The Cystic Stage of Octomyomermis itascensis (Nematoda: Mermithidae) in Chironomus plumosus (L.) (Diptera)

Abstract

Surveys of Chironomous plumosus larvae in Lake Itasca, Minnesota, in the summers of 1959 and 1961 document the existence of a cystic larval stage of Octomyomermis itascensis. The cysts are concentrated in the thoracic hemocoel and vary in number from 1 to 5 per insect. Each cyst contains a single juvenile nematode. Apparently the nematode reaches a given stage of development in the cyst and must excyst to continue development. Evidence is presented for the parasite origin of the cyst wall. Humoral factors of the host may play a role in excystation of the nematode. Couturier (1951) reported the presence of a capsule investing a mermithid juvenile in the development of Tunicamermis melolonthae in grubs of Melolontha melolontha in France. Couturier (1963) indicated that the preparasitic juvenile penetrates the grub and induces proliferation of host tissue around the parasite. The capsule enlarges as the nematode grows in size. The final capsule is large, irregular, and attached to the host by a stalk. This is the only record of encapsulation of mermithids as a normal developmental event. In July of 1959 an examination of 1,844 fourth-stage larvae of Chironomus plumosus from the benthos of Lake Itasca, Minnesota, revealed the presence of a small encysted nematode in 34.4% of the insects. Johnson (1963) described the adult of this form, Octomyomermis itascensis. Three other undescribed species of mermithids are known by the investigator to be present in the same host in the same habitat. One of these nematodes is imagocidal and the other two larvicidal. Early stages of the two larvicidal forms are known and do not have a cyst enclosing them. The small, round, flattened cysts of O. itascensis contain a single juvenile reaching a maximum size of 1.7 mm in length. The cysts are contiguous with muscles, fat bodies, imaginal discs, salivary glands, and other structures of the host. The cyst wall is not connected to host tissues or the juvenile. The red, transparent larvae of C. plumosus possess a head capsule, prothoracic prolegs, 12 body segments (3 thoracic, 9 abdominal), Received for publication 12 July 1973. posterior prolegs, and posterior gills. A sample of 172 infected host larvae was documented as to the number of cysts per host; 121 or 70% had one cyst, 34 or 20% had 2, 8 or 5% had 3, 7 or 4% had 4, and 2 or 1% had 5. Despite the presence of more adequate hemocoelic space posteriorly the cystic nematodes were more abundant in anterior segments. Two hundred twenty-six cysts were di tributed as follows: thoracic segment I, 51; II, 65; III, 45; abdominal segment I, 30; II, 8; III, 5; IV, 2; V, 1; VI, 1; VII, 0; VIII, 1; IX, 5; and posterior prolegs, 12. As many as three cysts have been found in one segment. No mechanical problems were observed in the insects. The acellular membrane composing the cyst wall is transparent, homogeneous, without pigmentation, and about twice the thickness of the cuticle of the nematode. The entire cyst can be readily dissected out of the host. The staining properties of the membrane resemble those of the nematode cuticle very closely. The size and structure of the cystic nematode was uniform. Measurements of six cystic juveniles indicate the following ranges: Length 1.4 to 1.7 mm, greatest diameter 87 to 97 It, stichosome length 600 to 900 u and gonad length 87 to 230 j/. The prominent structures in the cystic juvenile (Fig. 1) are the two large granular cells with oval nuclei anterior to the stichosome, the 16 linearly arranged irregular nuclei of the stichocytes, a rudimentary gonad posterior and ventral, and a trophosome occupying most of the posterior pseudocoel. The cuticular esophagus was present but could not be traced posterior to the granular cells.