June 21,1924 
Morphology of the Honeybee Larva 
1183 
upon itself at every such point, the fold extending toward the middle of the 
opening in such a way that only a small circular aperture remains. The cells of 
the wall of the spiracular branch next to the hypodermis, although long-pris¬ 
matic in form, are of the same size as the latter, and are also covered internally 
by a smooth layer of cuticle. A short distance within, however, the internal 
surface of the wall becomes raised up into annular ridges, thus again reducing 
the diameter of the lumen of the branch so that a distinct antechamber is made, 
more or less spheroidal in form, bounded externally by the chitinous fold border¬ 
ing the spiracular opening, and internally by the ridges just mentioned. The 
section or zone of the branch bearing the ridges comprises about a half of the 
total length of the branch. The cells are here much larger than elsewhere and 
usually much vacuolated, particularly at their bases. The ridges are here formed 
not so much by thickenings of the cuticle as by the contour of the inner ends of 
the cells themselves, except toward the inner end of this zone, where a number 
of coarse taenidia are found which in sections present a more or less pinnatifid 
form. These intergrade with the taenidia of the usual type which are found 
farther centrad within the branch. Centrad the zone of large cells rather sud¬ 
denly gives place to the flat pavement type of cells common to tracheae in general. 
The description just given applies to the larva of intermediate size, about three 
days old. In mature larvae the zone of large cells is even more sharply differen¬ 
tiated, but the cells no longer combine to form annular ridges on their inner 
surface. They are very much vacuolated at their bases, and this portion of the 
spiracular branch presents the appearance of having contracted, reducing the 
. lumen to little more than a cleft (fig. 3, B). The significance of this reduction of 
the lumen is not evident. The general aspect and the arborescent mode of 
branching of the tracheae is shown in Figure 3, A. At their tips the branches 
hreak up rather suddenly into a tuft of delicate filaments, the tracheoles. At 
the point of origin of the tracheoles and surrounded by them is a nucleus, 
of large size as compared with those of the tracheae and of somewhat irregular 
form (fig. 3, B, Nu). Its chromatin is abundant and distributed in the form of 
coarse granules of fairly uniform size. Surrounding the nucleus is an exceedingly 
delicate layer of cytoplasm, which accompanies each tracheole and in fact forms 
part of its wall. This is most clearly seen in transverse sections of tracheoles. 
The intima, however, is that which gives to the tracheoles their sharp contour, 
and since they are still visible in caustic potash preparations it is to be inferred 
that this intima is chitinous. Near their point of origin the tracheoles frequently 
branch; farther on in their course branching becomes infrequent. The tracheoles 
of course vary in length. In a preparation of the muscular layer of the mid- 
intestine of a mature larva a single tracheole was traced for a distance of 0.548+ 
mm. In the pericardial cavity the tracheoles are much shorter than this (about 
0.300 mm.). After the preliminary branching a tracheole is quite uniform in 
diameter, and decreases gradually until the double contour is lost, the tracheole 
finally terminating in a slender point. The course taken by a tracheole differs 
in accordance with the structure of the organ or tissue supplied by it. For 
example, on the outer surface of the mid-intestine, which is very richly supplied 
with tracheoles, the latter run in straight lines or in gentle curves, the majority 
. being parallel with the circular fibers. In the fat body, on the other hand, they 
pursue a sinuous course, winding in and out between the fat cells. 
The relation of the tracheoles to the organs and tissues of the honeybee larva 
is essentially the same in every case, since nowhere has a tracheole been seen to 
penetrate a cell, the statements of Ley dig {32), Kupffer {30), Lidth de Jeude 
{33), Gilson {12), and Holmgren {16) to the contrary notwithstanding. In the 
case of the alimentary canal and the heart the tracheoles simply spread out over 
