Rhopalomyia thompsoni Felt 1907

Dorchin, Netta, Mcevoy, Miles V., Dowling, Todd A., Abrahamson, Warren G. & Moore, Joseph G., 2009, Revision of the goldenrod-galling Rhopalomyia species (Diptera: Cecidomyiidae) in North America, Zootaxa 2152, pp. 1-35 : 32-33

Rhopalomyia thompsoni Felt 1907

Rhopalomyia thompsoni Felt 1907b: 159 .

Adult: Antenna with 17 or 21–22 flagellomeres in male (spring and summer generations, respectively), 16–17 or 20–25 flagellomeres in female (spring and summer generations, respectively); necks of male flagellomeres III–VII 0.43–0.60 times or 0.85–1.00 times as long as nodes (spring and summer generations, respectively), female flagellomeres without necks. Palpus 2-segmented, long, setose and setulose; second segment at least 1.5 times as long as first, tapered distally. Wing length 3.3 or 3.1–3.3 mm in male (spring and summer generations, respectively), 2.8 or 2.6–3.0 mm in female (spring and summer generations, respectively). Male terminalia ( Fig. 25 ): gonocoxite robust, with apicoventral projection, strongly setose, with setose mediobasal lobe; gonocoxal apodeme broad, undivided; gonostylus short and wide, almost same width throughout length, strongly setose and setulose, with many setae close to the apical, brush-like tooth; aedeagus cylindrical, slightly truncated and rounded apically; hypoproct M-shaped, setose and setulose; cerci very wide, separated by deep notch, setose and setulose. Female abdomen (Fig. 39): tergite 7 rectangular, weakly to unsclerotized in mid-anterior part, with two anterior trichoid sensilla, several rows of setae posteriorly, and groups of setae mesolaterally; tergite 8 Y-shaped, very long and slender, proximal arms arched mesad, slender, less than third the length of shaft, each with anterior trichoid sensillum; ovipositor 11.2 or 7.3–7.8 times as long as tergite 7 (spring and summer generations, respectively).

Pupa: unknown.

Type material: Rhopalomyia thompsoni Felt. Syntypes: 1 male, 1 female, USA, Worcester, MA, 3/V/1907, M.T. Thompson, Felt # 1100, deposited in Felt Collection.

Other material examined: 4 males, 4 females, USA, Freese Rd., NY, 18–25/IX/1987, M.V. McEvoy, ex. S. altissima .

Host: Solidago altissima

Gall and biology: This species has two generations per year, both of which develop in rhizome galls. The spring generation galls are solitary or clustered, bulbous, fleshy masses with 1–8 chambers each. As the galls develop, they become apparent above the ground and burst open before adult emergence in early May ( Felt 1915). The second generation galls constitute brownish, globular, and usually multi-chambered swellings of the rhizomes, which were still completely subterranean in mid August, when they contained mature larvae. Pupation took place in late August to early September, at which time the galls became elongate and fleshier. Shortly before adult emergence the galls became apparent above the soil surface, and adults emerged from mid to late September. In the original description of this species, Felt (1907b) named Solidago rugosa and later (1915) S. juncea as the host, but in the present study we were unable to find similar spring galls on these plants. Given the similarity of the galls and the adults of the two generations, we conclude that they represent the same species and consider S. altissima to be the host plant of R. thompsoni . Distinguishing among different Solidago species in early spring can be difficult; hence it is not unlikely that the original host was misidentified. Confirmation of this conclusion will require locating spring-generation galls and verifying the identity of the host in which they are formed.

Remarks: This species resembles R. hirtipes both in male and gall morphology, but the two species occur on different hosts and show morphological differences in females. Males of both species have robust gonopods with wide gonocoxal apodeme and ventral, meso-apical projection of the gonocoxite. Females of R. thompsoni have neckless flagellomeres and very long and slender abdominal tergite 8 with mesally arched arms, as opposed to short-necked flagellomeres and thicker and widely splayed arms of abdominal tergite 8 in females of R. hirtipes .

The summer and spring generations of R. thompsoni differ in the number of flagellomeres in both sexes, in length of flagellomere nodes in the males, and in the length of the ovipositor. However, the descriptions of the spring-generation galls and the manner by which adults emerge from them ( Felt 1915) are similar to what we observed in the summer-generation galls, and adults from both generations are otherwise similar morphologically. Similar differences between spring and summer generations in number of flagellomeres and length of their necks are also found in R. solidaginis .

Figures

FIGURES 20 – 25. Male terminalia (in dorsal view unless otherwise noted). 20. Rhopalomyia inquisitor. 21. Rhopalomyia lobata. 22. Rhopalomyia racemicola. 23. Rhopalomyia solidaginis. 24. Rhopalomyia solidaginis, ventral. 25. Rhopalomyia thompsoni. Scale bars = 0.1 mm