14 THE RICE WORM (tYLENCHUS ANGUSTUS) AND ITS CONTROL 



breathed upon. In moderately dry air, or even under such conditions of 

 humidit}^ as ordinarily occur in the laboratories at Pusa during the monsoon 

 months, movement on a dry surface is apparently impossible. In air that is 

 approximately saturated, moisture is condensed round the body of the worm, 

 appearing as a film or sometimes a minute droplet, in which movement is 

 relatively free. In such conditions worms have been found to pass round to 

 the under surface of the slide, and even to travel on to other slides on benches 

 above or below that on which they were placed in the moist chamber. Not 

 only can they move on dry surfaces in a saturated atmosphere, but tliey can 

 enter and leave drops of water, though the latter cannot be easy if the resist- 

 ance caused by surface tension be taken into account. >Some batches examined 

 seem to have had a much greater tendency to leave the drop of water in which 

 they were sown than others, and it is tempting to suppose that this is due to a 

 copulatory instinct. At any rate, it has been observed that the worms that 

 have left a drop often come together in twos or threes, and in several 

 cases a male and a female have been found applied together, either extended 

 or partially coiled over one another, though actual copulation has not been 

 seen. Under no other circumstances has the writer observed anything of the 

 kind, and he is fairly satisfied that copulation must take place in a saturated 

 atmosphere, ^^^thout free liquid, though perhaps confined to worms on the 

 living host plant. It should be mentioned, however, that larval stages have 

 occasionally been seen to possess the same instinct as the perfect forms. 



Several of the earlier inoculations carried out at Pusa failed owing to 

 ignorance of the above facts. They were made on plants growing in pots in 

 the laboratory with no precautions to keep the air around the plants near the 

 saturation point. In some the worms were inserted within the leaf sheaths ; 

 in others worms or eggs or infected pieces of paddy were placed in water around 

 the base of the plants. Field inoculations carried out in the same way 

 succeeded in every case when performed during the monsoon months. During 

 this period the humidity is always high, and the air surrounding the plants, 

 especially when, as was the case, these grow close together, is probably 

 approaching saturation. It has now been found that to get successful 

 results in laboratory inoculations the air must be kept moist by the use of 

 bell jars or other similar means. Only under very exceptional conditions 

 of humidity have attempts at inoculation within-doors on uncovered plants 

 given any result {see Expt. Ill on p. 20). 



We can now understand several of the peculiarities in the incidence of the 

 disease under natural conditions. As already stated, the boro paddy, Avhich 

 is grown during the period from January to April, escapes the disease. This 



