NATURAL PARASITES OF INSECTS 741 



infection of G. palpalis with T. gambiense depends almost entirely on the 

 presence of adult trypanosomes (short forms) in the blood of the verte- 

 brate ; such a hypothesis appears to assume that these forms represent 

 some sexual stage, otherwise it is difficult to explain why they alone can 

 produce infection in the invertebrate. A sexual cycle has never yet been 

 demonstrated. Another instance is to be found in the behaviour of 

 Herpetomonas culicis, a natural parasite of Culex fatigans. The early 

 stages of this organism are passed in the larva of the mosquito, and the 

 fully formed flagellates are found in the newly hatched imagines of both 

 sexes. In caught specimens from infected localities, however, only 

 the males and a small percentage of females are found to harbour the 

 flagellate, and if bred females known to be infected are given a meal of 

 blood the parasites are found to have totally disappeared ; this suggests 

 that vertebrate blood is an absolutely unfavourable medium for this 

 organism. Many interesting problems suggest themselves with regard to 

 the effect on the contained parasites when a blood-sucking invertebrate 

 feeds on a series of different species of hosts. 



NATURAL PARASITES 



Under the term 'natural parasites' are included those organisms which 

 pass through the whole of their life cycle in the invertebrate, and have 

 no alternation of hosts in the sense of the foregoing pages. Such 

 parasites are of very wide-spread occurrence, and are of the greatest possi- 

 ble importance in the study of the pathogenic and alternating forms, 

 since they constitute the most frequent and serious source of fallacy 

 in experimental observations. Apart from this, a considerable interest 

 attaches to them on account of the close relationship which exists 

 between many of them and the pathogenic forms. 



Almost all forms of protozoa, including gregarines, coccidia, rhizopods, 

 microsporidia, spirochaetes and flagellates, may be found in the tissues 

 of insects ; of these the flagellates are of perhaps the greatest practical 

 importance, on account of the very close resemblance between them 

 and the stages of trypanosomes of animal origin which are found in 

 the same situations in the insect. Such parasites are found in all classes 

 of Arthropods, blood-sucking and non-blood-sucking ; for experimental 

 purposes, in fact, it is a safe rule to assume that all insects have their 

 natural parasites, though all individuals are not affected. Even such 

 exclusively parasitic forms as ticks and lice are infected, though not 

 so commonly as the less specialized groups. 



The life history of these parasites here referring particularly to the 



