Comparison of the head patterns in the Tylenchoidea (Nematoda).

The first comparative study of head patterns in tylenchs was that of Sher & Bell (1975). De Grisse (1911) and Coomans & De Grisse (1981) srudied rhe underlying nervous system. The new information added since then is reviewed in this paper to see if the various patterns are characteristic of higher groupings (e.,9. families) within the Tylenchoidea. Therefore I used published and unpublished scanning electron micrographs (sEM) to produce schematized drawings. Comparing the photographs it was obvious that variation was found in the expression of pattems and sensilla. The schematized drawings give the maximum infbrmation obtained: e.g. whatever sensillum, distinct on a single photograph of a species or genus, was added on the drawing. In contrast to other nematodes there are no distinct lips in tylenchs. The head is an amalgamated lip region, internally strengthened by a cephalic framework and often including annuli, perhaps derived from ones formerly part of the body. The head can be continuous with the body, or variously offset but is always small to very small and unspectacular. It bears the mouth opening, which is nlinute and situated apically and centrally. There are two consecutive openings: the mouth opening seen at the outside is the prestoma opening: small, rounded to oval in Tylenchoidea and Aphelenchina, dorso-ventral slit to I-shaped slit in Criconematoidea. The stoma opening is found less than a micrometer to the