Rhoenanthus (Rhoenanthus) speciosus Eaton, 1881

Figures 10F, 11E, 13, 14

Materials examined.

5 larvae, Thailand, Narathiwat province, Klong Aika Ding stream, 5°47'45.9"N, 101°50'05.5"E, 22.IV.2018, B. Boonsoong leg.

Diagnosis.

The larvae of Rhoenanthus speciosus can be distinguished from those of other Rhoenanthus (Rhoenanthus) species based on the following characteristics: i) lateral subapical spine of the mandibular tusk present (Fig. 10F), ii) absence of medial rounded setae on the mandibular tusk, iii) length of the mandibular tusks ca 1.7-2.3 × length of the head, iv) length of the foretibiae ca 1.2-1.23 × length of the forefemora and about 2.72-2.82 × length of the foretarsi, v) leg with colour marking as in Fig. 11E, and vi) 4 or 5 bipectinated lateral setae on the mandibles (Bae and McCafferty 1991).

Egg (dissected from mature larva). Oval; with two large conical polar caps, (Fig. 13A); chorion with numerous scattered tubercles, with knob-terminated coiled threads at the equatorial zone; tagenoform micropyle; sperm guide circular (Fig. 13B).

Distribution.

Narathiwat and Songkla provinces.

Remark.

The larvae of Rhoenanthus speciosus were reported by Bae and McCafferty (1991) based on specimens from Indonesia and Malaysia. Parnrong et al. (2002) reported this species from Songkhla province (southern Thailand). In the present study, we found the larva of this species in the nearby Narathiwat province. The distribution of R. speciosus seems to be restricted to the south of the Isthmus of Kra, as was found for another mayfly species, Prosopistoma wouterae (Boonsoong & Sartori, 2019). These findings constitute the northern limit of the known distribution of this species.