54 Dr. O. Zacharias on the Development 



Fig. 12. Front view of tlie head of Cicada sp. i^l, plates with which 



the maxillae articulate. 

 Fuj. 13. Transverse section through the proboscis oi Sarcopsylla penetrans, 



middle. 

 Fig. 14. Transverse section through the rostrum of Notonecta glauca, 



apex. 

 Fig. 15. Transverse section through the proboscis of Pulex irritans, base. 



VI. — New Investigations on the Development of the Viviparous 

 Aphides. Bj Dr. Otto Zacharias *. 



Since the appearance of Metschnikoff's ' Embryologische 

 Studien an Insecten ' (1866) the development of the embryo 

 of the viviparous Aphides has not again been made the sub- 

 ject of a monographic investigation. What the Russian 

 author established with regard to the mode of development of 

 the "pseudova" of Aphis Rosce and A. Pelargonii -passes pretty 

 generally for all that is observable at present. Metschnikoff's 

 description of the development of Aphides (at least in its 

 fundamental features) is regarded as a " rocher de bronze," 

 which presents no point of attack for an incisive criticism. 

 This, however, is not the case, and I will, in a memoir that 

 will appear very shortly, furnish the proof that Metschnikoff's 

 description of the Jirst developmental stages (as far as the 

 formation of the S-shaped germinal streak, and even some- 

 what later) by no means agrees with the facts. For the 

 subsequent stages I have also obtained quite different results 

 of investigation, which I shall venture to summarize at the 

 conclusion of this preliminary note. 



The observation of the embryonic development of the vivi- 

 parous Aphides is for many reasons a difficult matter. 

 Besides the minuteness and delicacy of the objects with which 

 we have to do, there is a third condition which causes many 

 obstacles to the investigation, namely the clearness and strong- 

 refractive power of the protoplasmic contents of the egg. If 

 in the case of the eggs of many other insects we have to 

 contend with the obscurity of their yelk, it is in the Aphides 

 the crystal clearness of the latter which frequently acts very 

 prejudicially : prejudicially, inasmuch as under the circum- 

 stances indicated the upper half of the egg constantly acts 

 upon the lower half (or vice versa) ^ like a lens with a very 

 short focus, and not only enlarges but also distorts those 



* Translated from the ' Zoologischer Anzeiger,' no. 168, May 26, 1884, 

 pp. 292-296. 



