734 MEDICAL ENTOMOLOGY 



of an organism from the time it is ingested with the food to the 



time at which it passes back to the vertebrate, noting 

 Habitat of Parasite . . , . , , . , M-I-*- 



in passing, the cases in which theoretical possibilities 



fall into line with experimental observations. 



The stage of the parasite which is infective for the invertebrate always 



passes into the alimentary tract with the food, but there are several 



routes by which the stage infective for the vertebrate 



Passage of Parasite m reac h its destination, and it will be convenient to 

 from Invertebrate . 



Host to Vertebrate summarize these at the start. The following possibil- 

 ities present themselves : 



1. The parasite may leave by the route at which it entered, that is, 



the food canal in the proboscis. 



2. It may leave by the salivary duct in the proboscis, which, it should 



be noted, is entirely shut off from the food canal. 



3. It may leave by the anus, being passed out with the excreta. 



4. It may leave by the genital aperture ; only the case of the female 



need be considered here. In this event it passes to the next 

 generation, and must still leave by one of the other routes to 

 reach the vertebrate. 



5. It may leave by rupture of the body wall. This may take place at 



any part, or at a particular part, and may result in the entry of the 

 parasite at an abrasion of the skin, or into the alimentary tract 

 of the vertebrate when the latter swallows the invertebrate host. 

 It will be evident that, with one point of entry and several of exit, 

 the possibilities as regards the situation of a parasite within the body of 

 its insect host are considerable. The simplest case is that in which 

 the whole cycle is passed in the lumen of the gut. According to 

 Bouffard and Roubaud, Trypanosoma cazalboui never passes beyond 

 the proboscis of the tsetse fly which transmits it, the whole cycle 

 being carried out in the space between the labrum-epipharynx and 

 the hypopharynx, which in these flies is constantly filled with a liquid 

 believed to be salivary secretion. The infective stages pass into the 

 new host at the time of feeding. In other cases the organisms pass 

 into the mid-gut and hind-gut, and may leave the body either by the 

 proboscis or the anus. According to Roubaud, Trypanosoma dimorphon 

 and pecaudi, after a period of development in the hind-gut, pass again 

 forwards towards the proboscis, through which they leave the body 

 for the vertebrate host. Robertson has recently shown that the same 

 cycle is observed in the case of Trypanosoma nanum and T. pecorumn 

 in G. palpalis. 



