Review of Mesocallis Matsumura from China (Hemiptera, Aphididae), with one new species

Abstract The aphid genus Mesocallis Matsumura in China is reviewed. A total of seven species are recognised using morphological characteristics, including six known species, Mesocallis (Mesocallis) alnicola Ghosh, M. (Paratinocallis) corylicola (Higuchi), M. (M.) pteleae Matsumura, M. (M.) sawashibae (Matsumura), M. (P.) yunnanensis (Zhang) and M. (M.) taoi Quednau, and one new species, M. (M.) platycaryae Qiao, sp. nov. The new species, collected on Platycarya strobilacea (Juglandaceae) in Anhui Province, China, is described and illustrated. A key to Mesocallis species from China is presented.


Introduction
The aphid genus Mesocallis was erected by Matsumura (1919), with Myzocallis sawashibae Matsumura, 1917 as the type species. The genus has distinct morphological characteristics; a narrow body, antennae much shorter than the body, antennal segments IV-VI scarcely imbricated, segment III of the alatae with one row of oblong, secondary rhinaria along all or most of its length, and empodial setae distinctly longer than the claws (Higuchi 1972;Quednau 2003;Qiao et al. 2005). Currently, this genus includes 10 species placed in two subgenera: Mesocallis (Mesocallis) alnicola Ghosh, M. (Paratinocallis) corylicola (Higuchi) (Zhang), and M. taoi Quednau (Blackman and Eastop 2020;Favret 2020). These species all are associated with plants of the family Betulaceae and are mainly distributed in East Asia. Six species are hitherto recorded from China: M. alnicola, M. corylicola, M. pteleae, M. sawashibae, M. taoi, and M. yunnanensis. Recently, some apterous specimens on Platycarya strobilacea (Juglandaceae) were collected in Anhui Province (Dabieshan Mountain), which were identified as a new species in this genus. Herein, the genus Mesocallis from China is reviewed, a key to Chinese species is provided, and the new species is described and illustrated.

Materials and methods
The brief procedure of making aphid slide-mounted specimens follows that of Jiang et al. (2016). The descriptions and drawings provided here were produced from slidemounted specimens using a Leica DM4000B and drawing tube. The photomicrographs were prepared with a Leica DM2500 using DIC illumination and processed with Automontage and Photoshop software.
Fourth instar nymph. Body oval (Fig. 15), pale brown. Cauda circular at apex, otherwise similar to apterous viviparous female. Embryo. Dorsal body setae thick, long, and with terminal large round knobs. Head with three pairs of anterior dorsal setae, and two pairs of posterior marginal setae; pronotum with two pairs of spinal setae and one pair of marginal setae, some 20714-1-1-1, on Platycarya strobilacea, coll. J.J. Yu (NZMC). Paratypes: 1 apterous viviparous female, 1 third instar nymph, and 1 fourth instar nymph (NZMC), the collection data is the same as in the holotype.
Taxonomic notes. Based on the following morphological characteristics in apterae and nymphs of dorsal body setae with round knobbed apex, 5-segmented antennae, much shorter than the body, hind tibial distal setae similar to other setae on the segment, distal part of tibiae and tarsi spinulose, and abdominal tergites I-VII each with one pair of marginal setae, the new species should clearly be placed in Mesocallis. The species is characterised by the dark-brown dorsal body setae, which are placed on unsclerotized tuberculate bases, and by its colonisation of Platycarya strobilacea (Juglandaceae). Mesocallis platycaryae resembles M. taoi in the number of antennal segments, the ratio of antennae to body length, the length and the number of accessory setae of ultimate rostral segment etc., but apterae differ from those of M. taoi as follows: dorsal body setae dark brown, not arising from sclerites (M. taoi has the dorsal body setae pale but on pigmented sclerites); shafts of dorsal body setae largely smooth, only sparsely spinulose on part of length (vs long dorsal body setae with spinulose shafts); antennae and tarsi pale (vs distal part of antennal segments III-V and tarsi brown). The new species differs from M. carpinicola and M. pteleae in: ultimate rostral segment 0.06 mm long, and with two accessory setae (M. carpinicola and M. pteleae: 0.10-0.14 mm long, with four or more accessory setae), head vertex and antennal segments I-III pale (vs blackish), cauda with 6-8 setae (vs 7-15 setae). In addition, the new species may be distinguished from M. obtusirostris by: antennae 0.46-0.48× as long as body (M. obtusirostris: antennae 0.61-0.75× as long as body), ultimate rostral segment 0.74-0.78× as long as second hind tarsal segment (vs 0.50-0.55×). The difference between the new species and other species of subgenus Mesocallis may be found in the key below.
Distribution. China (Anhui). Host plant. Platycarya strobilacea (Juglandaceae). Biology. The species lives scattered on the underside of leaves of host plant.