NO. 6 INSECT ABDOMEN SNODGRASS II 



segments, and Wheeler points out that " the reproductive organs of 

 Xiphidium are therefore truly metameric in their origin." Later, how- 

 ever, with the rapid multiplication of the germ cells, the successive 

 gonad rudiments on each side of the body elongate and unite, result- 

 ing in the " formation of a continuous strand of germ cells with their 

 accompanying epithelial cells ". 



The recent account by Lautenschlager (1932) of the development 

 of the female gonads in Solenobia triqueirella (Psychidae) is in 

 essential agreement with the observations of earlier writers on Or- 

 thoptera. The paired segmental groups of germ cells, lying in the 

 fourth, fifth, and sixth abdominal segments, condense and unite on 

 each side into a single group having the position of the definitive gonad 

 in the fifth segment. Here each composite group of germ cells be- 

 comes enclosed in a layer of small mesodermal cells. The latter form 

 a thin sheath about the germ cells dorsally and laterally, but ventrally 

 they form three thick cellular masses, which later give rise to the 

 ovarial pedicles and the lateral ducts of the ovaries. According to 

 Lautenschlager the follicle cells of the definitive ^gg tubes of Solonohia 

 also take their origin from the cells of the mesodermal sheaths of 

 the gonads. 



Seidel (1924), on the other hand, finds that in Pyrrhocoris apterus 

 the segmental groups of germ cells, formed at an early embryonic stage, 

 become surrounded individually by mesodermal epithelium, and remain 

 thus, up to a relatively late period of development, as a series of dis- 

 tinctly separated gonadial rudiments segmentally distributed on each 

 side of the body in somites // to VIII of the abdomen. Eventually, the 

 posterior rudiments migrate forward until all are assembled in seg- 

 ments // and ///, where those of each lateral group develop directly 

 into the seven genital tubes of the adult organ. The only union that 

 takes place between them is in the lower tubular parts of the primary 

 elements, which unite to form the lateral duct. Seidel claims that this 

 condition in Pyrrhocoris apterus demonstrates the direct origin of 

 the definitive genital tubes from the segmental gonadial rudiments 

 of the embryo. However, since the case appears to be exceptional, 

 and since the Hemiptera are in many ways specialized insects, the 

 suspicion is created that the facts observed are to be interpreted as 

 the result of elimination in some of the earlier stages of development. 

 Yet, as we shall see, there is evidence to suggest that the definitive egg 

 tubes and sperm tubes were first formed as segmental outgrowths 

 from continuous gonads produced by the union of the primitive seg- 

 mental gonads. 



With most of the higher insects the reproductive organs appear 

 from the beginning as continuous genital ridges. In the honey bee, as 



