INSECTS AND HUMAN DISEASE 129 



proboscis, when at rest, is carried in a forward position 

 (fig. 32) ; and the presence of this awl-like piercing organ 

 renders confusion with the house fly, with its soft, sucking 

 proboscis, impossible. Common in Europe, the Americas, 

 and parts of Asia and Africa, this insect is almost as widely 

 distributed as the house fly. Both males and females 

 attack men and animals and suck their blood. The eggs, 

 to the number of about sixty, are laid on stable manure 

 and hatch out in two or three days, whilst the larvae, which 

 feed on the nidus, are full grown in about a fortnight. 

 They closely resemble house-fly maggots, but may be dis- 

 tinguished by their having quite circular spiracles on the 

 posterior segment. The pupa, which is reddish brown and 

 tub-shaped, closely resembles that of the house fly. 



TSETSE FLIES AND SLEEPING SICKNESS 



Sleeping sickness is one of the most appalling diseases 

 carried from man to man by the agency of insects. It has 

 been known to exist on the West Coast of Africa for more 

 than two hundred years, but not till 1901 did medical 

 men know that it was caused by a trypanosome, which its 

 discoverer, Dr Dutton, named Trypanosoma gambiense ; and 

 two years later it was proved that a tsetse fly, Glossina 

 palpalis, acted as the carrying agent. As trypanosomes 

 are responsible for some of the diseases of live stock, 

 which will be considered in a later chapter, it is as well 

 to describe the organisms briefly, without attempting to 

 distinguish between the various species causing different 

 diseases. Detailed descriptions can be found in many works 

 on tropical medicine, of which a list is given in the biblio- 

 graphy. Trypanosomes are very minute Protozoa, that 

 is to say, animals consisting of a single cell or a small 

 collection of similar cells; they belong to the class of 

 flagellates, so called because they are provided with a fine 

 thread-like structure, known as a flagellum (fig. 33). An 



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