166 H. M. WOODCOCK. 



reader is referred to this work. It must suffice here to give 

 some idea of the manner in which the passage of the Try- 

 panosomes to and fro, and their wandering through the body 

 of the Insect, is effected. 



The first act after penetration of the proboscis or upper lip (ul., fig. i) 

 is a sudden, particularly vigorous, respiratory contraction of the abdomen, 

 which causes the blood in the body of the gnat to rush foi-wards. Pressure 

 is thus exerted on certain sac-like diverticula of the oesophagus (od.) and 

 on the salivary glands (s. g.), the gaseous and liquid contents of which are 

 thereby expelled through the long tubular bypopharynx {hj).) into the 

 wound, carrying with them, in the case of an infected Culex, loose masses 

 of agglomerated Trypanosomes situated at the junction of the pharynx and 

 cesopbagus (shown by the cluster of stars in the figure). In this way is 

 brought about the entrance of tbe parasite into the Vertebrate host. The 

 quantity of saliva secreted is small, and serves principally to digest the 

 blood. Schaudinn finds that the poisonous effects caused by the gnat's bite 

 are due, not so much to the saliva, as to the irritant enzyme of a Fungus 

 (related to the Entomopbthorea)), whicb is a very common commensal 

 of the Insect, and lives in tbe cesophageal diverticula (the so-called "suck- 

 ing-stomachs "). Here it gives rise, during the processes of metabolism, to 

 carbonic-acid gas, and when tbe contraction of these "gas-reservoirs" takes 

 place, the secreted gas, together with a small quantity of the Fungus in a 

 pullulating condition, is injected into tbe wound. The author also thinks 

 that rapid coagulation of the blood (before it can pass into the Insect's 

 stomach) is prevented by tbe gas rather than by the salivary juice. 



With the cessation of the respiratory contraction, a small area of negative 

 pressure occurs at the junction of the gas-bubble in tbe wound and the 

 capillary-like hypopbarynx, and the liquid (chiefly blood) rushes up the 

 nariow tube into the pharynx, carrying with it any Trypanosomes it may 

 contain. From the pharynx the blood is pumped through a valve into the 

 oesophagus and its reservoirs, whicb become filled before the next respiratory 

 contraction takes place. When this happens the blood is driven from these 

 oesophageal diverticula into the stomach [st.), where digestion goes on. 

 The whole process may be repeated until the stomach becomes filled, often 

 to overflowing. 



The chances against a successful infection of the gnat 

 appear to be, however, considerable. The author found from 

 experiment that frequently infection did not occur at all, 

 many of the gnats either not biting the birds, or at other 

 times being unable to digest the blood, which was then 

 evacuated practically unaltered together with the parasites. 



