REPOKT ON THE DISEASES OP SILKWORMS IN INDIA 37 



■or that some new conditions have upset the normal balance which 

 the two had established. 



Now with these few general points in mind we may try to examine 

 some of the features of the resistance of silkworms to 'Nosema 

 attack. In the first place, what is the kind of injury caused to the 

 caterpillar by the parasite ? If one feeds caterpillars heavily on 

 pebrine spores and after a few days examines them, they will show 

 almost certainly no outward sign of disease, and yet when they are 

 killed and their organs carefully examined it will be found that a 

 caterpillar may have thousands of cells in the gut-wall packed with 

 meronts and spores — half the gut may be infected — (Plates III and 

 VII, figs. 3 and 2), while other organs of the body such as the nerve- 

 chord, the excretory tubes (Malpighian tubes) and the silk-gland 

 may also be seriously attacked. The parasites may be present 

 in countless numbers, so that one at first cannot understand how 

 the caterpillar could live for a day — much less be apparently 

 quite well. The only and obvious inference from this observation 

 is that at all events no toxin is being produced by the parasite ; 

 or to put it more cautiously, that the waste products of the 

 parasite are not particularly deleterious to the caterpillar. It 

 seems possible, as has been already mentioned, that the planont, 

 at all events, produces some digestive ferment which helps it to 

 make its way through the peritrophic membrane and through 

 the gut-wall cells or other parts of the body, and some workers 

 have supposed that the meronts also produce a ferment which 

 helps them to break down the contents of the cell in which they 

 live. The enzyme produced by the meront, however, must be 

 very weak, as it does not enable it to break down the wall of the 

 cell in which it is multiplying and so spread from one cell to 

 another. On the whole it must be admitted that, whatever ferments 

 are secreted or waste products excreted by Nosema bombycis, 

 they do singularly little harm- to the silkworm. 



How then does the parasite cause the disease pebrine ? It 

 seems certain the damage caused is chiefly due to the first and third 

 causes given above — mechanical injury and lessening of food supply. 

 The gut-wall cells invaded are prevented from producing digestive 

 juice and also from absorbing food by the great mass of parasites 

 which fill them, and the parasites further absorb their food from the 

 sheltering cell and so still further rob the host of its food (Plate III, 

 fig. 3). -Thus arises the typical symptom of pebrine, namely, the 

 slow unequal growth of the worms (Plate IV) : they are being 

 starved, their food is not being properly digested nor can they 

 make full use of what is digested. The excretory tubes may be 

 attacked and the cells in them prevented from functioning so that 



