ON THE DEVELOPMENT OF DERMATOBIA HOMNIS. 491 



The single pair of mouth parts consists of conspicuous heavy black hooks, 

 strongly recurved in a ventral and posterior direction. Their points, which alone 

 project, are close together, but their deep-seated bases diverge slightly towards the 

 sides of the body. They measure about 200 micra from base to bend. In a lateral 

 view (PL XXXV, Fig. 4) one can distinguish most clearly their general form. From 

 the base of each hook a large accessory piece, shaped somewhat like an anvil in lateral 

 aspect, extends caudad into the body, as far as the posterior margin of the second 

 somite. In longest diameter, that is, on its ventral extension, which forms the base 

 of the anvil, this piece measured 430 micra. 



There certainly are no anterior stigmata in this specimen, for it is well expanded, 

 and one can examine with ease the entire surface of the first somite, in the lateral 

 groove of which the stigmata are figured in later stages. Examination of it also by 

 transmitted light shows the entire absence of the large respiratory sacs such as are 

 conspicuous in a later stage among the internal organs. 



Somite II. The denticles of the second somite are decidedly larger than those 

 of the first, measuring 25 to 40 micra in length and 15 to 20 micra in diameter at the 

 circular base. Among them there are, however, those which have attained only one- 

 third to one-fourth of this size. The form of the individual denticle is the same as 

 that of those on the first somite, and like the latter these also all point caudad, and 

 slightly ventrad as well. They are, however, noticeably more numerous, the band 

 being everywhere about four rows deep, while on the dorsal and lateral aspects this 

 increases to six or eight rows; there is also a slight increase on the mid- ventral fine. 

 The anterior margin of the band of denticles is nearly a straight line parallel to the 

 somite groove, but the posterior margin is very irregular and isolated denticles or 

 groups of two or three stand here and there behind the main band. Within the 

 band, the hooks do not stand in circular rows, but in short series, traversing the 

 band at an oblique angle; in the series the dorso-posterior hooks are usually appre- 

 ciably larger, though numerous exceptions to the general rule may be noticed. This 

 band covers roughly one-third to one-half the width of the somite. 



Somite III. Here again the denticles have gained appreciably in size; for while 

 those near the anterior margin are not larger than those of the preceding somite, they 

 grow larger as the series proceed towards the posterior margin, and the average hooks 

 of this somite measure 50 to 55 micra in length and 30 to 35 micra in diameter at the 

 base. The largest hooks occur on the posterior lateral aspect of the somite, where 

 they are also most numerous. On this somite the band of denticles covers three-fourths 

 or more of the entire width and the arrangement of the denticles in irregular oblique 

 series is also evident. On the lateral and dorsal aspects of the somite the posterior 



