6i 



diaphragm. This membrane is attached to the heart on its 

 dorsal and its ventral surface. The membrane and sinuses 

 can be especially well seen in cross sections of the body. 



For an account of the circulation in insects see the Cam- 

 bridge Natural History, Vol. V., pp. 132-136. 



The histoblasts of wings and legs.— Returning to the 

 first specimen (that opened along the dorsum) carefully re- 

 move the muscle fibers from the body-wall of the three front 

 (thoracic) segments of the body, not including the head. Be 

 careful not to remove certain small whitish bud-like bodies 

 lying between the muscles and the body-wall. In specimens of 

 suf^cient age \\\^ histoblasts (or itnaginal buds or imaginal discs 

 as variously called) of the wings and legs of the adult fly and 

 of the external prothoracic respiratory tubes may be seen as 

 small sac- or bud-like bodies lying against and attached to 

 the inner surface of the body-wall of the thoracic segments. 

 There are two pairs of these imaginal buds in each thoracic 

 segment corresponding respectively to the prothoracic legs 

 of the imago and prothoracic respiratory tubes of the pupa, 

 the meso-thoracic legs and wings of the imago, and the meta- 

 thoracic legs and halteres of the imago The study of the 

 morphology and development of these imaginal buds is too 

 difficult to be referred to here. It is sufficient to know that 

 the legs and wings of the fly begin thus to develop under the 

 surface of the body and continue this internal development 

 until the larva changes into a pupa. These buds may be 

 seen to be intimately connected with the thoracic skin, and 

 are, in fact, actual invaginated parts of this skin. 



The head and its appendages.— After finishing the dis- 

 section of the internal organs remove the head entirely from 

 the rest of the specimen and examine with simple microscope. 

 Each of the short antenncB arises from a small lobe on the plate 

 covering the top of the head. This plate is long, tapering and 

 decurved behind. It is united along the anterior parts of its 



