THE METAMORPHOSIS OF IXSKCTS 



20., 



the hypodermis of the body-wall by a slender stalk-like portion. In 

 the young pupa (B) the peripodal membranes of the histoblasts of the 

 legs are greatly shortened and the leg-buds are near the surface of the 

 body; and in the old pupa (C) the leg-buds are evaginated. The 

 wing-buds are omitted in all of the diagrams. 



In the first two segments of the body of the larva (A) there is a 

 cavity (pk) which has been termed the "pharynx"; this is the part in 

 which the oral hooks characteristic of the larvae of the Muscidas 

 develop. The name pharynx is unfortunate as this is not a part of the 

 alimentary canal; it is an invaginated section of the head, into the 

 base of which the oesophagus (&} now opens. 



In the figure of the larva (A) note the following parts: the 

 oesophagus (&) ; the ventral chain of ganglia (vg) , the brain (6) , and a 



Fig. 220. Development of the head in the Muscidae. A, larva; B, young pupa; 

 C, old pupa (From Korschelt and Heider after Kowalevsky and Van Rees). 



sac (ba) extending from the so-called pharynx to the brain. There are 

 two of these sacs, one applied to each half of the brain, but only one of 

 these would appear in such a section as is represented by the diagram. 

 These sacs were termed the brain-appendages by Weismann. In each 

 of the "brain-appendages" there is a disc-like thickening near the 

 brain, the optic disc (od) ; this is a histoblust which develops into a 

 compound eye; in front of the optic disc there is another prominent 

 histoblast; the frontal disc (//), upon which the rudiment of an antenna 

 (at) is developed. 



In the larva the brain and a considerable part of the "brain- 

 appendages" lie within the third thoracic segment. In the young 

 pupa (B) these parts have moved forward a considerable distance; 

 and in the old pupa (C) the head has become completely evaginated. 

 The part marked p in the two diagrams of the pupa is the rudiment 

 of the proboscis. 



