412 THE BIOLOGY OF INSECTS 



Trypanosoma) which is the actual cause of the deadly 

 " nagana " disease in cattle and horses. A trypanosome 

 when fully grown is a narrowly elongate cell, the locomotor 

 whip-like process (flagellum) arising at the hinder end, is 

 turned forward alongside the cell-body to which it is 

 attached by a delicate *' undulating membrane," and beyond 

 which it projects as a kind of appendage (Fig 84, e). Such a 

 protozoan is well adapted for swimming through the blood- 

 plasma, either in the blood-vessels of the beast or in the 

 digestive tract of the tsetse-fly. Swallowed by the latter 

 when drawing blood from one animal the trypanosome may 

 be readily injected into another where it is " capable of 

 reproducing itself " at an alarming rate. It is of much 

 interest to note that Trypanosoma hrucei is present in 

 numbers in the blood of many African wild beasts, in which 

 it causes no symptoms of disease. These creatures, accus- 

 tomed to act as its hosts through countless generations, are 

 immune to its effects, while closely allied animals imported 

 into Africa from Europe succumb rapidly to the poisonous 

 action of the tiny parasite's secretion. 



In many cases of the infection of the large animals with 

 Protozoan parasites, the blood-sucking insect serves simply 

 as a ** carrier " transmitting possibly many individual 

 protozoans from one host to another. Frequently, however, 

 the insect-host is of great importance in providing scope 

 for some special and necessary phases in the parasite's life- 

 history. For example, there lives in the blood of rats a 

 parasitic flagellate known as Trypanosoma lewisi whose 

 alternate host is the Rat Flea (Ceratophylliis fasciatus). 

 When trypanosomes are swallowed with blood sucked into 

 the flea's stomach, processes of cell-division begin which 

 result in the production in the flea's hind-gut of a large 

 number of parasitic forms, known as Crithidia, smaller than 

 the typical Trypanosoma and without flagella. These 

 migrate forward along the digestive tract, and injected from 

 the mouth into the blood of other rats grow there into adult 

 Trypanosoma lewisi. (See E. A. Minchin, 1912.) 



Of all the Protozoa that live in the bodies of insects the 



