502 ON THE DEVELOPMENT OF DERMATOBIA HOMINIS. 



rows a number of very minute denticles, 10 to 15 micra in diameter at the base and 

 of about the same length. These denticles form a distinct group below the oral aper- 

 ture and an equally noticeable group on each lateral face. These are the chin and 

 cheek patches already described for the same somite on the first larva. None of these 

 denticles appear to be replaced on the new skin save a very few near the mid-ventral 

 line, and these are not on the external surface, but within the mouth cavity, where, 

 however, conditions are somewhat confused, so that an exact determination of their 

 number and position cannot be made; yet they may be followed down inside the 

 cavity as far as the anterior margin of the third somite. 



Two sets of mouth parts are easily seen (Fig. 15). The smaller hooks, ventral in 

 location, more sharply uncinate, and so placed that their points are almost directed 

 caudad, are the mouth parts of the earlier stage and agree in full with those described 

 in the first larva. Dorsal to these are two hooks, much less sharply curved and two 

 to four times as long and heavy. They are the mouth parts of the later stage. It 

 is not easy to determine their exact size in the confused mass of structures which 

 surrounds them. In the anterior region a pair of large respiratory sacs, which were 

 lacking in the earlier stage, are conspicuous; there is, however, at this time no con- 

 nection with the exterior completed, or even begun, as one can easily trace the 

 unbroken periphery of the sacs throughout. Possibly an indistinct lobing of the 

 anterior margin indicates the position from which the connection is to grow out. 



Somite II. The external skin carries many small hooks, measuring 20 to 30 micra 

 in diameter at the base and about the same in length. These hooks are distributed 

 irregularly in a band along the anterior border of the somite, covering one-half to three- 

 fourths of its width. The band is broadest in dorso-lateral aspect, where also the 

 individual denticles are somewhat larger. To replace the denticles just described 

 one finds on the new skin a group of about six in the mid-ventral line and two other 

 groups of about fifteen hooks each at the dorso-lateral angles of the body, while the 

 surface of the somite otherwise is devoid of any projections. These new hooks are 

 three or four times as large as those on the outer skin. 



Somite III. On the outer skin the small denticles, which are only a little larger 

 on the average than those of the previous somite, form a band covering the major 

 portion of the surface. On the mid- ventral line the band is narrowest, about half 

 the width of the somite, and the individual denticles are also decidedly smaller than 

 at the sides of the body. The anterior row of hooks forms a fairly distinct line along 

 the anterior margin of the somite and a few of these are directed outward or even 

 cephalad as if the somite had begun to roll in by some slight contraction at this point. 

 The larger hooks occur in the lateral and posterior portions of the band . 



