ON THE DEVELOPMENT OF DERMATOBIA HOMINIS. 505 



twice as large; they run nearly parallel to the main series and on the dorsum a single 

 such hook appears anew in a similar relation. 



The posterior limit of this somite is faintly indicated and near it one finds two 

 small hooks on the new skin, one on each dorso-lateral corner. These are directed 

 forward. Almost immediately behind these hooks there is an indication of what may 

 be the limit of the succeeding somite (VIII). In view of the close correspondence 

 between this groove and that shown on the largest larva of my series I am led to regard 

 them both as somite limits and not as chance folds. Other somite limits do not appear 

 in this posterior constricted region until one reaches the end. 



Somite X. The form of the old skin is that of a collar, and this seems to be equally 

 true of the new. The hooks on both are distinctly visible. They depart somewhat 

 in form from those found on the rest of the body, as already described for the first 

 larva, and they differ on the two skins only in size. Those of the outer skin measure 

 20 to 25 micra in length and 12 to 15 micra in basal diameter; the new hooks are 40 

 to 50 micra in length and 20 to 25 micra in basal diameter. 



Somite XL The general form of this somite is precisely that already described 

 for the first larva and is well shown in the drawing (Fig. 12). The hooks are visible 

 on both outer and inner skin, and agree in form and size with those just described for 

 the preceding somite. The spiracles of the old skin have the form of two small 

 slits in a circular field located on the posterior face of this somite just dorso-lateral 

 to the anus. This field measures 55 micra in diameter. Just within these orifices 

 one may see a tubular cavity corresponding in diameter to the field and continuing 

 some little distance cephalad in the body without change of diameter. On the new 

 skin one sees three slits of similar appearance, though much larger, measuring 250 

 micra in length. The relative positions and size of these spiracles is well shown in 

 the camera drawing (Fig. 19). The slits of one side are connected internally with a 

 sac in the form of a truncated cone whose transverse diameter at the surface is 300 

 micra and at the inner end nearly twice this. The posterior margin of these sacs has 

 a curious fringe of small chitinous rings, and the entire wall is heavily chitinized as 

 shown by the color. 



It is worth notice that in the anterior somites (II to V) one finds a pair of sacs 

 of very similar appearance but certainly without external orifices. I have been unable 

 to find the anterior stigmata postulated for this larva by some observers. Perhaps 

 they are developed later than the stages described in this paper. The other internal 

 organs could not be made out satisfactorily. 



Between the outer skin of this larva and that of the smaller larva first described 

 there is an evident agreement in armature which extends even to details, so that one 



