200 MR P. BRUCE WHITE ON 



further, the procedure may occasion atrophic muscle changes which are only known 

 to occur in that disease. When the first pair of spiracles is closed, a state of com- 

 plete incapacitation results, ending rapidly in death. 



Though too close a parallel must not be drawn with the natural disease, these 

 experiments are illuminating in that they give a basis to the view that the role of 

 parasites in partially preventing thoracic respiration is of prime importance in the 

 disease — possibly in itself capable of occasioning all the symptoms by which we are 

 wont to diagnose the disease and the muscle atrophy so often associated with it. 



The pathological syndrome of Isle of Wight disease is undoubtedly complex. 

 Apart from the sapping of the host fluids and the probable injection of a venom, 

 the mites may impair the mechanism of the bee either by destroying the respiratory 

 supply of the individual organs or by cutting oft' that of the nerve centres which 

 control and co-ordinate their activities. It is possible that the indirect eff'ect 

 through the nervous system, possessed as this is of a dual respiratory supply, is 

 particularly acute when there is considerable bilateral obstruction of the tracheal 

 system. 



Through the combined influence of these factors the power of flight is lost, and 

 a series of secondary conditions arise. 



The faeces normally voided on the wing accumulate, thus increasing the difficulty 

 of locomotion and compressing the abdominal air-sacs — another blow at the respira- 

 tory function. Jntestinal pressure must hinder the excretory activities of the 

 Malpighian tubules, and this excretory stasis, together with the absorption of toxins 

 from the stagnant gut, must be reflected back upon the body of the insect. , 



As soon as the power of flight is lost death of the bee becomes imminent, for 

 once it leaves the warmth and stores of the hive, unable to return, it perishes of cold 

 and starvation. Should it elect to remain within the hive it is faced with a prospect 

 of functional stagnation which cannot be indefinitely maintained. It would seem too 

 that in the colder months sick stocks often perish en masse through inability to 

 maintain the hive temperature. 



It seems that in rare cases individual bees may recover from the attack upon 

 being abandoned by the parasitic brood. Such cases are recognised by their bronzed 

 and blackened tracheae, which, however, contain no living mites. Bees in this con- 

 dition have been found foraging for infected stocks. 



Concluding Remarks. 



It is somewhat remarkable that the macroscopic changes of the thoracic tracheae 

 and muscle have so long escaped observation in spite of the detailed examinations of 

 several independent workers. 



Imms (l) held that " the disease is eminently one of the digestive system, and 

 might be described as a condition of enlargement of the hind ' intestine," while 



