130 MEDICAL ENTOMOLOGY 



of the wall, and this is neither very great nor is the wall in that part 

 which lies in the thorax and abdomen of such a nature as would render 

 it difficult of penetration by any parasite possessed of means of move- 

 ment. Once a parasite has penetrated the wall of the alimentary 

 canal and become free in the body cavity, then, whether it is motile or 

 not, it will, unless it is too large to pass through the lateral openings 

 into the heart, be drawn into the circulation and transported to the 

 head ; it may then, of course, find its way back in the slower backward 

 stream, or it may become attached or entangled in some part of the body. 



The relations of the haematocoele in the proboscis of the fly have 

 been described at some length in connection with the mechanism of the 

 mouth parts. If what was said on that subject be considered from the 

 point of view of the possible movements of a parasite which has per- 

 forated the w r all of the alimentary tract, it will at once be obvious 

 how intimate are the relations of the parasite to the blood in the 

 wound, or to the food surface in the case of the non-biting flies. Once 

 it is free in the blood of the abdomen it may be caught up at the dilata- 

 tion of the heart, and on the next contraction it may be forced along with 

 the blood stream into the aorta, and so on directly to the head. In the 

 eversion and closure of the labella, which together constitute the act of 

 biting in the Muscid flies, there is a forcible and rapid to-and-fro move- 

 ment of the blood in the lower part of the proboscis, and the chances that 

 the parasite will sooner or later be driven into the space between the 

 inner and outer walls of the labella are very great indeed. When it has 

 arrived there it is only separated from the blood of the presumptive 

 vertebrate host by the thin inner wall of the labella. Whether and under 

 what circumstances this inner wall could be perforated is one of the 

 questions to be decided in each case. 



The relations of the haematocoele of the labella are the most striking 

 in the case of Mitsca and its biting allies, but the same conditions are to 

 be found in the Orthorraphic biting flies. The labella of Tabantis, when 

 they function as feeding organs, are distended in exactly the same way as 

 those of Mitsca, and the distension can be readily demonstrated experi- 

 mentally by pressure on the head. In other forms, in which the labella 

 are not functional in this manner, it is extremely probable that something 

 of the same kind occurs in the eversion of the labella preliminary to 

 the insertion of the piercing parts. In the mosquito the position of 

 the tip of the proboscis, which appears to act merely as a guide and 

 support to the other parts, is a most suggestive one. 



The blood is easily obtained for examination by taking one of 



