The Tracheal System of Simulium. 709 



the body. Repeated examination of sections taken in 

 various planes tlirough the gill-base seems to make this 

 point certain. It is also noticeable in these sections that 

 the underlying epithelium is absent. In the developing 

 gill in the larva, and for some time after pupation, the 

 epithelium is present. It disappears eventually, and in 

 preparations of old filaments presents a disorganised 

 appearance. In an old pupa the filaments are frequently 

 broken across, and it is not uncommon to find that foreign 

 bodies, e.g. diatoms, have entered the cavity of the gill 

 through the accidental opening. From these observations 

 it seems likely that the interchange of gases between the 

 gill and the surrounding water is carried on without the 

 intervention of a cellular epithelium. 



The respiratory organs of the pupa of Simulium have 

 often been described as tracheal gdls. Tracheae, however, 

 are quite absent, and Vogler (No. 6, p. 33) has therefore 

 suggested the name tube-gills {Rohrcnhiemcn) for these 

 structures. The filaments are certainly tubular; since, 

 however, air does not occur within the cavity, but is 

 confined to spaces excavated in the cuticular wall, it 

 seems more appropriate to describe them as cuticular 

 gills. 



De Meijere (Nos. 1 and 2) has recently published 

 accounts of the compound spiracles of Dipterous larvas and 

 pupae. According to these observations, the original 

 spiracle becomes closed, and the trachea leading from it to 

 the adjacent tracheal trunk collapses and forms a solid cord. 

 The closed spiracle De Meijere names the outer stigmatia 

 scar (dussere Stigmcnnarhe) ; the solid cord, the scar cord 

 {Narhenstrang), and the place where it joins the tracheal 

 trunk the inner stigmatic scar {innerc Stigmcnnarhe). The 

 new larval or pupal spiracle arises as an outgrowth 

 from the adjacent trachea close to the inner stigmatic scar. 

 The outcrrowth orrows towards, and comes in contact with 

 the skin. It becomes hollow, and contains a special 

 chamber, the chitinous wall of which shows a retiform 

 thickening, and is often clothed with a felt of hairs. This 

 chamber De Meijere names the felted chamber (FilzJcccmmer). 

 At the place where the felted chamber becomes attached 

 to the skin, the external cuticle shows thin spots which 

 are rnxvaed 2nts {Tilpfcl), from their resemblance to the thin 

 places in the walls of plant-cells. From the presence of 

 these pits, the secondary spiracle is named a pitted stigma 



