730 MEDICAL ENTOMOLOGY 



habits of the invertebrate carrier is essential for a proper understanding 

 of the epidemiology of the disease, and forms the foundation for all 

 rational preventive measures. It is not, perhaps, quite so clearly 

 recognized that an equally intimate knowledge on the same subjects, 

 and in addition a close acquaintance with the anatomy of the inverte- 

 brate, often down to the minutest details of cell structure, is required 

 for the study of the parasite itself, during that stage of its existence in 

 which the body of the invertebrate constitutes its environment. A 

 study of the morphology of the parasite alone, though it may bring to 

 light, facts of great scientific interest, cannot be expected to lead to a 

 complete knowledge of its life history and conditions of existence, since 

 these are inevitably dependent on the life history, habits, and bionomics 

 of its hosts. 



The study of the internal structure of the invertebrate host is of 



particular importance, as it furnishes a valuable control for experimental 



observations, and enables one to avoid some at least of 



normal histology t ^ e manv pitfalls which beset the path of the worker 

 in this field. Unfortunately it is in this direction that 

 the greatest difficulty is met with. The subject has received little atten- 

 tion within recent years, and for much of the information at our disposal 

 we are indebted to a past generation of scientists, who carried out their 

 observations, often with very imperfect apparatus but with an accuracy 

 which is astonishing, at a time when parasitology in its modern sense did 

 not exist. Much more work in this direction is urgently needed, and until 

 it has been done some doubt must always attach to observations on the 

 more minute stages of protozoa parasitic in insect tissues. The disputed 

 points regarding the life history of spirochaetes will occur to the reader 

 in this connection : the phenomenon of ' granule shedding' in vertebrate 

 blood is not excessively difficult to follow, because the observer is 

 familiar with the normal appearance and constituents of the medium in 

 which the organisms lie, but phenomena of a similar nature taking place 

 in insect tissues demand the greatest reserve in interpretation. It cannot 

 be claimed that our knowledge of the normal histology and physiology of 

 these tissues is more than the merest fragment of the truth ; the study 

 of them has lagged far behind that of the protozoa which infect them. 



When dealing with the blood-sucking arthropods it has to be remem- 

 bered that they are exceptional members of their group, and cannot be 

 expected to conform to the usual type. The habit of feeding on blood is 

 associated with many other features both of structure and life history 

 which are common to many forms from widely, separated families, and 



