Carpentkr and Pou.aud — Lateral Spiracles in Hypoderma Larva. 79 



A series of sections cut tangential to the surface of the body-wall (figs. 

 7, 8, 9, 10) show the following features of structure. The spiracular cavity 

 (fig. 7) is lined with a dense but thin layer of chitin (ct.) continuous with the 

 outer non-staining layer of the primary cuticle. Around this is the thicker 

 stratum of deeply staining primary cuticle (ct. 1) of varying density. Sur- 

 rounding this is the translucent non-staining chitin which forms the thick 

 secondary cuticle (ct. 2) of the body-wall. Tracing the sections inwards, it is 

 found that the walls of the cavity become much folded (fig. 8), and that they 

 finally come together when the dense lining chitin is reduced to a few 

 crumpled sheets in the midst of a solid plug of the deeply staining primary 

 cuticle (fig. 9, ct., ct. 1). When the spiracular trachea leaves the body-wall 

 and enters the body- cavity, this plug becomes narrow and sinuous in cross- 

 section, and a continuation of the clear secondary cuticle surrounds it as a 

 sheath (fig. 10, ct. 1, ct. 2), the crumpled sheets of dense chitin having 

 altogether disappeared. There is no complete dense chitinous tube formed 

 around this as in the case of the tubes connected with the anterior spiracles 

 (see below, p. 80, fig. 18, ct. 2 a). The usual epithelial tube, continuous with 

 the sub-cuticular epidermis (figs. 10, 11, ep.), surrounds the whole, and in all 

 our preparations is not in contact with the secondary cuticular coat of the 

 solidified trachea. This structure persists until the solidified tube merges into 

 a short stretch of open trachea with the normal spiral thickening (fig. 20, sp. tr.) 

 which comes off as the outermost branch of the outer segmental tube as 

 already described. The presence of the normal spiral thickening at the origin 

 of the spiracular branch is convincing evidence of the nature of the structures 

 whose appearance is here described ; this is well demonstrated in the 

 photograph reproduced on Plate XIII (fig. 21). 



The Anterior Spiracles. 



The anterior spiracles are situated on the first thoracic segment, above and 

 on either side of the mouth (fig. 12). A marked groove (figs. 12, 13, p. g.) 

 runs above the mouth, and extends backwards to the dorsal aspect of the first 

 abdominal segment, indicating the suture bounding the lid of the puparium. 

 The anterior spiracles (fig. 12, a. sp.) are situated immediately dorsal to this 

 groove. The lower lip (fig. 13, v. I.) of each is crescentic in shape and raised 

 rather above the dorsal border of the spiracle, which has no definite lip. The 

 anterior spiracles are situated directly at the extreme anterior ends of the 

 longitudinal tracheal trunks. 



The structure of the anterior spiracles is best shown by a series of sections 

 through one of them, cut tangential to the body-wall (figs. 14-18). Tracing 



