490 ON THE DEVELOPMENT OF DERMATOBIA HOMINISj 



distinct under a low power than those of the anterior region, although larger than 

 many of the latter. Although superficially classifiable as large and small hooks the 

 structures on the anterior somites are in reality decidedly variable among themselves. 

 These differences will be outlined later, but they concern size rather than form. At 

 the anterior end one may note the chitinous oral hooks, of which only the points pro- 

 ject from the mouth opening; their further course in the body may be indistinctly 

 followed. At the posterior end the ultimate somite projects well beyond the penulti- 

 mate and the stigmata and tracheal sacs appear in ventral or terminal view. A more 

 detailed examination demonstrates the following points in the structure and armature 

 of each somite. 



Somite I. The hooks on the external surface are few in number, scantily distrib- 

 uted, and also the smallest found anywhere. They vary from such as are simply a 

 small dot under a magnification of 100 diameters to what may be called the standard 

 size of hook on this somite and to this the majority conform. This measures about 

 20 micra in length with a circular base 10 to 13 micra in diameter. These hooks 

 are few in number and so local in arrangement that one can hardly characterize them 

 as forming a band encircling the somite near its anterior angle and leaving the lateral 

 face entirely free, as on the more posterior somites. Here the hooks are most numer- 

 ous in the dorso-lateral regions and near the mid- ventral line, where they form thin 

 cheek and chin patches which are joined only by an irregular scattering line of hooks 

 of small size. They are also scanty on the dorsum. The points of the hooks are all 

 directed posteriad and slightly ventrad. 



From the dorsal half of the anterior face of this somite projects a low rounded 

 eminence which is distinctly divided into right and left mammiform protuberances 

 that suggest rudimentary antennse. The tip of each bears a small sensory tubercle 

 and dorso-median to each of these is an insignificant ring of pigment or possibly two 

 such. The mouth forms a transverse slit ventral to the projection just described 

 and is provided with a distinct ventral lip. It leads into a cavity, conical in ven- 

 tral view, the walls of which are abundantly supplied with muscle fibres. The ventral 

 wall of this cavity is thickly strewn with teeth which immediately follow those of the 

 chin patch already noted. These teeth begin at the oral margin as fine hooks only 

 5 to 10 micra in length and increase towards the fundus of the cavity, where they are 

 slightly larger than the characteristic external hooks of the somite. In appearance 

 they simulate the hooks of the external surface, while their arrangement recalls the 

 radula of a snail. On first examination they would easily be taken for teeth grouped 

 on the external surface with points directed caudad; upon more careful study their 

 true position was definitely determined. 



