Ecol:3 



reference to the mcanent of separating the nematodes from 

 the soil and not to the moment of sampling. Nematodes 

 might have been dormant in all of them. 



(2) Let us consider a field carrying a particular crop. The 

 field is infested by two potential parasites, one of which 

 is able to survive dry periods, while the second one is 

 more easily killed through prolonged or repeated desicca- 

 tion. This second type is not giving serious troubles, 

 but then the farmer starts an irrigation project whereby 

 he gives it a better chance, and he may end up by having 

 two serious pests. Or the fanner may give the first type 

 of nematode still better conditions, such that it is able 

 to increase the number of generations per year and, hence, 

 to do more damage. 



Sandy soils have a low water holding capacity and dry up easily while 

 there is more stabilizing effect to moisture fluctuations in clay soils. 

 A priori I should therefore think that an infestation of a given size 

 would be more serious in clay than in sandy soil. 



Another aspect of nematode ecology which I would like to say scmething 

 about is how nematodes actually live in the soil. We know that they live 

 in soil crevices, that is, in already existing pore spaces. It follows 

 from this that nematodes do not influence the soil structure mechanically 

 as do, for example, the earthworms. 



We also know a good deal about their food. Three groups can be distin- 

 guished: One group feeds on liquid substances j this holds time of all 

 species with a stylet or spear. Nematodes of this group pierce the 

 surface of cells and feed on the cell content, whether it be a plant, 

 fungus, or animal cell. Another group— and one very rich in species — 

 feeds on particulate food, especially bacteria and small soil algae. 

 A third group, comprising e.g. Hononchus and a few others, seems to feed 

 on larger organisms which they catch alive, 



I would like to point out that all nematodes feed selectively. They do 

 not ingest soil in order to digest what is digestible in it for them and 

 then pass out the residual matter as do oligochaetes. In agar cultures 

 it is quite easy to watch the particulate feeding type of nematodes pick 

 up single or small batches of bacteria. 



Having outlined what they do in the soil, it might be interesting to 

 inquire into their metabolism. Nematodes range in weight from about 0.02 

 micrograms to 60 micrograms, the majority falling iri. thin the range of 

 0.5-5 micrograms. Their respiratory metabolism varies largely :d.th 

 their size: 



Monhystera , Prismatolaiinus Approximately 1500 ml. O2 per kg. 



per hour 

 Plectus 1200 at 16° 0. 



