20 



PSYCHE. 



position as an extremity. It is an ap- 

 pendage of the body wall ; it has become 

 iiTegiilarly cj'lindrical in shape and pos- 

 sesses a number of constrictions and 

 folds, which in the case of the ventral 

 discs are equivalent to the joints of the 

 future leg. The proximal edges of the 

 discs, those in contact with the larval 

 hypodermis, grow and extend them- 

 selves and take the place of larval iiy- 

 podermis in proportion as this is de- 

 stroyed by phagocytes. 



The metamorphosis of the abdomen 

 is retarded and does not begin until that 

 of the head and thorax is well advanced. 

 Then in each abdominal segment the 

 two ventral and four dorsal discs (Van 

 Rees found two additional dorsal discs 

 in each segment) begin to grow and take 

 the place of the disappearing larval hy- 

 podermis. 



Kowalevsky * discovered that the discs 

 of the last segment do not take part in 

 the formation of hypodermis, but of the 

 endgut with the rectal glands, and that 

 they are situated in the vicinity of the 

 larval anus. The metamorphosis of the 

 larval internal organs was correctly re- 

 ported first by Kowalevsky t i" the year 

 preceding the publication of Van Rces' 

 paper. All of these organs are de- 

 stroyed b}' phagocytes except the 

 central nervous system, the heart, the 

 reproductive organs, and three pairs of 

 thoracic muscles. These with the excep- 

 tion of the reproductive organs remain 



* Kowalevsky. Beitrage z. Kenntniss d. nacliembry- 

 onalen Entw-icklung d. Musciden. I Tllcil. Zeit. f. Wiss, 

 2ool. 45 Bd. 1S87, p. 557. 



active, functional organs during the 

 pupal period and are not attacked by 

 phagocytes, but pass directly into the 

 imago without great change and be- 

 come imaginal organs. The organs 

 destrojed are reconstructed from imag- 

 inal discs in a way similar to that 

 already described. 



The only paper dealing with imagi- 

 nal discs which has appeared since 

 Van Rees' is one of my own published 

 in iS93.t It contains a description of 

 the larva of Melophagus ovinus. a pupi- 

 parous dipter. 



The pupipars are cyclorrhaphic bra- 

 chycera and very closely allied to the 

 muscids, so that we may expect to find 

 the same imaginal discs in their larvae 

 as in the muscids. And we do, in fact, 

 find in general similar conditions, but 

 there are several interesting differences. 

 The larva is apodous and acephalous 

 like the muscidian, but in manv respects 

 it is much less highly specialized ; 

 it seems, in fact, as if it might 

 represent the ancestral stage in dip- 

 teran phvlogenv at which the muscids 

 are beginning to draw away from 

 their relatives, to occupy a position be- 

 tween Corethra and Musca. In the 

 position of the thoracic discs, for in- 

 stance, it closely resembles Corethra. 

 We find these discs just beneath the 

 integument in two very regular rows 

 and not in the center of the larva as in 

 Musca. The accompanying wood cut 

 represents dorsal and ventral frontal 



X Pratt. Beitrage z. Kenutnis d. Pupiparen. Die Larve 

 von Melophagus ovinus. Archiv f. Naturgeschichte. Bd. 

 59. >S93. P- "S"' 



