386 MEDICAL ENTOMOLOGY 



at the anterior end of the egg, and that the walls of the uterus contract 

 so closely around their contents, by virtue of the large amount of muscular 

 tissue in the wall, that if the cavity were occupied the sperms would 

 neither be able to reach to and penetrate the egg, nor would they be 

 able to pass into the spermathecae to be in readiness to fertilize the next 

 egg. It is therefore essential that copulation, which takes place only 

 once in the life of the female, should be accomplished while the uterus is 

 empty. 



Unfavourable conditions of temperature or humidity may also affect 

 the reproductive function, either by causing the retention of the ova 

 within the oviduct, or by stopping their development. 



The egg, in the normal course of events, descends into the uterus 

 on the second day after the birth of the larva which preceded it. On 

 the fifth day the larva emerges, and thereafter grows 

 development ra pidly, unt ^ on tne ninth or tenth day it has attained 

 its full development and is born. During this period 

 it passes through three moults, as is the case in the larvae of most 

 Diptera. The first takes place very early, soon after the birth of the 

 larva from the egg ; the second is later, and takes place whe,n the 

 larva has reached twice or more its initial size ; the third takes place, 

 as in oviparous Diptera, at the time of pupation. In the first two moults 

 the integument is very soft and thin, and it is only in the last, when 

 the larva is about to be exposed to the external air, that the cuticle 

 assumes any considerable thickness. The larval skins are transformed 

 into a black compact amorphous mass, accumulated during the life 

 of the larva in the anterior and ventral portion of the uterus, and extruded 

 after the birth. 



The full grown larva is cylindrical in shape, but of constantly chang- 

 ing dimensions during the short period which elapses between its birth 



and pupation ; it consists of thirteen segments (Rou- 

 The larva . 



baud), and measures rrom 6 to 7 mm. in length, the 



breadth when at rest being less than one-third the length. In colour 

 it is white, or yellowish white, except at the posterior end, which is 

 highly modified and of a jet black colour. The head, as one would 

 expect from its mode of life, is very small and inconspicuous ; the 

 succeeding segments down to the twelvth, on the ventral surface 

 of which a minute vestigial anus can be detected, resemble one another, 

 and have a roughened integument. The last segment is highly modified, 

 and presents an appearance which is peculiar to the flies of this genus ; 

 the anterior or annular portion is strongly chitinized and densely 



