28 



The Irish Naturalist. 



March. 



found. In the fourth stage (Fig. 5) larva the anterior ends 

 of the hypostomal sclerites {h.s.) have become fused together 

 forming a comparatively wide chitinous band on the lower 

 lip or ventral aspect of the mouth. 





ph s 



Fig. 5. — Hypoderma bovis, fourth-stage larva, mouth armature, ventral 

 view, X 100. 



h, mouth hook ; ph. s. pharyngeal sclerite ; p.s. parasiomal sclerites ', 

 h.s. hypostomal sclerite. 



The pharyngeal sclerites (ph. s.) have largely lost their 

 opaque horny structure and have become a pair of wide 

 almost transparent plates. The mouth-hooks (//) themselves 

 have become greatly reduced in size and are further removed 

 from the central spine (p. s.) the sclerites composing which 

 have become more closely fused together. All the mouth 

 parts have both relatively and actually undergone a great 

 diminution of size. 



It is in the fifth and final larval stage that the greatest 

 change is noticeable (Fig. 6) . Each of the pharyngeal sclerites 

 {ph. s.) has become very broad with its outer margins 

 forming almost a semicircle. Their appearance now is 

 that of two wing-like outgrowths from the wall of the gullet, 



