1918] Morphology of Genitalia of Insects 115 



insect embryology, that there is present no genetic relationship 

 between the embryonal anlages of the ambulatory appendages 

 and the gonapophyses. 



The entirely different results obtained by Wheeler, from his 

 embryological work on Xiphidium, Heymons attributes to the 

 fact of an unusually early origin of the genitaha in Xiphidium 

 and other Locustidas, even as early as the embryonal period 

 before the atrophy of the anlages of the segmental appendages. 

 Also, in addition to this fact, Heymons lays emphasis upon the 

 very small size of the sternites in Locustidas, which would very 

 easily lead one to confuse the embryonic limb-rudiments and 

 the developing genitaha or gonapophyses; while in other 

 forms, he states, the sternites are much larger. In fact, Hey- 

 mons asserts, from his investigations on Lepisma saccharina and 

 certain Hemiptera, that the sexual appendages do not originate 

 even in the same parts of the sternites where the above-men- 

 tioned anlages of segmental appendages are found. It seems 

 unbelievable that an investigator who has had as much experi- 

 ence as Wheeler in the examination of insect embryos should 

 have made the mistake attributed to him by Heymons. 



The anlages of segmental appendages on the middle and 

 cephalic, as well as on the caudal somites, degenerate during 

 the embryonic stages, and some authors, Goosens and Knatz, 

 for example, have thought that, for this reason, the prolegs of 

 caterpillars first develop during larval life. To quote again 

 from Korschelt and Heider: "We should here have to suppose, 

 as Graber also has pointed out, an embryonic rudiment remain- 

 ing for a considerable time in a dormant condition. On the 

 whole, the embryonal data seem to support the view of Balfour 

 which Cholodkovsky adopted, and to which Graber was 

 incHned, that the abdominal appendages of the Lepidoptera and 

 Hymenoptera are to be regarded as true limbs. We have already 

 had several examples, in the Crustacea, of the disappearance 

 and redevelopment of a limb out of a rudiment which has mean- 

 while been latent (Mandibular palp of the Decapod larva, 

 Korschelt and Heider, Vol. II, p. 312; maxilhpedes of the 

 Stomatopoda, Vol. II, p. 300.) A similar example is afforded 

 among the insects by the thoracic hmbs of many Hymenoptera," 

 and most Diptera for that matter. "These appear as rudiments 

 in the embryo, disappear later, and reappear in the imago." 



