170 THE EMBRYOLOGY OF THE HONEY BEE 



with the main trunk is illustrated by figure 65, taken from a 

 larva just hatched. The spiracle is seen to be minute, in fact 

 scarcely perceptible. In older larvae it is relatively much larger. 

 This opening leads into a spherical chamber, as shown in the 

 illustration, and this in turn into a contracted passageway, the 

 spiracular branch (SpBr), which widens out just before reaching 

 the main trunk (TraTr). The taenidia (Tae) are evident in 

 the main trunk and at the inner end of the connecting branch, 

 but are replaced by a smooth chitinous cuticle for the remainder 

 of the course. This chitin becomes thickened as it approaches 

 the spherical chamber and is continuous with the cuticle of the 

 body at the external opening. Surrounding the cavity leading 

 inward from the spiracle is a single layer of long prismatic cells, 

 continuous with those of the adjacent hypodermis, and forming 

 a conical mass which may be considered as the stigma, and 

 which passes imperceptibly into the cellular portion of the con- 

 necting branch, the cells becoming progressively shorter up to 

 the junction with the main trunk. 



The rudiments of the tracheal system make their appearance 

 at Stage VIII (Figs. VIII, Villa) as two rows of pit-like in- 

 vaginations of the ectoderm, each row being situated about half 

 way between the lateral edges of the germ band and the ventral 

 mid-line. There are on the trunk ten pairs of these invaginations, 





H 



corresponding to the second to the eleventh trunk segments. The 

 two or three most anterior pairs appear first, the remainder in 

 rapid succession. The mouths of these invaginations are at first 

 irregular, the first pair being slightly larger than the others. At 

 Stage IX (Fig. IX) these have acquired the form of transverse 

 slits, the first pair however being turned obliquely to the long 

 axis of the embryo. Shortly after this — at Stage X (Fig. X) — 

 they have contracted to minute circular apertures which persist 

 as the spiracles. The invaginations themselves, at first shallow 

 depressions of the ectoderm, increase rapidly in extent and at 

 Stage IX assume the form of flat sacs wedged in between the 

 mesoderm and ectoderm (Fig. 67, TrTmv). At Stage X, each 

 of these sacs, with the exception of those of the second and 

 eleventh trunk segments, sends out four outgrowths or diverti- 

 cula, one of which is anterior, one posterior, one dorsal, and one 



