PROC. ENTOMOL. SOC. WASH. 87(1), 1985. pp. 135-141 THE BIOLOGY OF XYLOPHILIC CECIDOMYIIDAE (DIPTERA) Emily A. Rock' and Dale Jackson Department of Biology, The University of Akron, Akron, Ohio 44325. Abstract.— The general biology of six new Nearctic species of wood vessel-inhabiting Cecidomyiidae is described. Adults have fixed activity periods which vary with species. Females oviposit in exposed, non-decayed hardwood vessels. Host wood becomes progressively less attractive for oviposition as fungal growth on exposed surfaces increases. In all species larval development requires a min-imum of 14 days, but emergence from the vessel may be delayed for several weeks. Emergence requires rainfall sufficient to saturate the wood. Pupation takes place in the ground and the length of the period varies with species and sex; the minimum time between generations is 24 days. The insects overwinter either as larvae in the vessels or as larvae in cocoons in the soil. Sex ratios in two species are 3:1 and 4:1 respectively with a predominance of females. Larvae are parasitized by platygasterid wasps; the parasitism rate is approximately 5%. Ovipositing female midges are preyed upon by empid flies. Techniques for rearing the midges in the laboratory are described. Larvae of the family Cecidomyiidae have a great diversity of feeding habits. Most attack the meristematic tissues of a wide variety of plants such as hard and soft wood trees, root crops, grains, and fruits, and many are of economic impor-tance. In some species the larvae are zoophagous, attacking small prey such as aphids, scale insects, and mites. Others feed on decaying vegetation and some are mycophagous. Amidst this diversity there occurs a unique group of xylophilic larvae that live in the xylem vessels of freshly cut hardwoods. The adult females seek out newly felled logs and broken branches and oviposit into the exposed vessel openings. Kieffer (1900, 1913) described several species of xylophilic cecidomyiids and outlined the main events in the life cycle of some European species, but no significant contributions to the biology of this group have been published sub-sequently. We are reporting the general biology of species of xylophilic Cecido-myiidae found during a period of study in northeast Ohio. The study involved the collection and identification of xylophilic species and observations on the general life cycle. Materials and Methods Field and laboratory observations were made on larvae and adults collected from local populations. Quercus alba L. (Fagaceae) and Fraxinus amehcana L. ' Present address: The Wayne General and Technical College, 10470 Smucker Road, Orrville, Ohio 44667.