THE H^MOFLAOELLATES. 167 



Again, many individuals, as well as races of gnats, appear to 

 have acquired immunity against the parasite, i.e., they are 

 repellent to it. Moreover, even if the Trypanosomes, in the 

 requisite phase of their life-history,^ gain a footing in the 

 Insect, their further development may be hindered by the 

 existing presence of a different parasite. On the other hand, 

 if the infection is too strong and the development of the 

 organisms too lust}^, the gnats are unable to withstand them, 

 and, instead, succumb. A like consequence, it is interesting 

 to note, may ensue if the commensal Fungus (normally, as 

 has been seen, of much utility to the Insect) obtain too great 

 an ascendancy. 



The distribution of the Trypanosomes in the body of the 

 gnat is intimately connected with the process of digestion. 



As the imbibed blood passes through the posterior part of the oesophagus, 

 the cuticular lining of the latter becomes altered into a gelatinous layer, 

 which is cast cfl: and envelopes the blood in a kind o£ sheath, the whole 

 passing on into the mid-gut or stomach. Except during the actual time of 

 feeding the binder region of the cesopbagus is invaginated into the anterior 

 part of the stomach, the narrow neck (or proventriculus) of which is 

 expanded to receive it (fig. 4, prov.). The epithelial regeneration which 

 takes place at the conclusion of digestion begins in this region soon after 

 the reinvagination of the oesophagus. 



Towards the end of the digestion (which may take from two to six days) 

 the Trypanosomes," after a period of multiplication, enter upon a resting 

 phase, and are found either attached to or between the epithelial cells. 

 After a second meal, when the fresh quantity of blood has become digested 

 and ready to be assimilated, a second period of multiplication of the para- 

 sites takes place, and the organisms gradually collect in the anterior part 

 where, since the folds of the invaginated oesophagus are non-absorptive, the 

 nutriment remains longest unabsorbed. Here the parasites commence to 



^ It must be remembered that, so far as is known in the case of the 

 Hsemosporidia, only sexual forms are able to stand the transfer from the 

 Vertebrate to the Invertebrate (see the account of the malarial parasites by 

 Minchin [75]); and Scliaudinn finds the same to be true in the case of the 

 Trypanosomes he examined. 



- This account refers to the first of the two parasites described by 

 Schaudinn, namely Trypanomorpha (Trypanosoma) noctuae; certain 

 minor differences exhibited by the other one are mentioned below. 



