OF OECANTHUS AND TELEAS. 261 



From these figures it is apparent that in many if not all insects the serosa and amnion 

 play the same r.jle that they do in Oecanfhus, i. e., the serosa function? as a yolk sac 

 while the amnion is the dorsal wall of the insect. Hence the so-called dorsal organ is but 

 the remnant of the yolk sac. 



Note. — In the light of the important discoveries in the embryology of Tracheata made by the late 

 Professor Balfour (F. M. Balfour — The Anatomy and Development of Peripatus Capensis. Quart. Journ. 

 Micr. Sci., i). ser., No. xc, April 1883, pp. 213-259, pi. xm-xx) the process of gastrulation in Oecanthus is 

 more satisfactorily explained. In Oecanthus the original blastopore or gastrula mouth, existing near the 

 head end of the egg after the formation of the blastoderm, elongates with the formation and growth of 

 the germinal band into the form of a shallow furrow (the so-called mesodermic groove of insects). 

 It does not form, as in Peripatus, a slit-like opening within the limits of the germinal band, the lips of 

 which coalesce in the median line leaving at either extremity of the blastopore an opening into 

 an archenteric cavity — Balfour's sc-called mouth and anus,— but the posterior opening begins as a 

 shallow pocket and opens into the archenteron at a very late period. The mesoderm arises in the 

 region of the primitively circular mouth and grows backward, following the course of the groove in the 

 germinal band. The anus consequently is a part of the blastopore while the mouth is a secondary forma- 

 tion. The embryonic mouth persists until near the time of the closure of the body wall over the dorsum. 



These views may be stated as follows : — 



1. At the close of blastoderm formation the archenteric cavity is completed, and its mouth is on the 

 dorsum of the cephalic region (compare Oniscus, Mysis, Scorpio, Libellula, Calopteryx and Hemiptera par- 

 asita), hence the blastoderm equals endoderm, not ectoderm. 



2. The ectoderm arises simultaneously with, if not previous to, the endoderm ; but, instead of surround- 

 ing the entire endoderm, it covers only a small area on the dorsal side in the region of the gastrula mouth. 

 Subsequently, however, it surrounds the archenteron and it also becomes incomplete at the circular gastrula 

 mouth, where it unites with the endoderm. 



3. The mesoderm arises before the full completion of the gastrula as an unpaired plate in the region of 

 the fusion of ectoderm and endoderm, i. e., near the lips of the gastrula mouth, and grows both backward 

 and dorsad between the cctodermic and endodermic layers. 



4. The so-called mesodermic invagination is to be considered as connected with the blastopore — perhaps 

 in some cases the only indication of the previous existence of a gastrula mouth. 



5. On account of the pressure of an unwieldy mass of yolk these processes are somewhat modified, but 

 not so completely as to lose their identity. On this account the completion of the gastrula is retarded 

 until the organs of the embryo are well advanced. 



In Scorpio, Mysis, and Oniscus, the blastopore is dorsal in position. 



TELEAS. 



A parasitic Ichneumon fly, probably of the genus Teleas, infesting the eggs of Oecanthus, 

 presents highly interesting stages of development which were first made known by 

 Metschnikoff in 1866, and were more fully described for Teleas and a number of related 

 forms by Ganin about two years later. The results of my study on Teleas differ in some 

 points from those of Metschnikoff and Ganin on species of the same genus. De Filippi (16) 

 has described the embryonic changes of a Pteromalian, parasitic in the egg of a curculio 

 (Rhynchites), which, to judge from his figures, closely resembles Teleas in its younger 

 larval stage. 



Metschinoff (32) gives a short account of the development of a species of Teleas infest- 

 ing the eggs of Gerris lacustris, of which the following is a brief summary. The earliest 

 observed stage was that of the stalked egg in which the blastoderm (Keimhaut) was already 



