostemum fenestratum (Albarda), Ceraclea idaia Malicky & Chaibu, Oecetis tripunctata (Fabricius), and Parasetodes respersellus (Rambur). The upper Tapee River Trichoptera species included Rhyacophila petersorum Schmid & Denning, R. tantichodoki Malicky & Chantaramongkol, Ugandatrichia kerdmuang Malicky & Chantaramongkol, Chimarra spp., Pseudoneureclipsis spp., Psychomyia spp., Cheumatopsyche charites Malicky & Chantaramongkol, Diplectrona spp., Hydropsyche spp., Hydromanicus inferior Chantaramongkol & Malicky, Goera spp., Ganonema fuscipenne (Albarda), Goerodes abruptum Banks, and Marilia spp. The lower Tapee River Trichoptera species included Ecnomus talenoi, E. paget, Polymorphanisus spp., and Oecetis bengalica. The number of Trichoptera species was higher upstream and lower downstream.

Seventy-five species of caddisflies were found near eight waterfalls in the Tai Rom Yen National Park and the Khao Luang National Park, within Nakhon Si Thammarat Range, in southern Thailand. Rhyacophila suratthaniensis sp. n. is described and figured, based on adult males. Rhyacophila suratthaniensis sp. n. is distinguished from other species of the genus by the shape of its paramere: their basal part is broader than the apical one and is separated from it by a step; apex of the paramere bears many strong spines, which are arranged like in a toothbrush. Occurrence of Trichoptera species in the observed and neighbouring areas is discussed. http://zoobank.org/urn:lsid:zoobank.org:pub:336401B5-DAAA-4818-90EA-5DCB1944D1F0

Three new species of Trichosetodes, namely T. carmelae sp. nov., T. katiengensis sp. nov. and T. ratanakiriensis sp. nov. are described and illustrated by male specimens. The male genitalia of T. carmelae sp. nov. can be distinguished from the other 16 species of the genus found in Southeast Asia by the shape of the phallicata. The phallicata of T. carmelae sp. nov. bears a tuft of long hairs in the middle of the dorsal edge. Trichosetodes katiengensis sp. nov. can be distinguished from the other species in Southeast Asia by the shape of the phallicata which is divided into dorsal and ventral branches in lateral view, and T. ratanakiriensis sp. nov. by the characters of the left inferior appendage and the shape of segment IX. The posterior end of the left inferior appendage of T. ratanakiriensis sp. nov. is not forked and the ventral and lateral views of the posteroventral lobes of segment IX are rounded. Illustrations of male genitalia of Trichosetodes kampongspeuensis Malicky & Kong, 2020 are provided for comparison. The molecular diversity of new Trichosetodes species was analyzed using the mitochondrial large subunit ribosomal rRNA gene region (16S rRNA). In terms of their genetic divergence, T. ratanakiriensis sp. nov. and T. kampongspeuensis exhibited remarkable proximity, with only a 1.4% distance. On the contrary, T. carmelae sp. nov. displayed genetic disparity exceeding 6.3% when compared to both T. ratanakiriensis sp. nov. and T. kampongspeuensis.

Trichoptera were surveyed from four different physiographic regions of the Lower Mekong River, including the Northern Highlands, the Khorat Plateau, the Tonle Sap Basin and the Mekong Delta in Myanmar, Laos, Thailand, Cambodia and Vietnam. Twenty-three species of the genus Ecnomus, including a new species, were identified and mapped. Ecnomus mammus Malicky & Chantaramongkol, 1993 is a common species which is found from Tonle Sab Basin and Mekong Delta. Ecnomus alkmene Malicky & Chantaramongkol, 1997, E. volovicus Malicky & Chantaramongkol, 1993 and Ecnomus quordaio Malicky, 1993 are the common species in the area of the Northern Highlands and the Khorat Plateau. Ecnomus plotin Malicky & Laudee 2010 is found in the Mekong Delta. Ecnomus triangularis Sun, 1997 is a new species record for Southeast Asia. In addition, E. stungtrengensis sp. n. is described and illustrated. The male genitalia of E. stungtrengensis sp. n. are clearly different from those of other species in the genus Ecnomus by the shape of superior appendages which are slender and bent downwards distally in lateral view.