| Summary: | 碩士 === 臺灣大學 === 昆蟲學研究所 === 98 === Horsehair worms (Nematomorpha) are arthropod parasites. They are famous for their unique life cycles and host manipulations. About 300 extant species have been identified, but none in Taiwan been recorded. Based on the morphology of male’s tail, six types of areoles, vertical pigment line, and mitochondrial COI partial sequence, I described a new common horsehair worm species in Taiwan, Chordodes formosanus n. sp. and their morphological polymorphisms on the heads and tails. Chordodes formosanus parasitized with mantids Hierodula formosana and H. patellifera. When matured adult worms left their hosts, then mated and laid eggs in water. The hatched larvae infected paratenic hosts such as aquatic animals. The infection was detected in larval midges Chironomus sp. from Taipei Zoo. The horsehair worms were found in the host’s gut wall, trunk, posterior parapod, anal tubes, and ventral tubules. Logically, the horsehair worms land with aquatic insects’ imagoes. This was confirmed by the detection of infection in adult caddisflies (Chimarra formosana) and one unknown midge (Chironomidae) by using polymerase chain reaction (PCR). The horsehair worms invaded and developed inside the definitive host after the infected paratenic hosts were preyed. From the morphological comparison, the infected mantids, H. formosana, showed shorter wings in both sexes with shorter antennae and a reduction in the numbers of antennal sensilla in the infected males. This is the first discovery of a host’s sense organ change in the parasitism of horsehair worms. The reduced antennal sensilla made the infected male look similar to the nymph’s antennae. This may be the result of parasite-induced juvenilization. Additionally, the reduction in antennal sensilla was also observed in male H. patellifera fallen in water.
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