34 1 
(4) The glands have often suppurating points due 
to streptococcus infection. 
Transmission *—From an infected to a healthy 
individual is effected by Gl. palpalis , and almost 
certainly by other species, though these have not as yet 
been accurately determined. The transmission is 
direct, i.e., the fly passes straight from the sick to the 
healthy person; in fact, according to experimental 
evidence on animals, the fly cannot infect a second 
animal, as the adherent trypanosomes are cleared off in 
the skin of the first animal bitten. Further, the fly that 
has bitten an infected animal ceases to be infective on 
the next day, and probably much earlier (? a few hours). 
In spite of this fact, the trypanosomes are said to 
undergo the following changes in the mid-gut of the fly. 
(i) In twenty-four hours two kinds of trypanosomes 
appear, (a) the female, with sluggish motion, large with 
granular and deeply staining cytoplasm ; the nucleus 
is large and spherical, the free flagellum is short, and the 
blepharoplast is some way from the posterior end (Fig. 
108). These are rare in blood films, (b) The male 
actively motile and slender, cytoplasm non-granular, 
nucleus usually compressed, free flagellum long. These 
are common in blood films. (2) In forty-eight hours 
trypanosomes of an indifferent type appear. These are 
the type that prevail in blood, and have a short free 
flagellum. (3) In ninety-six hours all trypanosomes have 
disappeared, nor can they be found in any other of the 
flies’ tissues. If development now proceeds in some 
unknown way, then the fly must become infective again 
later. It does not appear that experiments have been 
made with flies kept for long periods. 
Acertain percentage, one to seven percent., of tsetse 
flies, moreover, that have never fed on human blood and 
possibly not on blood at all, contain trypanosomes in 
* Also occurs through sexual intercourse in some cases (Koch). 
