PROC. ENTOMOL. SOC. WASH. 88(2), 1986, pp. 316-319 HOST SELECTION IN XYLOPHILIC CECIDOMYIIDAE (DIPTERA): VESSEL SIZE AND STRUCTURE Emily A. Rock and Dale Jackson (EAR) Wayne General and Technical College, 10470 Smucker Road, Orrville, Ohio 44667; (DJ) Department of Biology, The University of Akron, Akron, Ohio 44325. Abstract. — Larvae of xylophilic Cecidomyiidae develop only in woody angio-sperms with vessels of 75 n or greater diameter. Because vessel diameter varies with trunk or branch diameter, mature parts of some species of hardwoods may offer suitable niches for larval development while smaller branches or twigs may not. Vessel diameters of the herbaceous angiosperms tested were less than 75 n and gymnosperms lack vessels; hence neither are hosts. Xylophilic Cecidomyiidae are an unusual guild of gall midges that use freshly exposed xylem vessels of hardwoods as a larval niche. The biology of two European species was described in detail by Kieffer (1900). We reported on the biology of several nearctic species that live in the vessels of hardwoods in Ohio (Rock and Jackson, 1985). All known species have similar multivoltine life cycles. Females oviposit in sapwood vessels of freshly cut hardwoods where larval development is completed in approximately two weeks. Mature larvae emerge from the vessels when rainfall is sufficient to soak the wood. Pupation occurs in soil and summer generations of adults emerge 10-16 days later. Host plant suitability probably depends on several factors including: olfactory attraction to exposed wood, suitable vessel diameter for oviposition, vessel di-ameter adequate for larval development, and available nutrients. This paper ex-amines the significance of vessel diameter on host tree suitability. Data is reported for the following species: Xylodiplosis longistylus Gagne; Trogodiplosis flexuosa Gagne; Ledomyia ernilyae Gagne; Ledomyia mira Gagne; and Ledomyia parva Gagne. Unless otherwise indicated, X. longistylus was used for laboratory studies because of its ready availability. Methods and Materials We have previously described methods for collecting and rearing the adults and larvae (Rock and Jackson, 1985). Freshly cut samples of 17 species of hard-woods and one species of softwood were collected for this study and placed at field sites where natural populations of the midges occurred (Table 1). Stems from some of the herbaceous angiosperms found at or near the field sites were also examined (Table 1). Quercus and Fraxinus have been described as suitable hosts (Kieffer, 1900, 1904; Huggert, 1980; Rock and Jackson, 1985) and we initially used them as
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