302 COMPAEATIVE ANATOMY. 



Diptera, wliere uot uufrequently there is only one (Fig. 160, o), 

 thougli more commonly two or tliree. In many Coleoptera and 

 Hemiptera also the number of chambers is small. The ovarian 

 tubes are longer in most of the Hemiptera and Hymenoptera ; the 

 largest number of chambers obtains in the Neuroptera and Orthop- 

 tera, and lastly in the Lepidoptera, where there are four ovarian tubes 

 made up of a large number of chambers, which look like a string of 

 pearls. 



The arrangement of the ovarian tubes on the so-called oviduct 

 also varies very greatly. They are sometimes united into tufts, 

 sometimes broken up iuto groups, and sometimes arranged in rows. 



The so-called pseudova have been distinguished from the eggs 

 (ova); these structures are partly characterised by the absence of the 

 germinal spot, like the products of the female generative glands in 

 certain generations of the Aphides and Coccidse. As the organs 

 resemble those in which true egg-cells are formed, and as the same 

 individual is able to produce pseudova and ova at diiferent times, it 

 is best not to regard the gulf between these two products of the 

 ovary as a very wide one. These structures are links in a chain of 

 pha3nomena, which are very common among Insects ; the chain 

 begins with the arrangement known as parthenogenesis, and 

 extends to an apparent alternation of generations. The whole 

 phfenomenon depends on the emancipation of the ovum from the 

 influence of the male reproductive elements. The simplest case is 

 that in which there is no anatomical difference between the eggs, 

 some of which are developed without previous fertilisation, while 

 the rest require to be impregnated. The parthenogenesis of Bees, 

 Wasps, and many other Insects is of this kind. The arrangement in 

 Avhich the same individual no longer produces these eggs at one and 

 the same time is a further differentiation ; the emancipated ovarian 

 products are then, as a rule, differently formed (pseudova). Still 

 more peculiar is the formation of these eggs in different individuals, 

 when whole generations can do without the influence of the semen on 

 the reproductive elements, and at the same time fall to a lower grade 

 of organisation (Aphides). These structures, finally, may be formed 

 in an earlier stage in the development of the animal, and from the 

 still indifferent gdrminal gland; this arrangement, just like the rest, 

 with which it is directly allied, is derivable from sexual differen- 

 tiation (Cecidomyia). 



§ 235. 



The two, ordinarily short, oviducts seldom open separately into 

 a depression of the integument (Bphemerida). As a rule this 

 depression is further developed into a common efferent duct 

 (Fig. 159, ov), the vagina; with this accessory organs, receptaculum 

 seminis (Fig. 159, rs) and bursa copulatrix (be), are connected. The 

 seminal receptacle is seldom absent; it is formed of a stalked and 

 sometimes much-coiled vesicle. The receptacle is often a proper- 



