40 REPORT ON THE DISEASES OP SILKWORMS IN INDIA 



of infection in the hypodermis or inner layer of the skin (Text- 

 fig. B) become gradually surrounded and infiltrated by a deposit 

 of chitin — the hard, non-living substance that forms the outer 

 skin and all hard parts of insects (Plate II, figs. 3 and 4). It is a 

 sort of cyst-wall being formed round the nest of parasites — the 

 parasites are enclosed in an impervious, dead cyst and so prevented, 

 from doing further injury to the host. This is a common response 

 of insects to parasitic attack. These black spots — or dark brown 

 rather because chitin in mass is that colour — are in India found 

 chiefly in caterpillars from diseased layings, as indeed the hypodermis 

 does not as a rule get infected very heavily with the parasite unless 

 it acquires its infection in the egg. At times, too, minute black 

 specks are found on the outside wall of the gut of caterpillars that 

 have been very heavily infected with pebrine (Plate II, fig. 2). On 

 examination these are seen to be of the same nature as the spots 

 in the skin- — chitinised cysts containing pebrine spores. 



Such then is a short review of the facts that we can observe regard- 

 ing the resistance responses of the silkworm to attacks by Nosema, 

 and it must be admitted that the natural immunity of the cater- 

 pillar does not seem very effective in preventing or arresting the 

 onset of this particular parasite. But if we consider a moment this 

 is hardly to be wondered at. Reference has already been made to 

 the probable, almost certain, antiquity of the association of the 

 caterpillar with Nosema. In dealing with the history of the causal 

 organism of pebrine it was argued that Nosema bombycis was an 

 "old established parasite of the silkworm and probably was present 

 in the silkworm long before its domestication. If this be so it 

 explains certain points that we have been dealing with above. 

 We have seen that it is characteristic of an efficient parasite that it 

 should not under ordinary circumstances greatly injure its host — 

 especially that it should not produce toxins — and that in turn the 

 host should be tolerant of the parasite. It might also be added that 

 in a case of parasitism such as a wild Nosema infection it would be 

 essential for the infection to be easily acquired, for under normal 

 conditions the risk of infection would not be great and if infection 

 were not readily accomplished the parasite would tend to die out. 

 All these conditions are really found in pebrine. The balance estab- 

 lished between the host and the parasite in the distant past is in 

 the main lines still preserved. Nosema easily infects the caterpillar 

 and in mild infections does relatively little harm, while in turn the 

 caterpillar is very tolerant of the parasite and does not, so to speak, 

 resist it in force. The changed conditions, however, from wild life 

 to domestication have enabled the parasite to infect the host much 

 more heavily than it could in nature, and the environment under 

 domestication is not so favourable to the caterpillar, so that probably 



