80 Proceedings of the Royal Irish Academy. 



the sections inwards from the exterior, we see that at first the spiracle is a wide 

 open cavity ( fig. 14, c) in the substance of the body-wall, lined by a thin layer 

 of the dense feebly staining chitin <<7.) and surrounded by an area of deeply 

 staining chitin ( et. 1 ). Further in (fig. 15 ) the cavity is much narrowed, and the 

 walls have become folded, though the same areas of chitin are still discernible. 

 The walls of the cavity now begin to approach each other, so that the aperture 

 becomes smaller and smaller (fig. 16) until finally it disappears entirely, 

 leaving the deeply staining chitin as a central core (fig. 17, et. 1). Before the 

 cavity is finally obliterated, curved pieces of dense, hard chitin, apparently 

 specialized portions of tl lary cuticle (fig. 16, et. 2a), have begun to 



appear round the deeply staining area. When traced inwards, these close 

 round the above-mentioned core, so as to form a continuous tube fig. 18,cl. 2a). 

 This structure continues when the tube has left the body- wall and entered 

 the body-cavity, a layer of translucent chitin of the secondary cuticle (fig. 18, 

 interposed between the solid core and the walls of the tube 

 abo'. i tube of hard chitin entirely filled by a deeply staining 



core with a covering I chitin, but showing no trace of the spiral 



structure as in the ordinary tracheae. After some distance the trachea 

 widens considerably, the solid core o ruptly, and is replaced by a 



cavity many tine imeter, the walls of which are strengthened by the 



usual spiral .-tincture (fig 



ire, the interior spiracles in llypoderma or any of its 

 near allies ' ribed except for Brauer's drawing of their 



position in ■■'*>. viii, fig 4 . It is of interest to find 



that the air-l _ -tinctures are plugged up with a chitinous 



core, so that they cannot be functional; they are indeed vestigial like the 

 lateral spira in the present paper. In this respect the anterior 



spiracles of II ma offer a striking contrast to the corresponding struc- 



tures in Gastrophilus, which have recently been well described and figured 

 by Enderlein ('99, Taf. ii , fig 2 27 These are provided with a number of 

 fine apertures, and m :.:idently regarded as functional. The divergence 



in larval strnctun 1 1 ■ poderma and Gastrophilus is thus considerable, 



and we have failed to find any trace of lateral spiracles in the latter. In this 

 connexion it is of inl note that some modern systematic students of 



the Diptera have ceased to regard the Oestridae as a natural family, assigning 

 llypoderma to the Tachinidae, and Gastrophilus to the Anthomyiidae. This 

 classification is adopted, for example, by Schnabl and Dziedzicki in their 

 monograph ('11) on the latter family, in which special importance is attached 

 to the male genital armature and the arrangement of the thoracic bristles in 

 the tlv. 





