742 MEDICAL ENTOMOLOGY 



flagellates is generally simple. They are confined to the alimentary 

 tract of the host, and usually to the hind-gut. In due course a stage is 

 reached when the parasite is fitted, by encystment or otherwise, for a 

 more or less brief exposure to the external atmosphere, and it is then 

 passed out with the faeces. This occurs in such a situation that the 

 young form which is to start a fresh infection can reach its host, for, as 

 in the protozoa of a more complex life history, the parasite is depend- 

 ent for its continued existence on the habits and bionomics of its host. 

 Defaecation may occur at the time of feeding, so that the infective stages 

 fall upon the food and are in a position to be taken up by the next fly 

 which feeds there, as in the well known instance of Herpetomonas muscae 

 domesticae ; or the parasites may, in the blood-sucking forms, be passed 

 out with the faeces on to the skin of the host of the arthropod. In the 

 latter case their chance of reaching the alimentary tract of another insect 

 is much greater than would at first be thought. It has been pointed 

 out in connection with several groups of Arthropods in previous chapters 

 that particular species exhibit a marked preference for certain parts of 

 the skin of the host ; some species of Tabanidae are constantly found 

 feeding on one part, other species on another, and the same is the case 

 with lice and fleas. The localization of the individuals of the host 

 species is all to the advantage of the parasite where infection is dependent 

 on such contamination, as is the almost universal occurrence among 

 blood-sucking arthropods of the habit of defaecating while feeding. 

 Tabanus striatus is one of the flies which has been observed to select a 

 particular site on the skin of the host, and its parasite, Crithidia tabani, 

 is almost certainly transmitted in this way. A similar means of 

 transference is readily understood with regard to gregarious insects. 



The parasite may not be confined to the imaginal stage, but may 

 occur in the larva, and may be transmitted from larva to larva and 

 from larva to adult. This occurs in the case of Herpetomonas culicis, 

 already referred to. Here again the parasites are dependent for their 

 propagation on the habits of the host. Culex fatigans, like most of the 

 members of its family, rests during the day in dark and preferably damp 

 situations, and the male may often be seen in considerable numbers hear 

 secluded collections of water, probably attracted there by the newly 

 hatched females. Their faeces are passed into or near the water in 

 which the eggs are laid, and the contained parasites find their way into 

 the gut of the larvae along with its food. The parasites are still pre- 

 sent in the larva when it pupates, and pass to the imago, where they 

 undergo their final cycle, and produce forms which are capable of infect- 



