THE DIPTERA BREEDING ON SKUNK CABBAGE, SYMPLOCARPUS FOETIDUS (ARACEAE)

The larvae of 19 species of flies belonging to 7 families were found to feed in the rotting portions of leaves, petioles, and flowers of skunk cabbage. Among flies bred from this plant, drosophilids were by far the most numerous, but there is considerable geographic and seasonal variation in composition and abundance of the fauna. The sex ratio of one species, a Bryophaenocladius sp. (Chironomidae), reveals it to be parthenogenetic. The breeding biology of the flies, with particular reference to other dipteran-aroid relationships, is reviewed. Symplocarpus foetidus is a common perennial plant of wet lowland areas in eastern North America. Inflorescences appear in early spring, the leaves expand about one month later, and seeds are formed in the fall, when the above-ground portion of the plant dies back. This species forms established patches that may vary in size from a few square meters to several hectares. Symplocarpus foetidus would thus seem to be an "apparent" plant to po tential consumers, and as such one might expect it to be protected by "quan titative" defenses (sensu Feeny, 1976). It is: the tissues contain barbed crys tals of calcium oxalate, which may deter a variety of herbivores (B. Meeuse, pers. comm.). An herbivore that would cope with this defense, however, would be rewarded with a predictable supply of food. Hence, specialization of some herbivore species on S. foetidus might be expected to evolve. Although a few Coleoptera, Psocoptera and Collembola breed in skunk cabbages, the majority of insects bred from the plant are a variety of dip terans. We have never seen visible signs of serpentine or blotch mines, chewing areas, or petiole girdling which might be indicative of insect activity. All flies in this study were bred from rotting portions of plants and most likely fed on bacteria harbored therein. This does not, however, rule out the possibility that diptera may be the cause of decay in the portions of some plants. We show here that Symplocarpus foetidus is probably the major breeding site of the Holarctic species Scaptomyza graminum. Another sig nificant finding is that Drosophila recens, previously thought to breed ex clusively in mushrooms, also utilizes Symplocarpus as well.