140 MEDICAL ENTOMOLOGY 



manner of its kind. Musca corvina (var. vivipara) appears to have 



^ .. a similar life history; in all probability the habit is 

 Musca bezzii J * 



by no means a rare one. 



In Musca bezzii there are only two ovarioles, each constituting the 

 ovary of its own side. As in the other Muscids, the ovariole is divided 

 into a number of follicles, in each of which there is an ovum, more or 

 less mature. But as only one larva can be contained in the uterus at one 

 time, the ova in the two tubes are never in the same state of develop- 

 ment, the lowest one of one side being always more advanced than that 

 of the other. The upper follicles, also, are not stretched out in line with 

 the first, but are bent over and appear to be attached to the side of the 

 large lowest follicle containing the mature or nearly mature egg. This is 

 a result, partly at least, of the alteration in the position of the parts of the 

 reproductive organs in consequence of the presence of the larva, which 

 thrusts the ovaries forward and to the dorsal side as it grows. The two 

 ovarioles distal to the last follicle are continued downwards as oviducts, 

 which unite in the ordinary manner to form a common oviduct, which 

 leads in turn to the genital opening. - The walls of both the paired and 

 unpaired oviducts are very well supplied with muscular fibres, which are 

 arranged in definite transverse and longitudinal bands. The transverse 

 bands are very well developed at the distal end, and form a sort of 

 sphincter around the genital orifice. 



The appearance and relations of the common oviduct, which may 

 be termed here the uterus, differ entirely according to the contents. 

 When it is empty it forms a simple elongate and rather spindle- 

 shaped chamber, resembling that of the oviparous species of Musca, 

 but of greater length. But when it contains a larva ready for birth it 

 is enormously increased in size, and is pressed forward into the anterior 

 and ventral region of the abdomen ; its upper end, at which the mouth 

 of the larva is situated, is expanded beyond the point at which the 

 paired ducts unite, so that the latter appear to join the uterus some 

 distance behind the anterior end. The whole chamber is bent a little in 

 accordance with the attitude of the larva, which does not lie stretched 

 out in a straight line, but is arched so that its lower border is concave. 



When the uterus is empty it is usual to find a nearly mature egg in 

 one of the follicles, and, vice versa, when the uterus contains a growing 

 larva the ripest egg is still immature. The two ovarioles function 

 alternately, first one and then the other producing a ripe ovum and 

 passing it into the uterus. 



The stay of the larva in the uterus is not a long one, and there is 



